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    1462 International DriveTraverse City, MI 49686

    231-947-0110www.tellurex.com

    Frequently Asked Questions About

    Our Cooling And Heating Technology

    1. How does this technology work?

    The basic concept behind thermoelectric (TE)technology is the Peltier effecta phenomenonfirst discovered in the early 19th century. ThePeltier effect occurs whenever electrical currentflows through two dissimilar conductors.Depending on the direction of current flow, the

    junction of the two conductors will either absorb orrelease heat. Explaining the Peltier effect and itsoperation in thermoelectric devices, is a verychallenging proposition. It ultimately keys onsome very complex physics at the sub-atomiclevel. Here we will attempt to approach it from aconceptual perspective with the goal of givingreaders an intuitive grasp of this technology (i.e.,without getting too bogged down in the minutia).

    In the world of thermoelectric technology,semiconductors (usually Bismuth Telluride) arethe material of choice for producing the Peltiereffectin part because they can be more easilyoptimized for pumping heat, but also because

    designers can control the type of charge carrieremployed within the conductor (the importance ofthis will be explained later). Using this type ofmaterial, a Peltier device (i.e., thermoelectricmodule) can be constructed. In its simplest form,this may be done with a single semiconductor'pellet' which is soldered to electrically-conductivematerial on each end (usually plated copper). Inthis 'stripped-down' configuration (see Figure 1),the second dissimilar material required for thePeltier effect, is actually the copper connectionpaths to the power supply.It is important to note that the heat will be moved

    (or 'pumped') in the direction of charge carrier flowthroughout the circuitactually, it is the chargecarriers that transfer the heat. In this example, 'N-type' semiconductor material is used to fabricatethe pellet so that electrons will be the chargecarrier within the molecular structure. With a DCvoltage source connected as shown, electrons willbe repelled by the negative pole and attracted bythe positive pole of the supply; this forces electronflow in a clockwise direction (as shown in the

    drawing). With the electrons flowing through theN-type material from bottom to top, heat isabsorbed at the bottom junction and activelytransferred to the top junctionit is effectivelypumped by the charge carriers through thesemiconductor pellet.

    In the thermoelectric industry, 'P-type'semiconductor pellets are also employed. P-typepellets are manufactured so that the chargecarriers in the material are positive (known inelectronics as 'holes'). These 'holes' are placeswhere electrons can easily fit when a voltage is

    applied and they enhance the electricalconductivity of the P-type crystaline structure.Positive charge carriers are repelled by thepositive pole of the DC supply and attracted to thenegative pole; thus 'hole' current flows in adirection opposite to that of electron flow.Because it is the charge carriers inherent in thematerial which convey the heat through theconductor, use of the P-type material results inheat being drawn toward the negative pole of thepower supply and away from the positive pole.This contrasting heat-pumping action of P and N-type materials is very important in the design of

    Figure 1 Figure 2

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    practical TE devices (as will be explained in thenext question). While the illustration hereforsimplicity's sakeshows 'hole' flow through theconnections to the power supply, in reality,electrons are the charge carriers through thecopper pathways.

    2. Why are two types of material (P and N)required?

    Unfortunately, while you can make a simplethermoelectric device with a single semiconductorpellet, you can't pump an appreciable amount ofheat through it. In order to give a TE devicegreater heat-pumping capacity, multiple pelletsare used together. Of course, the initialtemptation would be to simply connect them inparallelboth electrically and thermallyasshown in the Figure 3. While this is possible, itdoes not make for a very practical device. The 'flyin the ointment' here, is that the typical TE

    semiconductor pellet is rated for only a very smallvoltageas little as tens of millivoltswhile it candraw a substantial amount of current. Forexample, a single pellet in an ordinary TE devicemight draw five amps or more with only 60 mVapplied; if wired in parallel in a typical 254-pelletconfiguration, the device would draw over 1270amps with the application of that 60 mV(assuming that the power supply could deliver thatmuch current).

    The only realistic solution is to wire the semi-conductors in series, and doing so in a way thatkeeps them thermally in parallel (i.e., pumpingtogether in the same direction). Here, we mightbe tempted to simply zig zag the electricalconnections from pellet to pellet (see Figure 4) toachieve a series circuit. This is theoreticallyworkable, however, the interconnections betweenpellets introduce thermal shorting that significantlycompromises the performance of the device.Fortunately, there is another option which gives us

    the desired electrical and thermal configurationwhile better optimizing the thermoelectric effect.

    Figure 4

    By arranging N and P-type pellets in a 'couple'(see Figure 5) and forming a junction betweenthem with a plated copper tab, it is possible toconfigure a series circuit which can keep all of theheat moving in the same direction. As shown inthe illustration, with the free (bottom) end of the P-

    type pellet connected to the positive voltage

    potential and the free (bottom) end of the N-typepellet similarly connected to the negative side ofthe voltage, an interesting phenomenon takesplace. The positive charge carriers (i.e, 'holes') inthe P material are repelled by the positive voltagepotential and attracted by the negative pole; thenegative charge carriers (electrons) in the Nmaterial are likewise repelled by the negativepotential and attracted by the positive pole of thevoltage supply. In the copper tabs and wiring,electrons are the charge carriers; when theseelectrons reach the P material, they simply flowthrough the 'holes' within the crystalline structureof the P-type pellet (remember, it is the chargecarriers inherent in the material structure whichdictate the direction of heat flow). Thus theelectrons flow continuously from the negative poleof the voltage supply, through the N pellet,through the copper tab junction, through the Ppellet, and back to the positive pole of thesupplyyet because we are using the twodifferent types of semiconductor material, thecharge carriers and heat are all flowing in the

    Figure 5Figure 3

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    same direction through the pellets (bottom to topin the drawing). Using these special properties ofthe TE 'couple', it is possible to team many pelletstogether in rectangular arrays to create practicalthermoelectric modules (see Figure 6). Thesedevices can not only pump appreciable amountsof heat, but with their series electrical connection,are suitable for commonly-available DC powersupplies. Thus the most common TE devices now

    in useconnecting 254 alternating P and N-typepelletscan run from a 12 to 16 VDC supply anddraw only 4 to 5 amps (rather than 1270 amps at60 mV, for example).

    Of course, in fabricating devices with multi-pelletarrays, you must have a means to mechanicallyhold everything together. A solution is to mountthe conductive tabs to thin ceramic substrates (asshown Figure 7); the outer faces of the ceramicsare then used as the thermal interface betweenthe Peltier device and the 'outside world'. Notethat ceramic materials have become the industry

    standard for this purpose because they representthe best compromise between mechanicalstrength, electrical resistivity, thermal conductivity,and cost.

    3. Do these P and N couples function likediodes?

    No. It is easy to see why many people expectcouples to operate like diodes, given the pairing ofP and N materials, but there is a crucialdifference. In the manufacturing of diodes, adepletion region is created between theimmediately adjacent P and N layers. When the

    diode is forward-biased, charge carriers are drawninto the depletion region and the diode becomesconductive; when reverse-biased, charge carriersare drawn away from the depletion region and thediode acts like an open circuit. Without adepletion region, a TE couple cannot act like adiode; the couple will conduct in both electricalpolarities and there is no fixed voltage drop acrossthe couple (unlike the nominal 0.6 to 0.7 VDC

    typically dropped across a forward-biased silicondiode).

