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This sounds like a "chicken and egg" problem. Once the supply is running, there is probably an additional transformer winding and rectifier that powers the control circuits. To start the supply, a small amount of current is probably taken from the DC input through a large resistor. This may be only a milliamp or less. This current will be used to charge a capacitor up to a level where it can supply the TL494. Once the TL494 starts operating, the capacitor voltage will start to drop. After a few cycles, the winding on the power transformer will start to deliver power and allow continuous operation.

The storage capacitor is probably an electrolytic capacitor. This could very easily be open. (If it were shorted, the 9 volt battery wouldn't start the supply.) The charging resistor could also be open. I would expect a value of several hundred kilohms for the charging resistor and somewhere around 10 to 100 microfarad for the storage capacitor.

The charging resistor may be larger than most of the other resistors. With a charging current of 1 milliamp, the dissipation in the resistor would be 300 milliwatts. The voltage rating is also higher on the larger resistors. I would look for a half to one watt resistor wiht a value of several hundred kilohms.

If I'm getting this right, the PS has a cap that holds start up power for the next time you turn it on? That seems like a bad design to me. caps have leakage, even if small the supply sitting on a shelf will eventually become unusable. it also means they have to "jump start" them at final test in the factory.

The capacitor does not hold charge from the last operation of the power supply, but it is charged just before starting the power supply. The main effect is to delay starting the power supply for about a second as the capacitor charges.

Basically, you charge the capacitor with a few hundred microamps for several hundred milliseconds before starting the power supply. The supply control circuits may need tens of milliamps, but only for a millisecond or so until power becomes available from the main converter.

philba, If you mean the cap I was talking about, I didn't mean storage, I meant a cap from high voltage DC to Vcc to "kick" start it every time the power supply is turned on. 0 to 320 volts in a couple of cycles is a pretty good kick, that's why I asked if about diodes.

jollyrgr, it really is a pain working without a schematic isn't it? have tried shining a bright light through the board to follow the tracks on both sides at once? (of course ground plane center layer spoils that trick)

Cheers,

This is a single layer board and is not very complex. The TL494 has its own switched power supply. And I've used a flashlight to follow the traces and look for cold solder joints etc. I've reflowed solder all over the board, checked the resistance of almost every resistor, checked the semiconductors as well.

I asked one of the electronic techs at work about switching supplies. These guys repair TVs, VCRs, and medical devices down to the component level on a daily basis. He told me that he and a fellow tech worked on some switchers. They both went through normal testing and would get some results only to have the supply fail again. It became apparent to them that time spent was not worth the expense of buying a new supply. I knew this to be true with the power supplies from computers. If it were not the fuse or an apparent diode, toss the thing. They took it as a challenge just to see if they could find the fault(s) on the supplies they dealt with. He knew the exact problem I was describing as he'd worked on the supplies themselves and did a similar "jumpstart" trick.

Figuring I could not hurt anything I connected a 12VAC wall wart to the input of the line cord. I started probing the switcher and noted a 33V where the 320V should be (one tenth, right on the money) and could safely touch the board without fear of being hurt. I probed the high side looking for something around 2V DC that might kick off the switcher. No such luck. Likely I'm going to have to rig a bypass of some sort. Maybe a wall wart gluged in the case, next to the switcher, to serve as a permanent jump starter.

I have a peltier fridge with the same board, I just took out the temp control module and hooked it up to 5vDC Gnd and the white control white to an ADC on a microcontroller, and got 0 voltage. I turned the temp control knob and still nothing...

I want to turn this fridge into an incubator, I was thinking I would like to just flip the leads to the peltier, but I want to control the temp too, preferably within a tight range. I have a microcontroller that is programmable and also an LM35 temp sensor... but I couldn't trace how the peltier is switched to figure out where to output PWM from the TTL on my microcontroller.

There is a big heatsink on the "low" side of the board, what i thought was a transistor looks like a bi-directional diode, and in the previous posts it seems like it is a zener.

Dunno what to do... would like to use the supply as it is decently packaged and working as far as I know.

Edd >> where have you been hiding for so long, really missed you here..welcome back..

i have the same board.
someone took out components befor i got it.
i need to know what the component to the right of the double diode on the heat sink is.'lout" is printed under its location.the board looks burnt under it.
next to it is c10.missing.i need its description please

If the space you are referring to is the 1"x.5" LOUT area, it is an inductor. dont know what μH is tho..

Hey all, I am pretty much posting this possibly to help others out searching for answers on the cheap thermoelectric wine coolers, fridge, ect.. I was looking at my Vinotemp VT-28TEDS and it has pretty much a mirror image of the pcb50427e1 board, except that the fans that cool the hot side of the peltier heatsink have separate connectors instead of spliced into one. The replacement board is here for around 45bucks shipped:

http://www.vinotemp.com/View.aspx/3335/ ... -VT-28TEDS

It has been almost ten years since I got my EET degree and working on this board reminds me how fun it is figuring out the issues and correcting them with $10 of parts. But i swayed into computers (Linux Admin )and my knowledge of AC/DC circuitry is fading.

Take care,
JTG

Cheap Chinese Switch mode power supplies.
Very few will start without some load.
Look for bad electrolytic caps, try to use better quality than Chinese,
I have found several defective caps just by observation.
Good luck.

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comments on a 9 month old post?
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