Wine cooler flickers at startup and doesn't cool enough

[edugimeno]

Ok will do that today. Also I read on another post on this forum about this same that one guy fixed (Im talking about the original issue, once I replace the fried mosfet) 3 boards like this just by replacing C8. Will check that too
LINK
Also, trying to analyze the function of this board...As I don't have a valid schematic of it yet..I believe there should be a fixed output PS, probably driven by the small transformer. This contantly powers the digital electronics that show the temp onscreen, deal with buttons and temp selection, and generates the PWM signal, and there should be another PS that has a variable output, probably the one at the larger transformer, that's driven by a PWM signal to vary from 0-12V depending on the difference between set and measured temperature and applies to Peltier&fans. Does this make sense? In such a case I don't see how the PWM signal gets back to the primary side of the PS to drive it, there's nothing joining LOW and HIGH side of the PS other than a set of 2 ceramic caps but those only connect GND... This is something I don't get yet

Thanks

[edugimeno]

Ok the replacement mosfet arrived and I put it back in place. I measured resistance everywhere and found no short on diodes, and places that looked like shorts it was the transformer coil that was in parallel...

So what could be wrong before I turn this on again? The apparent short near the dual schottky diode was not a real short, both 1&3 legs measured low ohm but it was the secondary of the transformer. I took the dual schottky diode out to measure it and it was ok (infinite backwards,, 0,188 oh direct on both diodes).

I have a bunch of small capacitors out of the board to get a replacement especially since I read that C8 was very prone to fail

I have C7, C8 and C14 out awaiting for my local shop to open on monday and with my cheap ESR meter one of them measures 10 ohm while the other 2 give out >25 Ohm...

Also I realized that the primary twin large HV capacitors, that I replaced with spare parts I had around just in case, are not in parallel but in series, so I used different capacitance I found, which now to me seems like a mistake as one would be taking a much higher voltage than the other. Also what voltage were they originally for a 220v board? If 220v sine is 310v peak, divided by 2 each would take 155v, so 200v would be the right max voltage to order them?

Wow so complicated for a simple wine cooler...

Thanks

[stj]

if the mains caps are in series then they are usually 200v and the center-tap is used for 110/220v switching

[edugimeno]

Alright, I will get those. And what do you think of the other questions... Should I just turn it on as soon as I replace the caps? What could have caused the mosfet to instantly burn?

Thanks

[stj]

if a mosfet shorts it's gate then you need to check the drive circuit.
use a dim-bulb tester on it

[edugimeno]

Ok I'll get the caps from my local shop tomorrow monday and try it. What's the wattage used for the dim bulb series tester? It's going to be hard, filament bulbs have been forbidden to be sold here in Spain for many years now, I haven't seen a non LED bulb for a looong time
60W is fine?

[CapLeaker]

Ok I'll get the caps from my local shop tomorrow monday and try it. What's the wattage used for the dim bulb series tester? It's going to be hard, filament bulbs have been forbidden to be sold here in Spain for many years now, I haven't seen a non LED bulb for a looong time
60W is fine?

Yep. 60w or 100w incandescent light bulb. If the PSU consumes a large amount of current, the light will be bright constant and limit the current to the PSU preventing it from blowing mosfets etc.
If the light just lights up for 2 seconds and then goes out or is very dim, that's o.k. Comes from caps charging up.

[edugimeno]

Alright, will be back with some news hjopefully in a couple days, tnx

[CapLeaker]

Alright, will be back with some news hjopefully in a couple days, tnx

Actually the wattage of the light bulb depends on the load. Smaller power supplies less wattage, bigger power supplies larger wattage. Most of the time I use bulbs between 60W to 150W. You can use multiple bulbs too… like 2 x 60W bulbs to make 120W. Or 2x 150W to make 300W.

[edugimeno]

Yes this one feeds a peltier cell thats supposed to be 80-100W I believe...But I will be testing it without the peltier cell as I brought home the board only. I just came back from the shop to buy all caps, will test them later tonight, still need to find the bulb, nothing at the shop, will see at my dad's house

[CapLeaker]

you need to run the PSU without any load, otherwise the light will be constant on.