    Figure 6

    4. How is a typical thermoelectric (TE)system configured?

    Let's look conceptually at a typical thermoelectricsystem designed to cool air in an enclosure (e.g.,picnic box, equipment enclosure, etc.); this isprobably the most common type of TE application.Here the challenge is to 'gather' heat from theinside of the box, pump it to a heat exchanger onthe outside of the box, and release the collected

    heat into the ambient air. Usually, this is done byemploying two heat sink/fan combinations inconjunction with one or more Peltier devices. Thesmaller of the heat sinks is used on the inside ofthe enclosure; cooled to a temperature below thatof the air in the box, the sink picks up heat as theair circulates between the fins. In the simplestcase, the Peltier device is mounted between this

    Figure 7

    Figure 8: Conceptual Drawing of Air-to-AirThermoelectric Cooling System

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    'cold side' sink and a larger sink on the 'hot side'of the system. As direct current passes throughthe thermoelectric device, it actively pumps heatfrom the cold side sink to the one on the hot side.The fan on the hot side then circulates ambient airbetween the sink's fins to absorb some of thecollected heat. Note that the heat dissipated onthe hot side not only includes what is pumpedfrom the box, but also the heat produced withinthe Peltier device itself (VTE x ITE).

    Let's look at this in terms of real numbers.Imagine that we have to pump 25 watts from abox to bring its temperature to 3C (37.4F). Theambient environment is at 20C (68F). Toaccomplish this, we may have to take thetemperature of the cold side sink down to 0 C(32F) so it can absorb sufficient heat from the air.Lets assume that this requires a Peltier devicewhich draws 4.1 amps at 10.4 V. The hot side of

    the system will then have to dissipate the 25 wattsfrom the thermal load plus the 42.6 watts it takesto power the TE module (for a total of 67.6 watts).Employing a hot side sink and fan with aneffective thermal resistance of 0.148 C/W(0.266F/W), the temperature of the hot side sinkwill rise approximately 10C (18F) above ambient(67.6 w X 0.148 C/W). It should be noted that, toachieve the 17 C drop (30.6F) between the boxtemperature and ambient, we had to create a30 C (54F) temperature difference across thePeltier device.

    5. Can thermoelectric systems be used forheating, as well?

    Yes. One of the benefits of TE technology is thatyou can switch the direction of heat pumping bysimply reversing the polarity of the appliedvoltageyou get heating with one polarity, coolingwith the other. Thermoelectric modules makevery efficient heatersin fact, because of theunique properties of Peltier devices, any given TEsystem will have a greater capacity for heating aload than cooling it.

    6. Are TE systems used only for heating orcooling air?

    No. Systems are often designed for pumping heatfrom both liquids and solids. In the case of solids,they are usually mounted right on the TE device;liquids typically circulate through a heatexchanger (usually fabricated from an aluminumor copper block) which is attached to the Peltier

    unit. Occasionally, circulating liquids are alsoused on the hot side of TE cooling systems toeffectively dissipate all of the heat (i.e., a liquid-to-liquid system). Note that liquid cooling is neverachieved by immersing the Peltier device in thefluidthermoelectric modules are not theequivalent of 'electric ice cubes'.

    7. Do I have to use a heat sink in my design?

    Whether heating or cooling a thermal load, youmust employ some form of heat sink to eithercollect heat (in heating mode) or dissipatecollected heat into another medium (e.g., air,water, etc.). Without such provisions, the TEdevice will be vulnerable to overheating; once itreaches the reflow temperature of the solderemployed, the unit will be destroyed. When theheat sink is exchanging heat with air, a fan isusually required, as well, to minimize the size ofthe sink required.

    8. Can these devices be immersed?

    Only for cleaning purposes and never while underpower. TE devices should always be dry whenunder use to prevent thermal and electricalshorting.

    9. What type of products currently use thistechnology?

    There are an increasing number and variety ofproducts which use thermoelectric technologyfrom picnic boxes to water coolers, food serviceequipment, laser applications, and highly-specialized instrumentation & testing equipment.The compatibility of many TE's with automotivevoltages, makes them especially suitable for smallcooling jobs in that industry. With each new year,the imaginations of design engineers widen withthe immense possibilities of thermoelectric coolingand heating.

    10. Why would I want to use a thermo-

    electric system instead of compressor-based technology?

    Both technologies have their advantages anddisadvantages, but where thermoelectrictechnology really shines, is in making it feasible todo very small cooling jobsones which would bewholly impractical with a compressor-basedsystem. Can you imagine cooling an individualintegrated circuit with compressed gasses? What

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    about thermally cycling a test tube or cooling avery small enclosure? TE's are also strong inproducts which demand both heating and coolingin the face of a changing operating environment;here a simple switching of TE current polarityallows the system to shift to the mode required. Inaddition, unlike compressor technology, TEsystem components can be mounted in anyphysical orientation and still function properly. Ofcourse, one other advantage of TE systems, isthat they do not require evaporative chemicalswhich may be harmful to the environment.Thermoelectric devices open up a whole newworld to cooling and heating possibilities.

    11. Are there situations where compressor-based systems make more sense?

    Yes. Generally, whenever a small compressor-based system would clearly be 'overkill' inproviding a cooling solution, TE systems becomethe most viable choice. You find a 'gray area'amidst the medium-sized cooling jobs; heredecisions ultimately come down to criticalcost/benefit or design engineering considerationswhich are unique to each application. Given thepresent state of technologyunless there areunique overriding concernsthe compressor-based approach has distinct advantages in largercooling systems such as standard-sizedrefrigerators and air-conditioning systems forbuildings & vehicles. However, ongoing researchinto materials may one day make thermoelectrics

    practical for many of these larger applications.

    12. For heat-only applications, do thermo-electric devices have advantages overresistive heaters?

    Yes, but . . . Resistive devices create heat solelyby virtue of the power dissipated within them. TEdevices, on the other hand, not only provide thisI2R heating, but also actively pump heat into thethermal load; this, potentially, makes them muchmore efficient than resistive heaters.Unfortunately, the need for a DC power source

    and the generally higher cost of TE systemscompared to resistive heaters, precludes their usein most heat-only applications. Furthermore,Peltier devices have a far more limitedtemperature range than most resistive heaters.Generally, TE devices are only used for heating insystems that also require cooling or whereefficiency is extremely important.

    13. Why would I want to use thermoelectrictechnology instead of a passive coolingsystem (heat sink and fan alone)?

    With passive cooling, at best, you can only limitthe rise of temperature above the ambient

    condition. On the other hand, TE systems (likecompressor-based approaches) can actively pullheat right out of a thermal load; this makes itpossible to reach below-ambienttemperatures.

    14. How cold can these devices get?

    That depends upon a great many thingsambienttemperature, the nature of the thermal load,optimization of current delivery to the TE device,optimization of heat sinks etc. It is theoreticallypossible to get a T (hot side to cold side) ofaround 75 C working against a THOT of 35 C

    (note that this is the temperature drop across theTE device, itself, and does not include systemlosses such as the hot side's temperature riseabove ambient). However, this theoreticalmaximum only occurs if there is no thermal loadwhich is not going to happen in a 'real' system. Ina typical application, you will achieve about half ofthe theoretical maximum using a single-stage TEdevice. In order to reach colder temperatures, amulti-stage approach must be employed, either byusing multi-tiered Peltier devices, or by usingother technologies to create part of the desiredT. For example, you might use a compressor-

    based system to provide a below-ambientcondition for the hot-side of a TE device, thenemploy Peltier cooling to further reduce thetemperature of your loadthis is sometimes doneto get down into cryogenic levels. It should benoted, however, that TE devices become lessefficient at colder temperatures and the ratings forT will be markedly reduced when you operateunder extremely cold conditions. Furthermore,even though multi-stage Peltier devices canachieve greaterT's, they have much less coolingcapacity (in terms of watts pumped) than theirsingle-stage counterparts and are far moreexpensive to produce.

