Data Pack F
`
`Issued March 1999 232-3052
`
`Data Sheet
`
`Peltier effect heat pumps
`
`A range of semi-conductor thermoelectric devices
`workingonthe Peltier effect. When supplied with a suit-
`able electric current they can either cool or heat. When
`subjected to an externally applied temperature
`gradient these devices will generate a small amount of
`electrical power.
`Available in threesizes the larger devices can be used
`for cooling or controlling the temperature of sub-
`assemblies.
`
`RS stock numbers 618-724, 618-730
`
`Features
`
`@ Solid state, long term stability
`@ Capable of heating or cooling - dependent on
`current flow
`@ Generates no acoustic noise
`
`@ Capable of generating power.
`
`Figure 2 Generation of voltage
`
`HOT
`
`At open circuit a temperature gradient maintained
`across the device creates a potential across its
`terminals proportional to the temperature difference.If
`the temperature difference is maintained, and if the
`device is connected to an electrical load power is
`generated.
`
`APPLE 1015
`
`
`operating temperatures.
`
`Introduction to the Peltier effect
`In 1834 Jean C. A. Peltier discovered that the passage
`of an electric current through the junction of two
`dissimilar conductors can either cool or heat this
`junction depending on the direction of current. Heat
`generation or absorption rates are proportional to the
`magnitude of the current and also the temperature of
`the junction.
`
`Practical Peltier Effect Heat Pumpsconsist of many such
`couples connected electrically in series and thermally
`in parallel.
`
`Figure | Asingle couple
`
`Semiconductors doped both p and n type form the
`elements of the couple and are soldered to copper
`connecting strips. Ceramic faceplates electrically
`insulate these connecting strips from externalsurfaces.
`The semiconductor material used is bismuth telluride
`as this shows the most pronouncedeffect at moderate
`
`APPLE 1015
`
`1
`
`

`

`232-3052
`
`Figure 3 Use as a heat pump
`
`Peltier 18.8W (RS stock no 618-724) and
`
`68.8W (RS stock no 618-730) modules
`
`Electrical characteristics
`at hot side temperature Th = 25°C
`RS stock no
`RS stock no
`Parameter
`
`618-724
`618-730
`Maximum temperature
`
`difference (AT 65°C 70°C
`
`
`Maximum current 8.5A 9A
`
`
`Maximum voltage
`Heat pumping capacity (Q-)
`Operating temperature range
`
`15.4
`3.75
`68.8W
`18.8W
`-150°C to +110°C
`
`Specifications
`
`Universal multipliers
`Dimensions
`N
`B
`A
`No.of
`(mm)
`(mm)
`
`Couples
`RS donk no}
`a0
`#0
`
`(618-724)
`40
`40
`RS tak no|
`(618-730)
`
`Larger Peltier devices of 18.8W and 68.8W ratings.
`The 18.8W module takes a maximum current of 9A
`with the 68.8W device taking 8.5A. They are both
`suitable for cooling sub-assemblies.
`
`
`
`
`
`
`
`
`
`If, instead the device is connected to a dc source, heat
`will be absorbedat one end of the device, coolingit,
`while heat is rejected at the other end, where the
`temperature rises. Reversing the current reverses the
`flow of heat. Therefore the module can generate
`electric poweror, depending on howit is connected to
`external circuitry, heat or cool an object.
`A common misconception is that the Peltier device
`somehowabsorbs heat and carries it away, perhaps
`with the electric current. This is simply not true. The
`device only transfers or pumps heat from one of its
`sides to the opposite side. At the hot side, the heat must
`be removed throughthe use of a heat sink or by some
`other means.
`It is important to realise that the heat
`delivered to the hot side of the device includes the
`pumped heat plus the electrical power dissipated
`within the device.
`
`Back ¢—— 8—tm
`
`Figure 5 Dimensionsin millimetres
`
`Cold face
`18 AWGins. wire
`Ax B groundflatHot face
`(CP1.4 & CP2)
`16 AWGins.wire
`AxBground flat
`(CP2.8)
`10 AWGins.wire
`(CP5) 10cm long)
`
`Red
`(+)
`
`2
`
`

