Mini fridge build part 1Posted: May 11, 2011
Following the schematics I posted previously, I made up a little board containing all the external components, including the resistors, optoisolators, and isolated DC/DC converter.
The DC/DC converter in question is the XP Power IE1205S, costing £4, which is by far the most expensive component on this board. This little black square in the bottom left of the board in the image above, converts the 12V DC of the fridge’s power supply to 5V, which feeds Forebrain. In addition, there is isolation between the two sides.
The isolation is probably not strictly necessary since the fridge’s mains transformer board should also isolate its 12V supply from the mains too, but better be safe, particularly when I intend to plug Forebrain into my computer via USB. The isolation will prevent any unhappy “incidents” involving computers and mains electricity.
Beside this, there is also the rather fetching ivory-white chip, which is the optoisolator, a Vishay Semiconductor K824P, this chip allows the transistor on the fridge’s 12V side to be controlled, in an isolated fashion, by forebrain.
And there is also the TO-92 packaged BC237 transistor that drives the LCD’s backlight.
The three wires coming off this board are: orange (12V), black (PGND), and green (control signal). You will see these wires in later images.
The green control signal wire is attached to the base of this here TO-220 packaged BDX53B NPN darlington transistor, which in hindsight was probably a very poor choice (it has quite a large collector-emitter saturation voltage). I should have used a MOSFET instead.
However, by a quirk of thermodynamics, the large collector-emitter saturation voltage of the transistor doesn’t bite us in the ass half as hard as it should have; the high saturation voltage means that a lot of the power that was supposed to go to the peltier is instead lost as heat in the transistor. But because the peltier is a thermoelectric device, I can use it to pump the lost heat of the transistor back into the inside of the fridge! All I have to do is clip the transistor to the peltier’s heatsink, which will get cold as the peltier siphons the heat out of it and dumps it in the fridge like some kind of thermodynamic vampire.
But first, wiring the rest of the circuit:
This is the junction between the fridge’s mains supply board and the switch board, as per the schematics I posted previously, I have to wire our transsitor in the black wire there, to control the power flow from the mains supply board to the switch board (and therefore controlling the power supply to the peltier and fan). I also need to access the 12V supply as well to power Forebrain, the LCD, and the temperature probe.
Snip snip snip (somehow it seems wrong that I’m making sound effects to accompany my blog pictures)
CUT THE RED WIRE…THE RED WIRE!!! NO WAIT…THE BLACK WIRE…THE BLACK WIRE!!! AAAARGH…CUT THEM BOTH!!!
I cut the red 12V supply wire to solder our orange wire into it, reconnecting the severed ends.
Next, I strip and solder some leads to the bifurcated black PGND wire (I’m calling this black wire Power-GrouND to distinguish it from the isolated 5V GND that Forebrain uses). These are the brown and a blue wire in the image (no doubt previously scrounged from some mains lead in the forgotten past). The ends of these wires are connected to the collector and emitter pins of the transistor (importantly the collector comes FROM the switch board, the emitter TO the supply board).
As previously mentioned, I will attach this transistor, and it’s 7W heat loss to the peltier’s heatsink. If I had used a MOSFET or a BJT with less VCESAT, then this whole heat-sink attachment operaion wouldn’t have been necessary.
Heatsink paste must be applied as the side of the heatsink fin that the transistor is to be attached to is ribbed and doesn’t have a smooth surface. Unfortunately that nice wide flat area of the heatsink wasn’t quite wide enough to accept the TO-220 transistor.
I should mention at this point that the tab of the TO-220 transistor is connected to the collector pin. Since I’m not using any insulation between the tab and the heatsink, the heatsink is likely to be connected to the collector via the TO-220 tab, and therefore can be anywhere up to 12V relative to PGND. While I did check that the heatsink itself was not connected to any of the other wires, and therefore no chance of electrical short, I did find out after construction that the inside of the fridge itself is enameled metal. Given that the heatsink is bolted to the peltier, which in turn appears to be bolted to the metal interior of the fridge, there is a good chance that there could be an electrical connection between the heatsink and the metal interior via the bolts.. It would therefore be a good idea not to touch the inside of the fridge while the mains is plugged in in case I was wrong about the fridge’s supply being isolated.
I simply clip the transistor to the heatsink with this 100 year old invention, a baby-blue binder clip (I wonder if babies are really this blue colour). The great thing about the binder clip is that the handles are designed to be removable, leaving a semi-permanent atachment..