    15. Can I physically stack TE devices to get

    a greater T?

    Yes . . . but it is not so simple as merely stackingtwo identical Peltier devices, one on top of theother. The critical reality here is that the seconddevice must not only pump the heat from thethermal load plus its own internal power

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    dissipation (I2R), but it also must remove the heatdissipated within the first TE device. It is usuallymost sensible from a cost standpoint, therefore, toemploy a much smaller device on the first stagethan the second. Be aware in stacking modules,however, that the overall heat-pumping capacity(in watts) of the stack will be limited to thethroughput of the smallest device (while T isenhanced with multi-staging, Q is sacrificed).

    16. How hot can these devices get?

    This is purely a function of the meltingtemperature of the solder employed inmanufacturing the device. Our standard Peltierdevices are rated for 100C on the hot side. It isvery important that users keep temperaturesbelow that rating; if the solder reflows, the devicewill be compromised or destroyed.

    17. Can these devices be used at cryogenictemperatures?

    Yes, but they are far less efficient in this range.Note that you cannot achieve cryogenictemperatures from a single-stage Peltier deviceworking against a typical room temperatureambient.

    18. Does ZMAX offer any performanceadvantages over other thermoelectrictechnologies?

    Definitely! The patented ZMAX technology offersperformance which is unachievable with the moreconventional processes employed by othermanufacturers.

    19. How big can these devices get?

    Theoretically, there is no limit, but practicalitydoes impose some restrictions. Issues related tothermal expansion/contractionand costtend tokeep module sizes down. Typical devices rangeup to 50 mm (2") square and about 4 mm thick,

    but there are exceptions. In the general case,when greater cooling capacity is required, multipleTE devices will be employed rather thanfabricating some sort of gargantuan module.

    20. How small can these devices get?

    Here again, the theoretical limit goes far beyondwhat is practical. Devices are commonlymanufactured at sizes well below 8 mm square,

    many for such applications as laser diode cooling.Very small modules, however, are moreexpensive to produce because they are lesssuitable for automated processingmany ofthem, in fact, require manual attention under amicroscope. As a result, in creating designs forvery small cooling jobs, devices must be carefullyoptimized for cost as well as size and capacity.

    21. Is it possible to purchase customdevices?

    Yes, but the expense for new tooling (if it isnecessary) or special handling will be included inthe overall price. Whenever possible, it isgenerally more cost-effective to employ a stockitem. To explore the possibilities, contact aTellurex sales representative (231-947-0110).

    22. What does the specification, TCold, mean?

    This is the temperature at the cold-side mountingsurface of the Peltier device (see Figure 9).

    23. What does the specification, THot, mean?

    Figure 9

    This is the temperature at the hot-side mountingsurface of the Peltier device.

    24. How can I measure THot or TCold in athermoelectric assembly?

    This is somewhat challenging because you haveto get the measuring device as close as possibleto the outer ceramic of the Peltier device while it isin operation. The best choice of sensor here istypically a non-sheathed thermocouple fabricatedfrom fairly fine wire. One way to approach theplacement of the thermocouple, is to take the heat

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    sink or block which will be mounted to the TEdevice and cut a shallow groove at the interface.The thermocouple wire can then be placed so thatit will be in close proximity to the module's centeronce the system is assembled. Of course, exceptfor the exposed end of the thermocouple, thewires should be electrically insulated along thelength of the groove to prevent shorting. Thethermocouple should be potted into the grooveusing a thermally-conductive adhesive; be sure toclean off any burrs or excess adhesive to maintainthe flatness of the interface.

    A compromise which can sometimes be employedwhen using heat sinks, is to mount thethermocouple on the fin side of the sink (at thebase), opposite the center of the TE module. Thethermocouple should then be covered with a smallpiece of insulating foam. Some accuracy will belost with this method because of the inevitable

    thermal gradient between the thermocouple andmodule surface, but it can yield an acceptableapproximation. The insulating foam is critical orair movement will significantly affect the reading.

    Some people have attempted to mount finethermocouples on the inside of Peltier devices tomeasure these parameters, but this is generallynot a good idea. First of all, it would be difficult tosuccessfully mount a thermocouple on an interiorsurface and there would be significant potential fordamaging the device in the mechanicalmanipulation. Also, without any form of insulation

    on the inside, the thermocouple would actuallypick up a thermal gradient between the ceramicsurface and the air temperature inside the device.Other thermal gradients within the module couldalso effect the accuracy of the reading. If a dataacquisition system is employed to measuresystem variables, there would also be a potentialfor introducing a ground loop if the thermocoupletouches anything which is electrically live.

    25. What does the specification, VMax, mean?

    Contrary to what many people might expect, this

    quantity is not the maximum voltage that thedevice can withstand before failing. Actually, VMaxis the DC voltage which will deliver the maximumpossible T across the thermoelectric device at agiven THot. At voltages below VMax, there isinsufficient current to achieve the greatest T; atvoltages above VMax, the power dissipation withinthe Peltier device begins to elevate the hot sidetemperature and this diminishes T. Note that

    VMax is temperature dependentthe higher theoperating temperature, the higher the VMax ratingfor a specific device.

    26. What does the specification, IMax, mean?

    Again contrary to what many people might expect,this quantity is not the maximum current that thedevice can withstand before failing. IMax is thedirect current level which will produce themaximum possible T across the TE device.Operating below IMax, there is insufficient currentto achieve the greatest T; when drawing morethan IMax, the power dissipation (I2R) within thePeltier device begins to elevate the systemtemperatures and diminish T. Note that IMax andVMax occur at the same operating point, i.e., IMax isthe current level produced by applying VMax to aPeltier device. Unlike VMax, IMax is not especially

    temperature dependentit tends to be fairlyconstant throughout the operating range of adevice.

    27. What does the specification, QMax, mean?

    QMax is one of the more confusing specifications inthe TE world because its practical significance isnot obvious. Its definition relates to a fundamentalreality: as the thermal load (i.e., 'Q') increaseswithin a given TE system, the resultant Tdecreases. Within any given hardwareenvironment, for example, you will have a lower

    T with a thermal load of 40 watts than at 30watts. At a certain load specification, the T willbe reduced to zero; the load which produces thiscondition, is known as QMax and it is quantified inwatts. Note that this specification does notprojectthe maximum amount of heat that the device canhandle; if your load goes beyond QMax, you stillpump heat, but your net T is now in the oppositedirection (i.e., your cold side is now the hot side).The primary relevance of QMax as a specification,is that it is commonly used as an endpoint for'load lines' on performance graphs.

    28. What does the specification, TMax,mean?

    This is the maximum possible T across a Peltierdevice for a given level of THot. This point alwaysoccurs when the thermal load (Q) is at zero watts.TMax is the other endpoint for load lines onperformance graphs.

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    29. What does the specification, coefficientof performance (COP), mean?

    Coefficient of performance is an indicator of theefficiency of a thermoelectric cooling (or heating)system. It is essentially the ratio of:

    1) the heat pumped by a TE module (in watts),and

    2) the amount of power supplied to the TEdevice.

    Put mathematically, COP = Q / (VTE * ITE) whereQ is the number of watts pumped, and VTE and ITEare the voltage and current supplied to thethermoelectric module.

