MARCH 2003 1

By Ross Tester

Last century we described a Peltier-powered tinnie coolerwhich could be built into a reasonably large Esky and so keepyour food and drinks cool. Here’s another one – quite a bitsmaller, just right to build into a six-pack Esky to really cooldown the tinnies. As a bonus, it will also keep food warm!

Every true-blue Aussie knowsthere is nothing worse than awarm beer (or soft drink, for

that matter). But how do you get yourdrinks cold – and keep them cold?There’s the old ice-in-the-Esky rou-tine. But ice melts – especially if youkeep putting warm cans in.

Wouldn’t it be nice to have thecooler itself cool your cans? This onedoes! Just plug it into your car ciga-rette lighter (or any other 12V DC sup-ply) and it will silently cool cans downto the “aaaaaaahhhhhh” level.

Like the cooler we presented backin September 1999 (see, it was lastcentury!) this one is based on a PeltierEffect device. We explained this semi-conductor device and its operation insome detail in that issue (and in theAugust 1999 issue) so we won’t gointo too much detail again.

Suffice to say that it consists of anumber of P-N junctions sandwichedbetween two metal plates. Pass cur-rent through the junctions one way

and they absorb heat – one of theplates gets very much colder than theother. Pass current through the otherway and the reverse happens, the platewhich was cold heats up.

If you thermally bond the Peltier(that’s shorthand for Peltier Effect de-vice!) to another object, that objectwill either cool down or heat up, de-pending on the polarity of supply tothe Peltier. That’s why a Peltier can beused for both cooling and heating.

If you’d like more info on the opera-

tion of Peltier Effect devices, we sug-gest you refer to the issues mentionedabove.

Peltier “module”There are two major differences be-

tween this project and the earlier one.First, it’s much smaller – this one is

designed to fit into a 6-pack cooler(the earlier one fitted a larger cooler).It really is intended as a drink cooler,not a mobile fridge!

Second, and most importantly, this

Just in case you haven’t seen onebefore, this is what the Peltier devicelooks like. This is a lower rateddevice than the one in this module.

2 SILICON CHIP

project is based on an almost-com-plete Peltier module.

The earlier project required you tosource the bits individually; this onehas a pre-assembled Peltier modulewhich includes a fan, heatsink, ther-mal switch, aluminium block and gas-ket.

All you have to supply is an alu-minium plate (size to suit your cooler)and a small piece of, say, 15mm-widepolystyrene foam (such as used forpackaging).

The aluminium sheet needs to be aslarge as possible to ensure good heattransfer. We used a sheet 3mm thickbecause it was available – even thickerwould be better still.

Incidentally, the module comesfrom Oatley Electronics and costs theprincely sum of just $33.00 (plus p&p).Not real shabby, that: we’ve seen simi-larly rated Peltier Effect devices aloneadvertised for the best part of $150.00

Oh, nearly forgot: you also have tosupply the cooler!

Thermal shockBefore we go too much further, a

warning: one thing you cannot do witha Peltier is switch it between heatingand cooling (or vice versa) withoutwaiting for it to cool (or heat) back toroom temperature. This would placeenormous stresses on the device andwould quite likely damage or evendestroy it.

We have shown a “reversing” switchin our circuit of the device but bear in

mind the comments above beforeswitching over!

Thermal bondingThe reason we placed the module

in the lid of the cooler was that it wasconvenient to do so. It’s also true thatcold air falls, so theoretically the airinside the cooler would eventuallycool down via convection currents.

However, as every good cook willtell you, the secret in heating or cool-ing food or drinks is the thermal bondbetween the cold/heat source and theitem being cooled/heated.

Now we have to say that the ther-mal bond between the aluminiumplate and the cans in the cooler is notthat great. The reason for this is thatair is not particularly efficient at trans-ferring cold/heat.

It’s a bit like trying to boil a kettle byholding it above a hotplate rather than

(Above): the Peltier module attached to the top of the cooler lid while below isthe view from inside the cooler.

MARCH 2003 3

placing it on it. Sure, the kettle willeventually get hot – but nowhere nearas fast or efficiently as it would placeddirectly on the hotplate.

Similarly, it would be much betterif the cans could be placed in intimatecontact with the aluminium plate at-tached to the Peltier.

