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Just right for Summer! Last month, we introduced Peltiereffect devices and described an accurate temperature controller. This month, given the fact that those long, hot summer days are just around the corner, we’re putting a Peltier device to the most noble of uses – keeping those tinnies temperate! By Ross Tester A PeltierPeltierA Powered Powered Can Cooler Cooler Can 86  Silicon Chip O K, this project could (and probably would!) be used to keep cold a lot more than a few cans. But hey, we’re realists. We know what most dinkum Aussie blokes use their Eskys for. . . and for the benefit of our readers across the Tasman, that translates to what antipodean gentlemen use their chilly bins for . . . Well, this is one chilly, chilly bin bin! It’s not exactly in the league of most of the electronics projects you see in the pages of SILICON CHIP. Basically, we’re using just one electronic component – a Peltier device – and a bit of hardware. So even if you don’t know a resistor’s anode from its cathode (or maybe if you do?), this project should be right up your alley. Of course, if you enjoy a drop of the amber nectar, it’s even better. Fortunately, the project must be completed before you can cool a can, so there’s little danger of making a mistake due to over-sampling along the way. That’s just as well because if you get the connections wrong, you can actually warm the can instead of cooling it. No, we won’t get into any Aussie vs Pom beer discussions right now, thank you. The Peltier device Just in case you didn’t see last month’s issue where we discussed the Peltier device in detail, a word or two of explanation. It’s quite a simple device, basically a number of P-N junctions sandwiched between two metal plates. Pass current through the junctions one way and they absorb heat – one of the two plates gets very much colder than the other. Reverse the current and the junctions emit heat. This effect can be used to heat or cool. If you thermally bond the plate All you can see on the lid of the cooler is a large heatsink and 12V fan. The Peltier device is sandwiched between this assembly and an internal heatsink. getting cold to another surface, it will “suck” heat from that surface. And that’s exactly what we are doing in this project: we thermally bond a Peltier device to two heatsinks, one of which is inside the cooler. When power is applied the Peltier device cools, dragging the temperature of the heatsink down with it. The heatsink draws heat from its surroundings – the inside of the cooler. Therefore the cooler (and anything in it) cools down. (For a more detailed description of the Peltier device, see page 55 of the August 1999 SILICON CHIP). Peltier devices come in a variety of current ratings. The higher the current, the greater their cooling (or heating) capacity. While this is true up to a point, there are several limitations which stop the device operating at its maximum. Perhaps the most important of these is the ambient temperature – if the air on the “hot” side of the device is itself Reproduced from last month’s issue, this is what the Peltier device looks like. This particular one is rated at 12V, 4A. The side closest to the camera is the “cold” side when 12V DC is connected + to red, - to black. hot then there will not be anywhere near as much heat transfer as if the air was cooler. Linked with this is the amount of heat which can be drawn away from the Peltier device. In this project we use a suitably-sized 12V fan which has a reasonable flow but is certainly not hurricane strength. More airflow means more cooling; more cooling means greater efficiency. The “∆T”, or difference between the hot and cold side of the device, is given as 65°C. This is more-or-less the same for all of the types of Peltier device available in this size range. What this means is that at its rated power, the cold side of the device will be 65°C cooler than the hot side (or, obviously vice versa). In an ideal world, this would be the case – but this is not an ideal world. So don’t expect to get a 65°C difference in your cooler! The main difference you would note between the various devices is not so much in their cooling or heating ability but the time it takes to cool or heat. The higher the current, the faster the device will operate. DC switching? Last month we warned about switching a Peltier device with DC due to the thermal stress and shock which may occur between the two plates due to their difference in temperature. This September 1999  87 can be enough to damage or even destroy a Peltier device with continual usage. For this reason, in temperature control applications it is preferable to switch the Peltier device on and off at a rapid rate (>2kHz) and change the duty cycle (on time to off time) to achieve the appropriate power level. The net result is the same but the stresses are elminated. But guess how we are switching the Peltier device in our cooler? With DC, that’s how! The rationale is that in this application the Peltier device would not be switched on and off repeatedly – rather, it would be turned on and left on, for possibly hours on end. While there would be some thermal