    30. What is "AC resistance"?

    This is a term which has come into common use

    in the thermoelectric industry as a reference to theinternal resistance of TE devices. This label is notintended to imply that the resistance is in any waydependent on the type of voltage applied to adevice. The 'AC' refers to the fact that, short ofusing custom computer-based systems for testingPeltier devices, an AC voltage must be applied toa TE module to measure its resistance. Why? IfDC is applied, a T will develop across themodule and this will result in the build-up of aSeebeck voltage. The Seebeck voltage willoppose the applied voltage and make the deviceappear to be more resistive than it is. With AC

    voltage applied (preferably of fairly highfrequency), the polarity will change every half-cycle; as a result, only small Seebeck voltagescan develop and decay. Furthermore, becausethe Seebeck voltages are of opposite polarityeach half cycle, they effectively cancel each otherout over the full conduction cycle.

    31. Can I check thermoelectric devices witha conventional ohmmeter?

    No. The vast majority of ohmmeters apply DC tothe resistance being measured. This will result in

    the build-up of a Seebeck voltage which makesthe measurement inaccurate.

    32. Can I use an ohmmeter which applies apulsed DC voltage?

    No. This, too, will result in Seebeck voltageswhich make measurement inaccurate. The metermust apply a waveform which reverses polarity

    every half cycle to cancel out the Seebeck effect;this feature will be found most often in a high-quality LCR meter. B&K may still have a couplemodels of handheld meters that apply an ACpulse.

    33. Why are there separate performancegraphs for different values of THot?

    Because Peltier devices are temperaturedependent and have a positive temperaturecoefficient with respect to resistance, TMax, QMax,and other related parameters. These realitiesmust be reflected in the performance literature.

    34. Can these devices be connectedelectrically in series, parallel, or series-parallel?

    Yes. As long as you provide sufficient voltageand current capability from your power supply, TEdevices may be connected in any series/parallelconfiguration that makes sense for the application.

    35. Are there any advantages to connectingmodules in series vs. parallel or viceversa?

    Under ordinary circumstances, no. The amount ofheat pumped by each module will be a function ofthe voltage and current delivered to it; whetherthis delivery is a result of series, parallel, or

    series-parallel connections is of littleconsequence. Parallel wiring does, however,offer some protection against a TE device orconnection failurewith parallel TE modules therewill still be some cooling capacity even if part ofthe circuit becomes dysfunctional. With seriesconnections, if anything in the current path opens,all cooling power will be lost.

    36. Why do I need a DC power supply forthis technology?

    To keep heat pumping in the same direction, the

    polarity of the applied voltage must be maintained.This means that some form of direct current isrequired. If alternating current was appliedinstead, the polarity would change each half cycle,and so would the direction of heat pumping. As aresult, the net amount of heat moved would bezero and both sides of the system would getwarmer from the I2R dissipation within the Peltierdevice.

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    37. In varying power to these devices,should I change the voltage or thecurrent?

    According to Ohm's Law, I = V / R. Current is thedependent variable herewhile you can have

    voltage without current, you cannot have currentwithout voltage. Any time you apply voltage to aresistive load, current will flow, and if you vary thelevel of voltage, you will vary the level of currentwhich resultsOhm's Law must be obeyed! Inmost cases, you will choose a voltage levelwhichgives you the desired results. There are someapplication niches where 'current sources' arecommonly used, but in reality, these currentsources electronically control the applied voltagein order to guarantee a particular level of current;it only seems like the voltage results from theamount of current put through the TE device.

    38. Are the Peltier devices purely resistive?

    They are mostly resistive. While there is somecapacitance in a Peltier device, it is verynegligible, and given the fact that DC is applied, itpresents no real barriers in applications. Inductiveeffects are mostly confined to the leads of thePeltier device and typically present few problemsunless the device is being driven with some formof pulsed DC. If this type of power is employed, itmay be advisable to shield the leads and keepthem separated from any signal wiring to minimizedifficulties. You will see transient inductive effectsin the leads whenever power is switched on or off;if these could be problematic, take precautions.

    The most notable non-resistive characteristic isSeebeck effect. Just as charge carriers can moveheat, the movement of heat through an electricalconductor will carry charge carriers along with it.Thus, whenever a temperature difference isplaced across a TE device, a small voltage willdevelop. If an electrical load is placed across thedevice in this state, current will flow. Thisphenomenon is called "Seebeck effect" (see FAQon power generation technology).. Similarly,

    when a Peltier device is placed under power anda T develops, the T creates a 'back EMF' (or'back voltage') which opposes the applied voltage.This 'Seebeck voltage' is not something that youcan see or measure while external voltage ispowering the Peltier device, but the circuit acts asif the Seebeck voltage is subtracted from theapplied voltage. This, naturally, results in a lowercurrent level than you might otherwise expect and

    makes the Peltier device appear to be moreresistive than it really is. Incidentally, you canmeasure the Seebeck voltage when TE power isdisrupted, although as the T decreases from theback flow of heat, the Seebeck voltage willgradually decay to zero.

    39. Do I need a well-regulated DC powersupply for my thermoelectric system?

    This would only be required if you need well-regulated power for other circuitry that will runfrom the same supply. Nonetheless, manycustomers choose regulated switching suppliesfor thermoelectric systems out of consideration forsize, weight, and efficiency. In high volumeapplications, some users have effectivelyemployed custom, non-regulated switchers tostrike a good compromise between price and

    performance. Those who want to keep thingssimple, can use a non-regulated, linear supplywith a 'brute force' filter capacitor; these circuitsdo tend to be bulky, however. In this latter case,ripple should be kept below 15% to maximizepower output to the TE modules. UnfilteredDC(i.e., rectification only) is very inefficient and quiteproblematic if the peak voltage exceeds VMax (seenext question).

    40. If I have an available power source whichexceeds VMax, can I pulse-widthmodulate it to reduce the effective DC

    level?

    No! No! No! Because this sort of approachusually works with resistive heaters, a significantnumber of designers seize upon this idea to avoidhaving to translate their available DC supplyvoltage to a more suitable level. The onlyproblem is that it doesn't work withthermoelectrics. Why not? Let's say that we wantto apply a pulsed DC at twice VMax with a dutycycle of 50%. If this was powering a conventionalresistor, we would simply look at the effectivepower dissipation over the full cycle of the pulsedDC and proceed accordingly. Unfortunately,Peltier devices present some extra complexitieswhich cannot be overlooked. The crux of thematter is that the Peltier device can only pumpheat when current is flowing. We thus have tolook at what is happening in each separate phaseof the cycle.

    When power is on (voltage high), we are drivingthe TE device at twice its rated VMax; what

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    happens during this interval is no different thanwhat happens when powering a module at twiceVMax continuously. With so much powerdissipated (V

    2/R) within the device, there is no

    capacity left for pumping any heat from thethermal loadin fact, some of the excessive heatwithin the module will flow back into the load.When power is turned off in the other part of thecycle, it is true that no power is being dissipatedwithin the Peltier device, but without current flow,it's not pumping any heat either. Thus whenpower is on, you are operating a badthermoelectric system which creates heat ratherthan removing it from the load; when power is off,no active cooling work is being done. Nothinggood is achieved in the process.

    41. Can I use pulse-width modulation tocontrol my Peltier device if I keep the

    voltage at VMax or below?

