Missing Component Values on a Pectlight TX1775DH PSU

[Crystaleyes]

I was given a Chinese lighting SMPS to have a look at.

It's only a small thing with a single 24v 8.3A output, coming off a 127v mains input.

It came with a blown fuse and someone had already changed the filter caps and the switching Mosfet Source resistor, and it is these values which I am wondering how best to work out.

The FET is a FQPF12N80C N-Channel with the Source resistor connecting straight to ground.
The original resistor had been replaced with two piggy-backed 0.33Ω which measures around 0.18Ω on my tester. I am still learning with SMPS's, but this seems way too low.
Do I need to calculate this resistor for the 3.0A input rating, or does it need to be able to deal with the 8.3A output current?

As for the caps, they had been replaced with 100uF 450v which were physically oversized, so sat wonky.
What calculation can I use to find a suitable value for these?

Any help would be greatly appreciated.

Attachhing some photos...

[stj]

Re: Missing Component Values on a Pectlight TX1775DH PSU

you should have rejected the job,
nothing worse than something somebody else has fucked with!

the resistor is a mystery,
the fet probably originally shorted and fed mains to the driver circuit
but you dont know because somebody has been in already!

[R_J]

Re: Missing Component Values on a Pectlight TX1775DH PSU

I would have rejected the repair as well, but the source resistor could be close to 0.15Ω. BUT, when the mosfet shorts and the source resistor opens, the drive ic is usually bad as well. there should be a small resistor (R16) connecting the mosfet source pin to the CS pin of the ic. and when the source resistor opens, 160+ volts is applied to the ic CS pin. damaging it. ZD2 could be damaged as well. Sell them a new one

[Crystaleyes]

Re: Missing Component Values on a Pectlight TX1775DH PSU

Thanks for the replies, but I'm gonna try and get it working, it's not actually a paid job, more of a favour for a friend who runs a small lighting company

SMPS repair is something which I really want to crack so at the very least I'll be learning more than I know now..

It's not exactly the most complex of supplies and I can literally pull and test every component other than the ic.

Have checked every diode on the board, with no failures.

Three small polyester caps read lower than their rating however, and the suppression cap is also very low.

The chip is available online so I think I'll give it a go and see how far I get?

Maybe I'll contact the company and see if they can't give the missing values?

Nothing to lose....

It's not working now, is it?

[Crystaleyes]

Re: Missing Component Values on a Pectlight TX1775DH PSU

Think I may have found the source resistor value...

This photo is taken from a similar rating PSU and seems to show a 0.12Ω resistor.

The only real difference between the two PSU's is that this one has no smd components

[sam_sam_sam]

Re: Missing Component Values on a Pectlight TX1775DH PSU

I would highly recommend that when you think that you might have it repaired
I would use an incandescent light ( at least 100 watt ) bulb instead of the fuse and see if the light bulb light’s very brightly and is continuously ( it should dim down considerably from when you first power it up ) then you might still have an issue with the circuit board or if does not light at all then you have another issue

[Crystaleyes]

Re: Missing Component Values on a Pectlight TX1775DH PSU

Thanks for the advice.
For sure it'll be switched on through a lamp.


Been too busy to even look at it these last few days, yet I'm suspecting that the OB2269 PWM controller has failed.

I'll obviously test the smd 431 and the opto-coupler first, so let's see?