shock at each switching, it’s nowhere near as bad as switching on or off every few minutes or so, as happens in temperature control. We want the Peltier to work flat out – and that means staying on until the last tinnie is removed! The circuit The circuit, shown in Fig. 1, could hardly be simpler: the Peltier device is in parallel with a 12V fan – and that’s it. There is no on/off switch (it’s intended to run from your car cigarette lighter socket) and there’s no fuse (the cigarette lighter socket already has one). You just plug ’er in and away + – Fig. 1: the circuit diagram could hardly be simpler – a Peltier device in parallel with a 12V fan, both connected to 12V DC. she goes! Peltier devices are not all that common but Oatley Electronics have 4A, 6A and 8A models for between $25 and $30. They are all 40mm square with a ∆T of 65°C. Construction The Peltier device is mounted through the lid of the cooler and requires a suitable-sized hole to be cut in the lid. Fig.2 shows how this is arranged. The exact method of mounting depends to a large degree on the type of cooler you are fitting the device to. We used a “Willow” brand – it happens to be a 33-litre size but that’s unimportant. What is important is (a) enough clearance under the handle to allow the heatsink/fan assembly to fit and (b) a suitable flat area in the middle of the lid to allow the whole assembly to sit flat. Our cooler also had a The first step is to cut a hole in the lid of the cooler the same size as the aluminium block, then drill the mounting holes through the top heatsink, cooler lid and bottom heatsink, taking care not to foul any heatsink fins. The photo at right shows the aluminium block in the hole – a nice, tight fit. 88  Silicon Chip very handy recess in the underside of the lid which just fitted a large (140x 155mm) heatsink; very handy indeed! The heatsinks we used were preloved units from the junk box – it just so happened we had two on hand which fitted quite nicely. If you have to buy new heatsinks, the Jaycar Cat HH8592 (125 x 125mm) and the Altronics H-0566 (150 x 121mm) would appear to be amongst the most suitable. As we said before, efficient thermal transfer is absolutely vital if the system is to work and this depends on intimate contact between the inside heatsink, the Peltier device and the outside heatsink. Your cooler lid will almost certainly have some thickness, probably made of two sheets of plastic (polycarb-onate?) with either air or another insulator (eg, polystyrene foam) between them. In our case it was about 10mm thick. So you’re probably going to need a block of aluminium, slightly thicker than the lid of the cooler, to occupy the space. Again, refer to Fig.2. Exact size of this block of aluminium is not important as long is it is just larger than the Peltier device (which itself is is 40mm square) – say around 50mm square. As you can see from the photo, it sits inside four screws which hold the asssembly together, sandwiching the Peltier device between the aluminium block and the outside heatsink. Fig. 2: exploded view of all the components in the powered cooler. Not shown are the electrical connections: the Peltier device and fan simply wire in parallel (the wires can be hidden between the fan and outer heatsink and held in place with the cable clamp). To minimise stresses on the Peltier device, a gasket/washer of thin rubber (preferably heat-conductive) is fitted between the Peltier device and the outside heatsink. This has a cutout the same size as the aluminium block and Peltier device, the idea being to give shock protection for the Peltier device while still allowing intimate contact between the elements. The photograph shows this more clearly. The small block of aluminium shouldn’t be too difficult to obtain from either an aluminium merchant or perhaps a non-ferrous scrap yard; the rubber gasket may be a little more difficult. If all else fails, virtually any closecell rubber or foam material will do –believe it or not, we cut our gasket from an old Neoprene mouse mat!   Use the aluminium block as a template for carefully cutting the hole in the cooler lid. We used a fine-bladed jig saw to cut ours after first drilling a pilot hole. The block should be a snug fit in the hole. When ready for final assembly, this block can be held in place with some silicone sealant if you wish to maintain the cooler’s semi-watertightness. Then again, who ever tips their cooler upside down? If you do use silicone sealant, it’s important NOT to get any on the face of the aluminium block – this could act as a heat insulator. Place the Peltier device in the middle of the aluminium block and mark the position of the two screw holes along the centre line (size to suit the screws) allowing, say, 5-10mm clearance. You may need to look at your heatsinks before determining position to ensure that the holes are not going to line up with the fins of the heatsink. If the size and layout of your heatsink means that a clash is inevitable, you may need to do what we did and use over-length bolts with nuts tight- Upside-down view of the outside heatsink, with the Peltier device wired in and the rubber gasket in situ. The gasket compresses to ensure a good