Things can be fixed!

My son has a cooler box in his car.

For reasons too stupid to explain he managed to connected it reverse polarity and it stopped working.

To everyone's surprise there was no release of "magic smoke" in the process.

There is a PCB with a relay and some components.

The simple answer - get a new one.

The "Annoy Dad" answer - ask me to fix it. Well I did some physics and electronics, but I know a man that knows way more than I (Kev) and I thank him for his insight.

He worked out the schematic, and suggested we have fried one of the transistors.

Well, that is easy. Even a min quantity from CPC is a small price to get more.

So, today, we replace the transistors with new ones that cost 15p each. In fact, probably only one needed replacing, but when you have 5 why not be over cautious.

End result - bingo - it works.

Saves some shit going to land fill.

Thanks Kev...


  1. I hate how we live in such a throwaway society - well done for fixing it!

  2. Is this the Tesla? Does this mess up the warranty?

  3. What a bizarre design. With the zener diode it looks like it is detecting if the voltage is too high and will then refuse to turn on the relay. Trucks I believe can run off 24v so it would stop it being damaged. But with all those components why not add a additional diode to protect it from reverse voltage.

    1. It's a bit minimal and probably not that reliable in that configuration but I think D2 is the temperature sensor. a 13V BZX79 (say) has a temperature co-efficient of approx 10mV/K - at a nominal 13.8V supply the base of Q2 will be about 0.8V so Q2 will be in saturation and holding Q1 off. Raise the temp by 30°C and the voltage at the base of Q2 will drop to 0.5V turning it off, the base of Q1 will then rise to approx 3V turning it on and energising the relay. I assume that it is a Peltier cooler and the circuit is sensing the temperature of the hot side heatsink.

      Depending on value C1 could be a bit of noise filtering or a delay to stop the relay trring to turn on and off too quickly.

      Or I could be totally wrong :)

  4. Even more strange is that many plug in car coolers are designed to be reversible, or the heat when the polarity is reversed...

  5. These things exist in various forms but are generally based around a "thermoelectric cooling device", which uses solid state physics (specifically the Peltier effect) and DC electricity to implement a heat pump with the usual hotter side and cooler side but with no moving parts and (usually) little risk of failure.

    I've had several slightly different ones over the years. My first one, a relatively small and simple Electrolux-branded one, could change from heating to cooling just by reversing the DC supply connector (as already noted by Geekypenguin).

    My more recent ones have been unbranded boxes with a non-reversible power connector and have used a slide switch to provide the "cooling vs heating" selection, and LEDs to confirm the mode.

    The most recent one adds a power supply voltage sensing circuit, which flashes a warning LED and switches off the supply to the Peltier device (and associated fan) if the supply voltage indicates that the vehicle battery isn't being charged (ie if it's likely to risk draining the vehicle battery if the device remains powered).

    Handy things. For some reason the ones in the shops nowadays seem to be less robust and yet just as expensive as the ones bought over ten years ago.


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