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Identifying problem with a cheap SMPS.

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magic:
If it gives legit shocks and not just tingles the transformer may be shorted between primary and secondary.

Disconnects for a few minutes, short all capacitors with a screwdriver (they should already be mostly empty by that time) and check for continuity between primary and secondary transformer windings. There obviously should be none.

4kruby:

--- Quote from: Gyro on January 21, 2022, 05:47:21 pm ---The Y cap (if it is actually a safety rated) is the blue one alongside the transformer, I can't see the part number or device marking in any of the photos. C5 looks to be part of a snubber on the primary side.

It's likely that the shock you feel is Y cap (?) leakage, but the quality of construction is so poor that you cannot rely on the rest of the construction to be safe (eg. the transformer).

--- End quote ---
I think you are right. That capacitor is connected between DC output positive with the rectifier diodes. it is rated 470, 1KV


--- Quote from: Gyro on January 21, 2022, 05:47:21 pm ---Specific observations:

1. Bypassed blown fuse resistor on mains input (who would do that?  :o)
2. Incorrect PCB clearances between primary and secondary side.
3. Incorrect PCB clearances primary side (especially now the fuse resistor is bypassed).
4. Board wrapped in flammable expanded foam, the PCB obviously wasn't designed for that case.
5. Very poor soldering - solder splashes where there really shouldn't be solder splashes.
6.  Bodged components, eg, two diodes in parallel for the output rectifier - they will not share current properly, the hotter one will pass more current.


--- End quote ---
Very nice observations. Even I was shocked to find the foam and a cardboard inside it. (The cardboard is underneath the foam), Guys thought they are leaving a Easter egg for opener?


--- Quote from: Gyro on January 21, 2022, 05:47:21 pm ---Is there any way you can report the manufacturer to the authorities? At best, you should destroy it and crush the PCB to ensure that it is not salvaged and re-used (again?).

--- End quote ---
No one cares. TII.


--- Quote from: magic on January 21, 2022, 05:59:36 pm ---If it gives legit shocks and not just tingles the transformer may be shorted between primary and secondary.

Disconnects for a few minutes, short all capacitors with a screwdriver (they should already be mostly empty by that time) and check for continuity between primary and secondary transformer windings. There obviously should be none.

--- End quote ---
I can check that. Thanks. Just one or two learning before I destroy this thing.

Gyro:

--- Quote from: 4kruby on January 21, 2022, 06:06:01 pm ---I think you are right. That capacitor is connected between DC output positive with the rectifier diodes. it is rated 470, 1KV

--- End quote ---

Definitely not Y rated (or safe) then.


--- Quote ---No one cares. TII.

--- End quote ---

I feared as much.

To me, that board does have the look of one that has been salvaged, 'repaired', possibly modified for different output, and 'persuaded' into the case.




--- Quote from: HumbleDeer on January 21, 2022, 05:54:16 pm ---Solder splashes where there really shouldn't be solder splashes? So, everywhere? There are no places where solder splashes, potentially shorting things, are appropriate.  ;D

--- End quote ---

Ha, yes. I was highlighting that the ones between the closely spaced primary side tracks being particularly dangerous. All solder splashes/balls are bad, but some you get away with.

4kruby:

--- Quote from: magic on January 21, 2022, 05:59:36 pm ---If it gives legit shocks and not just tingles the transformer may be shorted between primary and secondary.

Disconnects for a few minutes, short all capacitors with a screwdriver (they should already be mostly empty by that time) and check for continuity between primary and secondary transformer windings. There obviously should be none.

--- End quote ---

It is a good amount of shock. Even the electrical tester glows pretty brightly when touched to the negative rail of the output.

I found the transformer has the pinout same as below with the four terminal on the high side and two terminal on the output side :


I checked it and 1&2 are connected, 3&4 are connected and 5&6 are connected. Apart from these, there are no connections between the terminals.

I was just curious to know how did the mains leak out.

Gyro:
Leakage through the capacitor is probably enough to light the electrical tester (I'm assuming you mean a Neon screwdriver type). As for the transformer, a continuity test won't reveal high leakage due to poor insulation that might occur at mains voltage. It's really not possible to tell without dismantling the transformer and seeing the clearances between the windings (often the ends are an issue where they come out to the terminals), and quality of the tape insulation between them.

The following video will give you some insight - in fact, if you look through his other videos, you will see several (at least 8 ) more...

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