Electronics > Repair

Why does SMPS take 20 minutes to power up?

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max.wwwang:
I got a sewing machine to repair, which is said to have a faulty power supply. However, after watching a YouTube Video online and trying the suggested trick, it works all fine. The only thing is, I needed to turn on the power switch (no response at first whatsoever) and leave it there for some time (it's said to be up to one hour) before it powers up. In my case, it took 20 minutes before the machine turned on without any problems. Next time (according to the video, as long as you don't leave it idle for long), it will turn on immediately without any problems.

It's the rubbish SMPS type, not the traditional linear transformer thingie.

What do you think is likely the problem that makes an SMPS work but takes a long time before waking up. Have you seen this kind of behaviour of SMPS before? I've attached photos of both sides of the board to get an idea of its design.

Practically, since now I know the trick, I don't need to fix it. But it's always good to bring it back to the way it should be.

[Edit] With a second thought, it is possible that the PS has no problem but there is some problem in the main board of the machine. Unfortunately, now I have no way to ascertain if this is the case, because without leaving the machine idle for I-don't-know how long it takes, I'm unable to reproduce the problem!

Kim Christensen:

--- Quote from: max.wwwang on September 10, 2023, 02:46:03 am ---[Edit] With a second thought, it is possible that the PS has no problem but there is some problem in the main board of the machine. Unfortunately, now I have no way to ascertain if this is the case, because without leaving the machine idle for I-don't-know how long it takes, I'm unable to reproduce the problem!
--- End quote ---

Well, you could check the DC voltages coming out of the power supply. If they are all there instantly at turn-on, then the problem is likely elsewhere in the machine.

xavier60:
Also, carefully listen for any faint ticking from the PSU while it's in the fault state.

amyk:
Almost certainly bad caps. Their ESR is far too high when cold, but once they've warmed up they are just barely OK to work.

xavier60:
Yes, likely the control ICs Vcc capacitor.

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