`

`232-3052
`
`  
`
`Using Peltier devices
`Whentrying to determine the heat pumping capacity
`required,
`two factors must be considered; active
`heating elements and heatleak.
`Active heating elements are any components which
`have a power consumption. All of
`this power
`consumption (input power) will eventually be
`converted to heat and should be considered as heat
`load.
`An objectthat is held at a temperature below ambient
`will draw heat from the surroundings onto its cold
`surface. The results of this additional heat is that a
`cooled object whichis not insulated will not be able to
`maintain temperatures as low as one which has
`insulation. This additional heat load requirement is
`called the heat leak. Three main factors affect the
`magnitudeof the heat leak. These are:
`1. The temperature difference between ambient and
`the cooled device.
`
`2. The surface area of the cooled device.
`3. The amountof insulation used.It is advantageousto
`thermally insulate the device being cooled in order
`to reducethe heat leak to a minimum.
`
`leak such as
`There are other sources of heat
`conduction heatfrom electrical wires or heat leak from
`the heat sink backto the cold plate ofthe Peltier device.
`Henceprecise calculation of heat leak is difficult andit
`maybe best determined empirically.
`Thetotal heat pumping capacity required is the sum of
`the active heat load and the heatleak.
`
` ÿ



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`
`Choosing the proper heat sink
`Once the required heat pumping capacity has been
`determined the next step is choosing the proper heat
`sink. A Peltier device is not a sponge which absorbs
`heat, rather,
`it is a heat pump. The heat which is
`pumpedout ofthe cold surface is deposited on the hot
`side of the module. This heat must be dissipated in
`someway.If it is not the hot side of the device will heat
`up the point whereit will stop functioning as a cooling
`device and actually begin to heat the cold surface.
`From fundamentals, a heat sink must be maintained ata
`temperature higher than ambient to transfer heat from
`its surface out into the surroundings. The higher the
`heat sink temperature above the ambient temperature
`the more heat can be transferred out of the heat sink.
`This points to choosing a heat sink which will get as hot
`as possible. However, reference to the performance
`curves shows that as the AT across the module
`becomeslarger(as a result of the increased hot side
`temperature) the heat pumping capacity and the
`coefficient of performance (COP) both decrease.
`Considering both these phenomenaa heat sink which
`rises to a temperature between 5 and 15°C above
`ambientis a practical choice.
`
`3
`
`

`

`232-3052
`
`Figure 6 Coefficient of performancevs current
`
`Howto use universal performance graphs
`Example:
`a device dissipates 31 watts of power. It is desired to
`maintain the device at a constant temperature of +5°C,
`the ambient temperature being +35° (ie. T, = +5°C, Ty,
`= +35°C). To select the appropriate module, approach
`with the following method:
`LAT=T,-T,
`= +35°- 5° = 430°C.
`
`Choose operating current (typically from 30G to 40G)
`at 35G (equals 35/50 = 70% of I max).
`2. From T, = +35°C graph (AT = +30°, 1 = 35G), obtain
`Q/GXN = 1.65; Then Gx N = Qv/1.65 = 31/1.65 = 18.8.
`3. Choose module with G x N= 18.8
`
`This implies that the 68.8W (RS stock no 618-730)
`module is the mostsuitable for the application.
`
`AMINA
`G = Thermoelement geometryfactor
`
`LeteltgCoyoh
`COAALAO
`HAAAAATY
`WA|WA|AA|AA|
`
`Figure 8 Universal performance graphs
`
`°F = -58
`°C =-50
`3.5
`
`-13
`+32
`-25
`a
`Th = +25°C (+77°F)
`
`+77
`+25
`
`—40
`—40
`Th = +35°C (+95°F)
`
`—13
`+32
`+77
`~26
`0
`+25
`Th = +50°C (+122°F)
`I
`|
`
`+122
`+50
`
`+414
`+59
`+104
`-10
`+15
`~40
`™= +65"errsF)
`
`+149 = °F
`+65 = °C
`3.5
`°
`
`T. = Cold face temperature
`N = Numberof thermocouples
`
`Th = Hot face temperature
`
`AT=Th-Te
`| = Module current amps
`
`V = Module voltage,volts
`
`Q, = Watts heat pumped
`
`4
`
`