    Yes, and this is one of the most electrically-efficient ways to control voltage to your devicealthough you must observe some precautions. Aslong as you keep the voltage at VMax or below,you will effectively pump heat whenever the dutycycle applies voltage to your system; when thepower is turned off, the heat pumping will stop.By pulse-width modulating a suitable voltage, youcan easily control the extent of heat pumping bysimply varying the duty cycle of the pulses. Thegreat thing about this approach, is that it allowsyou to minimize power dissipation in your control

    circuitespecially if you use power MOSFET's forswitching (a subject which goes beyond thisparticular question).

    Significant precautions must be employed withPWM, however. First of all, the PWM should beat a high enough frequency to minimize thermalstresses to the TE devices. While we like to keepthe frequency in the low killihertz (Hz) range, inmany applications these days we mustcompromise at around 120 Hz for the sake ofelectromagnetic compatibility. Another importantissue is the potential for generating electro-

    magnetic interference (EMI) in the wiring to the TEdevice. If you are using PWM, you may need toshield your power wiring or keep it away from anysensitive electrical signals. A stitch in time toconfront these issue early, can save a lot ofcorrective work deeper in the design cycle.

    42. If I use enough TE devices in series, canI just rectify voltage from the wall socketto get 120 VDC?

    This is, indeed, a very 'frequently asked question'when dealing with large-capacity systems.

    Unfortunately, the answer is not as straightforwardas it might seem.

    First of all, the only way that you will get "120VDC" from a wall socket (assuming that yourelectrical service is 120 VAC) is if you leave therectified voltage unfiltered(i.e., at 100% ripple). Itis important to note that the 120-volt rating is theRMS equivalent of the sinusoidal waveform; this isequal to about 70.7% of thepeakvoltage.

    Once you use a capacitive filter on the rectifiedAC to create 'steady-state' DC, the resulting

    output will only be slightly lower than the peakvalueapproximately 170 V (less diode dropsand ripple).

    To make this approach work, therefore, you wouldneed to place additional devices in your seriescircuit (about 40% more, in fact) to avoidexceeding the VMax rating of the thermoelectriccomponents.

    Figure 10

    There is another option for getting steady-stateDC from an AC line at a lower voltage, but it isseldom practical in TE systems. By using aninductor in series with the TE devices (and acapacitor in parallel with them), you can get afiltered voltage at around 108 VDC (less diodedrops and losses). Given the current demands fora typical TE device, however, this would require apretty 'beefy' and expensive coil. The losses inwire resistance and core currents would besignificant with this solution, as well.

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    Of course, some people would be tempted tosimply use the 120 VAC unfiltered. While thiscould be done, it is important to remember thatthe series circuit must still have a combined VMaxrating of at least 170 VDC or heat-pumpingperformance will suffer greatly (just as it doeswhen pulse-width modulating an excessivevoltagesee 41).

    The potential problems go beyond themathematics and filtering, however. One of thebiggest concerns is safety. Thermoelectricmodules are typically sandwiched between heatexchangers fabricated from electrically-conductivemetals. While ordinarily this should not cause aproblem, with the close proximity of components,there is a genuine risk that hazardous situationscan develop when the circuit is connected directlyto an AC line. It is always possible that a wirelead can get pinched or debris can cause a short

    between a module and the heat exchangers. Withlow-voltage DC, this is seldom a problem;however, when you have a series of TE modulesconnected to the AC line (or high voltage DC forthat matter), one of the exchangers can becomedangerously 'live'. If a designer pursues thisapproach in spite of the risks, a ground faultinterrupter is essential; an isolation transformer isalso highly recommended.

    43. Can I use linear drive (either voltage orcurrent) to regulate the temperature ofmy thermoelectric system?

    Yes. Typically this is done by placing a bipolarjunction transistor (BJT) in series with the TEdevice(s). Of all the options for regulating powerto TE's, linear control (done properly) is the onewhich imposes the least thermal stress on Peltierdevices. On the downside, a lot more power isdissipated in the linear BJT drivers than withMOSFET's in a PWM controllerand this willtranslate into greater costs for semiconductor heatmanagement. However, when PWM cannot beeasily accommodated in a system because of EMIor power conditioning considerations, linear

    control may be the only viable option.

    44. Can I cycle the device on and off fortemperature control?

    Thermostatic operation (on-off) is one of the moreaffordable modes of control and is a temptingoption (although a pulsed steady-state control canoften be done as cheaply). With this approach,

    cooling (or heating) power is turned on at onetemperature and off at another. This means thatthe system will continually bounce back and forthbetween two temperature limits; as a result, it isnota good alternative if steady-state performanceis desired. An additional issue here is cycle time.Moderately slow cycle times can be morethermally stressful to the modules than usingother types of control. If thermostatic control is tobe employed, therefore, it is probably best to keepthe cycle time in the range of tens of minutes ormore.

    45. Can I use a mechanical device like asnap switch for thermostatic control?

    Only if you are using it to switch the coil voltageon a relay or contactor. After years of attemptingto use these devices in low cost systems for directswitching of TE modules, Tellurex can no longerrecommend them for that purpose. Most snapswitches are designed for AC applications. Whenswitching DC for TE devices, we have continuallyseen these switches fail in a closed state whichleaves the system in a full-on condition. This isespecially problematic if that system is in heatmodeit can create some real safety issues. Wecannot stress this enoughdo not usemechanical snap switches for direct control ofthermoelectric modules.

    46. Can I configure a controller to switchbetween heating and cooling modes?

    Yes, this is commonly done. It is most oftenaccomplished with a DPDT relay (watch for theDC ratings on any device you are considering)wired in the classic polarity-reversal configuration,but sometimes designers will employ a transistorbridge (using MOSFET's) to combine theswitching and control architectures. You can alsochange modes manually by toggling anappropriately-rated DPDT switch. If usinganything other than a DPDT switch or relay, youmust design your circuit to guard against transient

    Polarity Reversal Configuration

    DC Supply

    DPDT Switch

    P N

    TE Module

    Figure 11

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    conditions that can occur when mode changes aremade, otherwise you may create damaging short-circuits across your power supply. In automaticcontrols, a suitable 'deadband' for triggering modechanges is also recommended to prevent'chattering' (i.e., high frequency toggling betweenmodes).

    Note that some situations may require a relaysimulation of a DPDT-center-off switch so therewill be a fail-safe mode. This can be done usingtwo SPDT relays. Each relay enables a differentmode of operation. When both are de-energizedor both are energized, no current is supplied tothe TE device(s).

    47. Can I use a solid state relay to drive myTE device?

    Yes, but this would not be the best choice in anysituations that come to mind. If you want to switchpower electronically, power MOSFET's are farmore advantageousthey're cheaper, smaller,and are often rated with remarkably low onresistances. If chosen properly, MOSFETs willdissipate much less power for a given level of TEcurrent.

    48. Can I control the temperature of my loadby just using a variable power supply toset a fixed DC level?

    Not under ordinary circumstances. This can onlywork if there is absolutely no change in yourthermal load or the ambient environment. Thehallmark of temperature control is keeping thetemperature steady despite disruptions caused byvarying load or environmental conditions. Toaccomplish this, you must employ some type oftemperature sensor in a closed-loop controller thatwill provide just enough power to the Peltier

    devices to hold the thermal load at the desiredtemperature. In a cooling application, if the roomtemperature increases, the closed-loop controlcircuit will detect this change and provide extracooling power to keep your thermal load whereyou want it. If the thermal load (in terms of watts)changes, the controller can detect this conditionand vary cooling power as required. Without atemperature sensor and a closed-loop controlcircuit, any variance in your operating conditionswill be reflected in a change of the temperature ofyour thermal load. If you want stability, you mustuse a 'real' control circuit.