heat transfer between the aluminium block (not shown) and the top heatsink. Note the liberal use of heat transfer compound! September 1999  89 ening down onto washers sitting on top of the heatsink fins. The use of liberal amounts of silicone heat transfer compound is vital – between the inside heatsink and aluminium block, the aluminium block and Peltier device and between the Peltier device and outside heatsink. The name of the game is to transfer as much heat as possible, as efficiently as possible. Mounting the fan Again, this depends on the layout of your heatsink and the mounting holes for the fan. The most usual method of mounting would be to drill some holes through the heatsink and fit the fan with nuts and bolts. It is important that the fan sits directly on the heatsink fins and draws its airflow through the fins. You can check the airflow most easily by connecting the fan to a 12V battery or supply. In some cases, reversing the connections will reverse the fan direction and therefore airflow but some fans will not operate with reversed connections – in this case, simply turn the fan over. If there is a protective finger guard, make sure it is on the outside of the fan. 120mm fans are available from all the usual suppliers. Which way is ­up? The convention is that when the Peltier device is lying flat with its leads pointing towards you and the red lead on the right side, the upper plate is the “cold” side. Needless to say, this is the side which goes on the inside of the cooler. The red lead, as you would expect, connects to +12V and the black to earth. But even if you do manage an up-stuff, no problem. Simply reverse Parts List 1 suitable cooler, preferably with plastic (polystyrene) lid. 1 Peltier-effect device 1 12V fan, 120mm square (or to suit your heatsink) 2 large heatsinks, size to suit cooler 1 aluminium block, approx 50 x 50 x 5mm (see text) 1 close-cell rubber (or similar) gasket, approx 75mm square 1 3m length heavy-duty polarised figure-8 cable 1 car cigarette lighter plug 1 cable clamp 4 assembly bolts, approx 1/8in or 3mm, length to suit heatsinks, with nuts & washers 1 cable clamp bolt, 3mm x 10mm with nut and washer 4 fan mounting bolts, 1/8in or 3mm, length to suit fan and heatsinks the connections and the heating/cooling plates are reversed. The Peltier device and the fan simply connect in parallel and are wired to a cigarette-lighter plug. Airflow of the fan is important: it needs to be wired so that it is sucking air through the heatsink and blowing it away. This gives the most efficient and effective set-up. We used a 3m length of heavy-duty figure-8 cable which came already fitted with a cigarette lighter plug. These are available from Oatley Electronics for $1.00 each or 10 for $4.00 (what a bargain!). The cooler end of the cable was anchored to the outside heatsink with a small cable clamp and the wire ends soldered to the Peltier device and fan leads, all insulated with small lengths The view inside the cooler lid after assembly. As you can see, this heatsink has been around the traps a little but the additional holes don’t cause any concern. Note the two bolts which mount on top of the heatsink fins with suitable load-spreading washers. 90  Silicon Chip of heatshrink tubing. Be careful with the wires connecting to the Peltier device – they are only soldered themselves and can break off (as we found!). Checking it out It is simply a matter of plugging it in and leaving it to run for, say, half an hour. The lid doesn’t even need to be fitted on the cooler. You should find after this time the inside heatsink is quite cool, even cold, to touch. If it is warm to hot and the outside heatsink is cold . . . congratulations – you’ve made a warmer, not a cooler! To cool, reverse the connections to the Peltier device. Incidentally, there is nothing wrong with connecting the cooler “back to front” if you want to keep food warm. It won’t cook it but it certainly will keep it warmer than the outside temperature. If you want to make a “universal” cooler/heater, simply fit a double pole, double throw switch in the power lead to reverse the connections to the Peltier and fan. One warning: don’t leave it connected for too long. Even with the lowest-rated (4A) Peltier, it will flatten your car battery in fairly quick time, especially if your battery isn’t quite up to scratch. This device really is intended for use when the car engine is running! And one last point: Peltier devices should NOT be operated from 12V battery chargers unless there is a battery connected as well. Most battery chargers have little or no smoothing; the output is usually half-wave rectified (50Hz) or full-wave rectified (100Hz) pulsating DC. That’s fine to charge a battery but by itself will effectively switch a Peltier on and off at either 50Hz or 100Hz – much too slow to avoid the mechanical stresses we mentioned before. Connecting a battery is like connecting a giant capacitor across the supply, smoothing it out to nearly SC constant DC. Where to get the parts: Peltier Device: Oatley Electronics Heatsinks: Jaycar, Altronics Fan: Jaycar, Altronics, Dick Smith Electronics Power Lead/Plug: Oatley Electronics Cooler: Big W, KMart, etc.