Reparing an Airam Cooler 262T cool box

We bought this "active" cool box to keep the food in good shape on longer car trips. The box can be used on both 12V DC and from a 230V AC outlet. The 12V side sports a voltage guard, keeping the ~60W peltier from draining the car's battery.

The Airam Cooler 262 T

The box haven't been used that much the 2 years we have owned it, but there already seemed to be something fishy with the voltage guard this summer. I didn't mind taking a look at it before our planned trip to Lapland was closing in in September.

Verifying the fault

I begun by testing it in the car, with the engine running, and as I feared the box didn't start but only complained about low battery level. I then took it inside and hooked it up to a wall outlet, and to my big surprise the same "low battery level" LED lit up. What?! Nothing else to do but to open it up and see what's going on. However, that was easier said than done, because I couldn't find any screws that kept the top part of the lid in place. I almost broke it trying to figure out how to open in, when I realized that the screws must be underneath the lid's foam gasket...

Lid screw position under the foam gasket. One screw per side.

Reverse engineering the PCBs

Inside the lid I found two PCBs (actually three, but the third one is the 230VAC/12VDC transformer). One with a couple of relays, seemingly some kind of power board, and the other with some finer electronic components.

Relay and heat-or-cooling switch PCB

Under-voltage protection and LED indication PCB

I figured that it's best to just trace the tracks on the PCBs and sketch a schema over the circuits.

Sketched schema over the power PCB

Sketched schema over the voltage sensing/illumination PCB

Nice, huh? ;)


So, I realized that relay 1 (REL1) is a switch for battery or outlet power and REL2 is the safety switch for preventing the battery from draining. The thing is that I could hear a *click* when the outlet power was connected, which meant that the REL1 worked but REL2 didn't or wasn't just energized.

Next I tried adding 12V directly on the relay coil, and it clicked, which meant that the relay was working. This also meant that some part of the under-voltage protection sensor electronics was broken.

Finding the faulty component

By looking at my schema I quickly found a transistor grounding the relay coil on signal from the comparator (IC1). By measuring the voltage on the transistor's (Q1) base I saw that it sure did get the signal comparator, but it didn't lead current through to the emitter. Ahaa! There's the problem. I then tried to short circuit the collector and emitter legs, and voila! the fans started spinning!

Faulty transistor.

I replaced the transistor with one that I found in my drawer, which had kind of the same specs, but the collector and emitter legs where switched, so I just had to turn it 180 degree compared to the original one before soldering.
Tested with both 12V DC and 230V AC power source and both now works again!

Green light and a spinning fan, the thing is working again!