    49. How closely can temperature becontrolled with thermoelectrictechnology?

    Figure 12: DPDT-Center-Off Simulation

    With a properly tuned, well-designed, steady-state

    controller (typically using some variant of 'PID'control), it is possible to maintain temperatures towell within 0.1 C of set point providing there areno 'dramatic' (i.e., large and sudden) interruptionsin the operating environment. Indirectmeasurements of stability, have suggested that itis possible to achieve constancy to within a fewhundredths of a degree (some even claimthousandths), but it is virtually impossible to verifythis level of performance directly. In discussingthe potential for stability, of course, it is importantto stress that any given thermal load will showtemperature gradients and parts of the load willexhibit variance from the set-point no matter how

    stable it may appear on a digital displayyoucannot assume a homogeneous response acrossyour entire load. Designers must also be wary ofsubtle long-term problems such as set-point driftor loss of calibration.

    50. Is it possible to use inexpensivetemperature controllers with thistechnology?

    That depends on what your objectives are. If youcan live with single mode control (i.e., just heatingor just cooling), a reasonable amount of error, nodisplay of temperatures, and a small amount ofvariability in resulting temperatures, you have areal chance at keeping costs down. If it's a high-volume application, as well, you may even get thecost to around $20 per controller on an OEMbasis (Tellurex can explore options with you). Onthe other hand, if you need tight control, minimalerror, a display, alarms, etc., these features will

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    There is one more consideration which imposes apractical limitation on cooling rateswithdifferential expansion/contraction responsesamong the component materials, excessive ramprates will destroy Peltier devices. Therecommended upper limit for Tellurex devices is1 C per second (heating or cooling) and even thiswill shorten the useful life of the module. Newmanufacturing and materials technologies holdout the promise that this range can be extended,but generally, designers should keep ramp rateswell within these specified limits to insure longproduct life.

    54. Are there any special considerationswhich apply to 'clamping' the device?

    Although spring compression systems aresometimes used on OEM products, generally,machine screws are employed to compress thePeltier device between the hot and cold sides.Because thermoelectric devices can be crushedthrough improper handling, some care must betaken in designing and implementing the clampingmethod. For optimal results, try to use only twocompression points per module (if possible) andkeep them in close proximity to the device (usuallywithin 0.25"). Naturally, the two compressionpoints should be along the center line of themodule. In bringing up tension on the machinescrews, take extra care to adjust slowly at eachcompression point and alternate between the twofrequently; in this way compression across the

    surface area of the module can be roughlyequalized throughout the adjustment process. Acommon error is to tighten one side so that it tipsbelow the top surface of the module (Figure 13);then when pressure is increased on the oppositeside, it creates leverage which crushes themodule (Figure 14). The greater the number of

    compression points or their distance from themodule, the greater the likelihood of damaging themodule in this manner. Patience is definitely avirtue here.

    With multiple device deployment, it is not alwayspossible to use two screws per device, andsometimes the space between the compressionpoints will span more than one module. Here, theuser must watch out for bowing of the mechanicalinterface; this can not only damage the Peltierdevices, but compromise the thermal interface, aswell. In these instances, it is usually best todecrease the amount of compression to insureflatness.

    55. Where do I start in designing a TEsystem?

    If you are doing a heat/cool application, focus firston the cooling side of the equation; if you haveenough capacity for cooling, you should haveplenty for heating. Establish some initial designparameters. What load temperature do you want?What is your worst-case ambient (always designagainst the worst case)? What is the targetedtemperature rise for your hot side above theambient? Do you have constraints on availablepower for your system? If so, what are they?What physical limitations do you face (space,weight, harshness of environment, etc.)? Arethere sources of radiant heat in the environment?

    Probably the most challenging area in design, iscoming to terms with your load. Thermal load ismade up of two distinct componentsactive andpassive. The active load is the part which actuallycreates heat. For instance, if your thermal load isan electrical circuit, its power dissipation would bethe active load. In some cases, there will be no

    Figure 13Figure 14

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    active loadthis is the situation with a picnic box,for example.

    Much of the load in any TE system will bepassive. Passive load is the amount of heat (inwatts) which must be pumped to maintain the

    temperature difference between the load and theambient environment. It is like bailing out a leakyboat; water (like heat) is continually coming in andyou must labor to pour out a comparable volumeto maintain the level of the boat. The greater theT you require between your load and theambient, the greater the passive load will be.

    Once you know all of these quantities, you canbegin looking for a thermoelectric solution whichconforms to your requirements.

    56. How can I determine the magnitude of

    my passive load?

    The transfer of heat from a load to the ambientenvironment, is largely a function of two thermalprocessesconduction and convection.Conduction is the transfer of heat through matter(insulation, structural components, seals,fasteners, etc.) and is a function of thetemperature difference (i.e., T) across thematerial, the physical dimensions, and the thermalconductivity of the material (K). Convection isheat transfer across the boundary layer of air atthe surface of a material. It is a function of the Tacross the boundary layer and the rate of airmovement at the surfacethe faster the airmovement, the greater the convection of heat.

    With a well-insulated thermal load (e.g., aninsulated enclosure), convection is a relativelyinconsequential component and you can oftenfocus exclusively on the conductive element. Thefollowing equation can be used to estimate apurely conductive load:

    L

    AKTQ

    = , where

    Q is the amount of heat conducted (it can beexpressed in either BTU/hour or watts,although in the themoelectric industry, mostsupport documentation is based onwattage);

    T is the temperature difference between thethermal load and the ambient environment(in F for BTU/hour calculations, in C forwatts);

    K (Kappa) is the thermal conductivity ofthe material expressed in eitherBTU/hour-feet-F or watts/meter-C;

    L is the thickness of the material (in feetfor BTU/hour calculations, meters forwatts); and

    A is the exposed surface area of thematerial (in square feet for BTU/hourcalculations, square meters for watts).

    If you want to include both the conductive andconvective components of the load, you can usethis equation:

    hK

    L

    TAQ

    1+

    = , where

    Q is the amount of heat conducted andconvected (expressed in either BTU/hour orwatts);

    K (Kappa) is the thermal conductivity of thematerial expressed in either BTU/hour-feet-F or watts/meter-C;

    h is the heat transfer coefficient (in still air,this ranges between 4-5 BTU/hour-feet

    2-F

    or 23-28 watts/meter2-C; in turbulent air, hfalls in the range of 14-20 BTU/hour-feet2-F or 85-113 watts/meter2-C);

    L is the thickness of the material (in feet forBTU/hour calculations, meters for watts);and

    A is the exposed surface area of the material(in square feet for BTU/hour calculations,square meters for watts).

    T is the temperature difference between thethermal load and the ambient environment(in F for BTU/hour calculations, in C forwatts).

    Note that the result that you get for Q with thisequation, will be lower than that obtained for theformula based only upon conduction. This isbecause the convection/conduction equationaccounts fortwo sources of thermal resistance toheat flow. With the calculation reflecting a slightlygreater series resistance to heat leakage, itlogically follows that fewer watts will be indicatedto compensate for passive load.

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    When you are dealing with an uninsulated load, oran uninsulated portion of one (e.g., a cold plate),then it becomes very important to explore theconvective part of thermal load. In thesesituations, convection may offer the primaryresistance to the leakage of heat. Remember thatin some situations (e.g., an uninsulatedenclosure), you will have air movement on boththe inside and outside; as a result, yourT will besplit between the two boundary layers (based onthe relative convectivity of each). As you will findin using the equation below to estimate yourconvective load, without insulation, you will needto pump a lotmore wattage with your TE system.