[edugimeno]

Ok Im completly lost at this time... Here's how it went:
I found a proper 60W incandescent bulb and a socket
I bought all the suspicious capacitors, including the HV pair to make sure they are the same capacitance
I went to where I have this board and put all the news caps in, replaced the mosfet, added the bulb in series and powered up
Nothing happened not even noise.. Mains current wasn't going too far. The fuse was blown too!
Ok replaced with a similar vale and tried again. 0,3v at 12V outputs...Traced from start to end. 160V DC on each HV capacitor, this is good.
Close to10v AC at all wires of primary of the larger transformer, nothing at the secontary...
While doing all these tests I realize it smells like "hot board", turn it off and as I touched the left trnasofmer heatsink it was burning hot. The right one was not
This transformer is the one fed by the mosfet that burned
Checking again resistance, I see leads at the RIGHT mosfet (not hot) show a very low ohm...but as there are 0.X resistors around and also they feed a primary coil, I expect a low ohm to be normal.
But maybe it's not, as I found another old peltier fridge that used a board which not being exact to this, shows a very equivalent schematic, with 2 mosfet driving 2 transfomers, dual schottly diode, etc.. and there is no shortcut at any mosfet leads...

I tested ALL diodes on the board, they are all good.
Also reviewing all the process:
1) Board has a "blinky" power up for a miunte, then would stay on but would only drive 3v to peltier instead of 12v
2) I replaced some caps and at some point one mosfet blew
3) I replaced mosfet and most of the caps and now the mosfet I replaced gets really hot.


Any idea?
Thanks!

[edugimeno]

ANyone? Thanks!!

[petehall347]

is this broken ?

[edugimeno]

is this broken ?

You mean the diode, that looks like it has a thin line across? I think it's just a camera reflection, but I tested it with my MM and it checks good in diode mode 0,5v- infinite on both ways

Thanks
Its got to be something closer to the chopper,,, I believe

[edugimeno]

Hi again!
Im still investing some time, not much as Im busy lately... on this project
My first issue, is that I dont' reallly understand how this whole circuit works,.
I've found the schematic, attached as 2 images.
So there are 2 main transformers, small and large. Small has 1 dual winding and another single winding on the primary side. Center tap on dual primary winding seems to be controlled by twin mosfets, 1 drives to Vcc, 1 drives to GND. But then the top winding comes in series with the other transformer, and this is what I don't understand why they get interconnected.
Then on the secondary side (different pictures), the large transformer seems to drive a dual diode to generate 12V DC which is fed directly to the fans, and peltier cell, and also a 8v regulator drives the OPamps and the front panel. Also the OpAmps., which seem to compare the set temperature and the measured temperature , seem to drive a signal to the TL494 IC that drives a dual transistor push-pull block to the small transformer. So is this transformer working in reserve, so the signal driven by these 2 transistors (T4/T5) alter the voltage in the right side (I guess, primary) so this changes the voltage in the left (I guess secondary) side, and this modifies the power applied to the primary of the large transformer to modify the output voltage (0-12v) sent to the fans and peltier?
If so what would be the best procedure to diagnose what's wrong now?

Hopefully someone understands the schematic and can shed some light on it...

Thank you!

[stj]

strange and cheap design
T6 and 7 can pull the transformer center tap to + or - and should alternate to drive the primary,
they are driven from the lower winding that also provides the feedback so it self oscillates.
problem with such designs is getting them to start up first.
those 4.7uf caps are probably critical to this,
also check the 2ohm gate resistors r35 and r38 arent damaged by the previous mosfets.

the second page makes no sense so i'll assume the schematic is wrong or mismatched - it is driving the output of the transformer as though it was an input.
i think they drew it wrong.

[edugimeno]

Thanks, I want to believe that the lower transformer is actually used backwards on purpose, like the top transformer provides a variable voltage that is regulated on the right side of the circuit, and a proportional voltage is applied to the right side of the lower transformer, which in turn drives and modulates the top transformer to alter the 0-12v voltage to the peltier cell & fans?
Not sure thou...