TEC power supply, no current

[trex66]

Hello everyone, I have a power supply from a wine cooler that went bad.

The history with this is when it first stopped working I took a look and replaced a blown cap 'B' in the picture. It worked for some time then stopped again, I replaced 'B' again but still not working. Then I found there was very low current(amps). Fans and controller will work if the TEC is off and NTCI is unplugged. It will measure 12.5v but it will fall to 0 current when I try to measure volts. Anyway, bought a new TEC and replaced the caps at 'A' with same results. The original Caps at 'A' were 200v,120uf and looked like they had leaked a little.

So can someone point me in the right direction, where should I look next, why am I not getting any current?

[kaboom]

Re: TEC power supply, no current

The small caps need to be changed, too. Especially those two 'back-biasing' ones in the base ckt's of each switcher.

Edit- Even though it may look ok, resolder everything. Those sweat shops aren't the cleanest places, and who knows with the original soldering, dirty leads/pads, etc.

This is a half-bridge, with a larger than normal inductance in the output inductor. When these work properly, the 494, with a signal from the thermistor (possibly aided by a transistor), drops the duty way down. Very narrow pulses are still present on the rectifier (SBL3040?) side of the inductor- the inside LED, if it has one is tapped off here. But when duty cycle is low, hardly any voltage appears at the capacitor side of the output inductor, thanks to the integrating that happens.

When there's a call for cooling, the duty cycle is ramped up and the pulse width is much higher. Now, we have our 12-ish volts at the output filter cap. There is also either a fast or Schottky diode from the common cathode of the SBL3040, leading to a small cap, maybe 68 uf, feeding the 494 and temp control.

I've rebuilt these supplies. A mini 'mini-fridge' had line voltage for about three years nonstop. The garbage caps were degrading nonetheless. Last winter, the unit had no power for a few hours. When power was restored, the unit was even more pathetic than usual. It barely cut it with its undersized Peltier module and useless fans. But now the junk caps 'deformed' in that short period without bias, that it would never reach setpoint. I'm not sure if it was even run for long. Owners said it 'acted funny' and unplugged it.

When rebuilding the board, I saw that the two filter/doubler caps leaked. No traces were damaged and I cleaned the board up. Replaced all caps with Nichicon PW. The two filter/doublers were replaced with Panasonics- have to look for Mouser order to find the series I used for them. The 1A line rectifiers were replaced with 2A units. Also cleaned up some wiring- these chinese things can get sloppy. Replaced the pointless fans with 'real' ones and this thing made it down to 36-38 degrees, then cycled. I also changed the value of a feedback resistor so the output voltage was 14.2 instead of 12.0.

When I first got the unit, I tried the module and original fans with a bench supply. Ran constantly and never got below 52 degrees. No wonder milk went bad in the thing!

-Paul

[trex66]

Re: TEC power supply, no current

Thanks for the help, I ordered new schottky and rectifiers but now I am also going to replace all the small caps so I don't have to question their working or not. Looks like a lot of junk component used on here

[trex66]

Re: TEC power supply, no current

replaced the two rectifiers and schottky - still no current... waiting on small caps

[trex66]

Re: TEC power supply, no current

Replaced all caps, it ran fine for 15 min then fans started to surge and display blinked as did both red and green LEDs on PCB.

I took a closer look at PCB for dry solder joints, everything looks good, double checked cap V and uf... tried it again and this time same surging then after a while blinking slowed down and got steady fans and LEDs - But voltage is only 10.7v not 12-13v.

Kaboom, where's that feedback resistor and how do I change the voltage to 14? I think the TEC units are for 15v

There are two LEDs on the PCB green and red, they were both blinking and clicking when I had no current. Now they both stay on, but pretty sure only the green is supposed to be on. Does anyone know what the red LED means?