    BTAhQ = where

    Q is the amount of heat conducted andconvected (expressed in either BTU/houror watts);

    h is the heat transfer coefficient (in still air,this ranges between 4-5 BTU/hour-feet2-F or 23-28 watts/meter2-C; in turbulentair, h falls in the range of 14-20BTU/hour-feet2-F or 85-113watts/meter2-C);

    A is the exposed surface area of thematerial (in square feet for BTU/hourcalculations, square meters for watts).

    TBB is the temperature difference across theboundary layer at any exposed surfaces(in F for BTU/hour calculations, in C forwatts).

    When dealing with enclosures, you can alsoestimate your passive load empirically once aprototype is built. Simply place a known heat loadinside (make sure that the enclosure can 'take theheat'), then monitor the temperatures of theambient and enclosure interior. Once the Tbetween the inside and ambient has stabilized,you can use the following equation to determinethe passive load:

    ENCAMB

    DES

    TTTPQ

    = , where

    Q is the passive load, expressed in watts;

    TAMB is the ambient temperature afterstabilization (in C);

    TENC is the enclosure temperature afterstabilization (in C);

    P is the power dissipation within the heateremployed for the test (expressed inwatts); and

    TDES is the desired T for the system.

    57. Are there things I can do to decrease thepassive load?

    Definitely! It is generally a good idea to insulateyour load as much as possible from the ambientenvironment. The more insulation that youprovide, the less heat will 'leak' back into yourload. Try to seal any places where air can getaround the insulation. If you must have doors orhatches, use good quality seals to minimize

    leakage. If there are radiant sources of heat inthe outside environment, make the outside of yourload as reflective as possible. Try to eliminateany thermal shorts that might exist between yourload and the outside world.

    58. How can I determine what my heat sinkneeds are?

    Once you can quantify the nature of your load andidentify the power requirements for the Peltierdevices that will be employed, you can get asense of your heat sink needs. Again, we will

    focus on designing for cooling because that will befar more demanding than operation in heat mode.First, you determine the total number of watts thatmust be handled by your hot-side sinksimplyadd the wattage for your active load, passive load,and TE device power (VTE * ITE). Next, divide yourtargeted rise above ambient for this sink by thetotal wattage; this will give you an estimate of yourrequired heat sink/fan thermal resistance. Oncethis thermal resistance is calculated, it's time to'hit the catalogs'you will need to shop around tofind a heat sink/fan combination that can deliverthe performance that your design demands. Most

    heat sink catalogs will display graphs showing theinteraction between air flow (in CFMi.e., cubicfeet per minute) and the resulting thermalresistance. It is not likely that you will find acombo with the exact thermal resistance that youare seeking; typically you will have to choose thebest compromise and see how it impacts uponyour system's performance.

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    Identifying a heat sink for the cold side of an air-to-air system, is a very similar process. Again youmust target a temperature drop for this side ofyour system (i.e., the difference between thetemperatures of the heat sink and the aircirculating through it). On the load side of thesystem, the wattage is confined to your active andpassive loadsthat is what is passing through thesink; TE power dissipation is not an issue here.With far fewer watts to handle, the heat sink onthe cold side of the system tends to besignificantly smaller than the one on the hot side.

    There are certain 'rules of thumb' which should beobserved in dealing with heat sinks inthermoelectric designs. When it is important tosqueeze out as much cooling performance aspossible, you will want to minimize thetemperature drops across your sinks; good targetswould be about a 10 C (18 F) rise on the hot

    side with around half that on the cold side. Thismeans, however, using heat sinks and fans whichare more thermally conductive (i.e., less resistive)and that translates into more fin area, morealuminum (or copper), and greater cost. In someextreme cases, exotic heat sink designs may bechosen to take thermal resistance to an absoluteminimum. When cost is of critical concern,however, it may well mean compromises in thethermal performance of the heat sinks; this leadsto higher temperature drops across the sinks anda loss of cooling capacity. In the end, componentchoices will come down to cost/benefit analysis

    and every user's needs will be different.

    59. Do I need to be concerned about watercondensation within Peltier devices?

    In applications where cooling below the dew pointwill produce condensation, it is usually advisableto provide special protection for thermoelectricmodules to prevent shorting (thermal & electrical)and electrolytic effects. Although mostmanufacturers recommend perimeter seals todeal with this problem, Tellurex does not(although we will provide them for customers who

    insist). While perimeter seals are an effectivebarrier to water and dust, they are not imperviousto vapormigration. Once the vapor gets into themodule, it condenses and becomes trapped. Theseal is then an effective impediment to outflowingof the condensed moisture. Thus the perimeterseal winds up compounding the problem, oftenaccelerating the corrosive effects of electrolysis.Modules often do better with no seal at all than

    with the usual perimeter protection offered in theindustry.

    In recent years, Tellurex has pioneered the use ofZ-Coat, a proprietary conformal coating whichis applied to the interior surfaces of the modules

    using highly specialized equipment. This is simplythe best moisture protection available in the world.It not only retards or prevents electrolytic effects,but actually improves the mechanical properties ofthe deviceall while having minimal impact onthermal properties. Some stocks of these sealeddevices are usually kept on hand.

    Perimeter seals may be required in someproducts for hi-pot (i.e., high voltage) protectionor keeping debris out of modules. In those casesif condensation is also an issue, the application ofZ-Coat is highly recommended to protectagainst any trapped moisture.

    Contact Tellurex sales (231-947-0110) to discusssealing options.

    60. Are there any special safety concerns?

    There are two areas of concern here. First, neverpower up a Peltier device unless at least one sideis mounted to a suitable heat sink. By 'suitable',we mean one that can at least handle the wattagedissipated by the devicenot some little sinkmade for a small transistor package (like a TO-220). A typical Peltier device may dissipate 60 W

    or more internally. How long can you hold apowered 60-watt incandescent bulb in your handbefore it burns you? The hot side of athermoelectric device can get even hotterandfasterwhen it is not mounted to a proper sink.This is not just a safety issue, eithera devicepowered without proper sinking, can destroy itselfvery quickly.

    The other safety concern is electrical. Althoughthe electrical hazard potential associated withmost thermoelectric systems is very small, thereare some issues which deserve attention.

    Typically, Peltier devices are mounted to eitheraluminum or copper hardware (sinks, liquid heatexchangers, etc.). It is possible, therefore, fordebris or moisture to create a short-circuitcondition between the hardware and anelectrically-live part of the Peltier device. It is upto the designer, therefore, to prevent such aproblematic condition from occurring. From asafety standpoint, it is highly recommended that

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    designers employ DC power which is fully-isolatedand properly fused. The use of autotransformersor direct wiring to an AC service line is generallynot recommended; if employed, a ground faultinterrupter should be included in the design.

    Be sure to use appropriately-rated wire in makingconnections, too. Thermoelectric devices candraw appreciable current and inadequately-sizedwire may become very hot.

    61. Do I have to insulate between the hotand cold sides of the system?

    While there is no law requiring it, insulation ishighly recommended to minimize heat leakagebetween the hot and cold sides. For best results,use two-part, closed-cell foam; one-part mixturesdo not generally produce good results.

    62. Can these devices be used for powergeneration?

    Yes. The 'flip-side' of the Peltier effect is theSeebeck effectwhen thermal energy movesthrough an electrically-conductive material, chargecarriers are transported by the heat. Thus whenyou create a temperature difference across athermoelectric device, the movement of heat andcharge carriers creates an electrical pressure(called Seebeck voltage). If an electrical load isconnected across the device, current will flowif

    not, the pressure builds to a steady state conditionand a 'no-load' voltage will be present. While astandard Peltier unit can be used in this fashion, ifhigher temperatures are involved, specialthermoelectric modules are employed.