`

`Installation of Peltier devices
`Peltier devices are only as strong as the semiconductor
`materials used in their fabrication and thus may be
`damaged by the application of excessive stress.
`Modules should never be designed as a mechanical
`supporting memberof an assembly.
`Two mounting methods are recommended with the
`clamping method being generally preferred. Epoxy
`bonding should not be used when operation in the
`vacuum is required.
`
`Clamping method
`
`Figure 9 Fixing by clamping
`
`
`
`Heat sink
`
`Module
`
`Location screws
`
`compound
`
`Device to
`be cooled
`
`1. The mounting surfaces should be smooth to within
`+0.001 in.
`
`2. Clean the module and mounting surfaces to remove
`any burrs,grit, etc.
`3. Coat the module hotside with a thin film of heatsink
`compound and place the module onthe heat sink.
`Applying firm but even downward pressure, rock the
`module from sideto side until a slight resistance is felt
`and excess heatsink compoundis squeezedout.
`4. Coat the cold side of the module with a thin film of
`heatsink compound. Place the object to be cooled in
`contact with the module and rock the object slightly
`from side to side to squeeze out excess thermal
`grease.
`5. Both the object to be cooled and heat sink together
`using either stainless steel screws with spring washers
`or nylon screws. To ensure even pressure across the
`module surfaces,
`tighten all screws finger tight and
`then continue tightening in an alternate or diagonal
`pattern starting with the centre screws(if any) first.
`Maximum recommended compression loading is 15
`pounds per square inch of module surface. Do not
`overtighten.
`
`Expoxy bonding method
`
`Figure 10 Fixing by bonding
`
`Peltier
`mini module
`
`Leads of mini
`module are attached
`to sparecircuit
`padsof IC base
`
`232-3052
`
`Glass top
`IC can
`
`Chip circuit is
`attachedto cold
`surface of mini
`module
`
`Hotside of mini
`module is secured
`to base of
`IC assembly.
`(Base acts as heat
`
`sink for mini module)
`
`
`1. The mounting surfaces should be smooth, flat and
`free from greaseorburrs.
`2. Coat the module hot side with a thin layerof silver
`loaded epoxy.
`3. Place the module onthe heatsink and rockslightly to
`squeeze out excess epoxy.
`4. Weightorlightly clamp the module to hold it in place
`until the epoxy has cured.
`
`Power supply considerations
`
`Figure 11 Base power supplies
`
`Peltier devices operate from direct current and the
`power requirements are usually not stringent or
`precise. For most applications, unregulated dc power
`with a ripple content of 10%orlessis satisfactory andit
`is possible that higherlevels of ripple can be tolerated
`for certain non-critical applications. However, because
`this ripple will degrade module performance it is
`generally recommendedthatthe ripple component be
`limited ta 19% or less
`
`5
`
`

`

`232-3052
`
`There are many methods of temperature controlling
`Peltier devices in either open or closed loop modes.
`With open loop arrangements manual adjustment of the
`inpul current is made, normally by meansof a variable
`power supply, and the temperature is
`thereby
`maintained reasonably nearthe desired set point.
`
`Figure 12 Closed loop control block
`
`Control
`Potentiometer
`
`Very precise control can be achieved by this method
`but relative cost and complexity are greater than for
`open loop arrangements.
`
`Figure 13 Improved switch control
`Switching
`Device
`
`mien
`
`Device
`
`
`
`Thermoelectric
`Module(s)
`
`to
`* The resistor value if selected to reduce the current
`approximately 30%of the normal operating level. The optimum
`value may be determined experimentally.
`
`Thermal -?,
`Feedback
`
`Ss
`
`‘enso
`
`.
`
`With closed loop methods, a temperature sensor, such
`as a thermocouple, is used to sense the temperature of
`the device and appropriate electronic circuitry effects
`automatic control of the Peltier device current.
`
`the above control arrangements a
`In either of
`proportional on/off mark space control can be used but
`is has been found that premature module failure may
`occur because of
`the frequent expansion and
`contraction that results. When this meansof control is
`adopted
`the
`arrangement
`of Figure
`13
`is
`recommended to reduce the thermal expansion
`effects.
`
`The information provided in RS technicalliterature is believed to be accurate andreliable; however, RS Components assumesno responsibility for inaccuracies
`or omissions,orfor the use ofthis information, and all use of such information shall be entirely at the user’s ownrisk.
`No responsibility is assumed by RS Components foranyinfringements of patents or otherrights of third parties which may result [romits use.
`Specifications shown in RS Componentstechnicalliterature are subject to change withoutnotice.
`
`6
`
`6
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