One way to achieve this would beto place the Peltier module not on thelid but on the side of the cooler, withan “L” shaped aluminium plate run-ning down the side of the cooler andacross the bottom, so that the cansactually sat on the cooling plate.

Even better would be to sit the cansin a small amount of water becausethis would achieve a much better ther-mal bond with the cans than the air inthe cooler.

Indeed, using a cooler is not theonly possibility. We’ve had thoughtsabout using a small length of suitably-sized aluminium tube, maybe a yachtmast extrusion, which often has a flaton one side suitable for mounting theblock (or maybe even the Peltier di-

rect). We haven’t tried this yet – butwill do so when we find a suitableextrusion.

We’ll leave you with those thoughtsin case you want to experiment. Plac-ing the module in the lid of a cooler,as we have done, is certainly not theonly possible approach.

AssemblyBecause the Peltier module is pre-

assembled, building this project is

pretty simple: you cut an appropriatehole in the cooler (slightly larger thanthe aluminium block), pass the coolermodule through it, fit the styrene foamseal and screw on your aluminiumplate. Connect power and it’s done.

The power lead would ideally be asuitable length (but shorter rather thanlonger) of polarised (red/black?) 10Afigure-8 cable, fitted with a car cigarlighter plug at one end. Note that thereare some “cheap” cigar lighter plugs

This very simple circuit could be made even simpler if you don’t wantthe “heat” option: leave out the switch and thermal switch.

Here's how the whole thing fits together. Asmentioned in the text, the majority of thisproject is pre-assembled as a single module –everything between the finger guard and thealuminium block, in fact. All you have to do isfit it to the cooler and attach the aluminium heattransfer plate.

4 SILICON CHIP

around that are real junk – their springsaren’t and deform badly after a fewuses. It’s better to pay a little bit moreand get a good’n.

For safety’s sake we have incorpo-rated a 10A fuse in the + line betweenthe changeover switch and the cigarlighter plug. 10A is more than isneeded by the Peltier and fan butshould protect in case of catastrophicshort circuit.

Speaking of catastrophic, we haveshown an “exploded” drawing of theassembly but that’s just in case youneed to know how it all goes together.However, you should not need to dis-assemble the Peltier module.

One thing you will note on the draw-ing is the use of heatsink compound,especially between the aluminiumblock and the aluminium plate. Thiswill ensure maximum heat transfer.

The thickness of the styrene blockdepends on the thickness of the lid(or wall) of your cooler. The idea isthat it compresses slightly when thealuminium plate is screwed to theblock, making a nice airtight sealagainst the cooler lid/wall but stillallowing the block to make intimatecontact with the aluminium plate.

The high density foam gasket (sup-plied with the module) forms the air-tight seal on the outside of the cooler.

The two seals probably won’t bewatertight but with a bit of work withsilicone sealant could possibly bemade so. Just make sure you don’t getany sealant between the heatsink andPeltier, the Peltier and block or theblock and plate – it is a good way tostop heat transfer!

The thermal switchThe project has a 65 degree thermal

switch mounted on the side of thealuminium block.

This only comes into play whenused as a heating device and is de-signed to prevent the thing getting sohot it starts melting (or at least de-forming) the plastic lid or body of thecooler.

If you are not intending to use thedevice as a food heater, it (and ofcourse the reversing switch) can beleft out.

Power supplyThe 50W Peltier device draws

around 3.5A at 12V (DC). Therefore itis going to place a fairly significantload on your car battery if the engineis not running – certainly enough toprevent you starting the car after a dayout.

Still, if you are caught out in thesticks with a flat battery, your drinkswill be beautifully cold. . .

Of course, you could also run itfrom a suitable 12V (or 13.8V) DCpower supply – a typical “CB” powersupply is rated at about 4-5A so wouldbe perfect.

1 Peltier Effect module (Oatley Electronics)1 6-pack plastic cooler1 3mm thick aluminium plate, size to suit coolerSuitable fig-8 connecting cable with car cigar lighter plugIn-line fuseholder and 10A fuseHeatsink Compound1 DPDT switch, 5A DC contacts (optional, see text)

Parts List – Tinnie Cooler

Upside-down view of the module as supplied by Oatley. Note the liberal use ofheatsink compound on the aluminium block – you cannot see it in this photobut there is also a good dollop of the stuff on both sides of the Peltier device.