    Because TE power generation devices are fairlyinefficient in converting thermal energy toelectricity, their use is largely confined toapplications where 'waste heat' is readily availableor in remote areas where dependability is moreimportant than efficiency. In these types ofsituations, the power conversion process may beless than ideal, but the 'fuel' is free. Furthermore,

    because their small size makes it possible tomount them in tight spaces, they can be used toreclaim energy in places where it would otherwisebe impractical. Potential users should be mindful,however, that a T still must be created acrossthe devicethere has to be a 'cold' side as wellas a source of heatand this can present achallenge to designers.

    In using power generation devices, one of theprinciple objectives will be to extract as muchpower as possible from the thermoelectricmodules. Because power generation deviceshave significant internal resistance, designers whowant to employ this technology should review theprinciples ofmaximum power transfer in electricalcircuits. It is essential to grasp that maximumpower will be transferred when the load resistanceequals that of the TE device configuration. In theend, a designer must come up with aseries/parallel array of modules that will assuregeneration of the desired voltage while coming asclose as possible to a 'matched load' condition. It

    is also critical to design to the worst-case Tand to make sure that THot never exceeds themaximum rating for the device. Furthermore, ifvoltage regulation is important and the T or loadwill be variable, a shunt regulator or DC-DC

    converter will be needed.

    Those interested in Seebeck technology shouldread our FAQ on the subject (available atwww.tellurex.com).

    63. If Peltier devices can be used for powergeneration, will they run in a self-powered mode?

    There is no such thing as a free lunch. Theamount of Seebeck voltage generated by a TEdevice at a given T, is a fraction of the voltage it

    takes to create that T with the Peltier effect.Furthermore, the voltage which is generated,opposes the applied TE voltage and, therefore,has an attenuating effect on TE current.

    64. What happens if I use a device with morecooling power than I need?

    That depends. You can just let your system settleto a colder temperature than was originallytargeted. If this is unacceptable, you essentiallyhave three choices: 1) if possible, you candecrease the amount of voltage in your system to

    bring the TE response in line with your needs;2) you can squander some of your excesscapacity by using a less optimal heat sink (whichmight save you money, as well); or 3) you canemploy a temperature controller to limit TE poweras necessary. Remember, if it is critical tomaintain your load at the desired temperature, thethird option is the only one which can insure thisresult.

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    65. Are these products tested beforeshipment?

    Yes, Tellurex electrically tests every modulebefore shipping.

    66. I need to route wiring through the Peltierdevice; can I drill a hole anywhere in themodule to facilitiate this?

    Semiconductor pellets are used in a rectangulararray throughout the device and are connected toone another through copper tabs. There are smallspaces between the rows and columns. Whiledrilling is not recommended, it is theoretically

    possible to put holes in these areas; this requiresprecision drilling and great care must be taken toavoid creating electrical problems such as shortsor opens. If you fracture a pellet or sever a tab

    connection, the module will become inoperable.

    It is possible, on the other hand, to manufacturedevices with holes in them. These modules dorequire extra tooling and special processing,however, so the cost is significantly greater than astandard device.

    67. I need to cool a tubular shape; can I get acustom device made in a cylindricalform?

    Given the present state of technology, this is an

    exceptionally challenging proposition. Just findinga way to configure the device so that it couldadequately manage thermal expansion andcontraction, would be demanding enough. If youcould satisfy that objective, you would then haveto find a means of coping with tolerance variationsin interfacing the device to a cylindrical heat sink.The biggest hurdle, however, would be creatingmanufacturing equipment and processes toassemble such a module. Can it be done?Possibly, but the cost would be extraordinarywell beyond the bounds of viability for mostprojects.

    The solution in most situations like this, is tomachine a thermally conductive block (usuallyaluminum or copper) to create an interfacebetween the tubular load and the flat surface ofthe thermoelectric device. While this is not aselegant, conceptually, it generally works very welland is affordable.

    68. My results seem to vary significantlyfrom what is predicted on the graphs atyour Web site. What would account forthis?

    Like other manufacturers, Tellurex bases its

    specifications and performance graphs onmodules tested under laboratory conditions.Great care is taken in the preparation of samples.We especially try to insure consistency in thermalinterfaces and insulation between hot and coldsides of the test system. Key temperaturereadings are taken right at the surface of the TEmodules. Using these methods, we find that weget very good repeatability.

    Results within a customers system are verydependent upon the details of the design andenvironment, from selection of heat sinks, to

    worst-case ambient conditions, power supplyspecification, nature of interfaces, use ofinsulation, active & passive loads, and so on.Placement of temperature sensors can often go along way toward explaining discrepancies.

    The best way to insure close correspondencebetween expectations and results, is tocommunicate with our sales staff from the outset(231-947-0110 or [email protected]). We canoften spot problems that you may not see andmake recommendations that can substantiallyimprove your design. We can also run yournumbers through our computer model, which willgive you a more accurate prediction of what yoursystem can achieve.

    69. Are all thermoelectric modules on themarket essentially the same?

    No. Each company takes its own approach withmanufacturing processes and materialformulations, some to assure the highest qualityand others to deliver the lowest price.

    70. What sorts of differences are seen?There are two major areas of difference and

    each impacts performance:

    a) Workmanship: In good quality devices,manufacturers strive for superior alignment ofsemiconductor pellets, strong bonds betweenpellets & conductor tabs, strong bondsbetween tabs & substrates, and superior

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    cleanliness. In poor quality devices,electrical and thermal shorting can becreated by bad alignment and debris;furthermore inadequate bonds cancompromise strength.

    b) Material technology: Each manufacturerjealously guards its formulation ofsemiconductor material. Some companies,like Tellurex, have become internationalstandards for the benchmarking of theirmaterials. In fact, Tellurex has exclusiveworldwide licensing to the highest performingpower generation materials. Othercompanies are not as successful in thedevelopment of semiconductor material andthe differences in performance can be highlysignificant.

    71. There are thermoelectric modulesavailable from third party sources athighly discounted prices. Are thesedifferent from other thermoelectricmodules?Yes. Low budget thermoelectric modules, inaddition to the problems noted above, may bescrap or defective products that a reputablecompany would never release to a customer.

    72.I purchased a thermoelectric modulewith the same specifications as the

    Tellurex product but it does not performas well. Why is that?

    We have seen specifications from anothercompany that make it appear that a device has

    an incredible capacity for heat-pumping asexpressed in watts. Upon closer examination,we have found that their published data actuallydescribes the heat that the module dissipates inits own resistance while running at VMax. Werecommend that you discuss all publishedspecifications with their sales support staff. Ifthat experience disappoints you, call us.

    73. Is Tellurex still the global leader in newthermoelectric materials, engineeringand technology?

    Yes.

    74. How long has Tellurex been in business?

    Since 1986.

    75. How can I order products from Tellurex?

    You can call us at 231-947-0110 or order fromour Web site at www.tellurex.com.

    76. Do you have a minimum order require-ment?

    Tellurex direct sales requires a minimum of $75.Exemptions may made for academic institutions.

    77. How can I get more information?

    The best place to get information is from our

    websitewww.tellurex.com. You can also call usat 231-947-0110 or write us at 1462 International

    Avenue, Traverse City, MI 49686. Email may bedirected to [email protected].

    2010 Tellurex Corporation