Author Topic: Identifying problem with a cheap SMPS.  (Read 666 times)

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Offline 4kruby

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Identifying problem with a cheap SMPS.
« on: January 21, 2022, 04:58:27 pm »
Hello,

I had a cheap SMPS wall adapter (9v 1A).
When it is powered, the output is around 10.05 V. But I am getting earthing (shock) on the barrel connector of the power supply. AC tester glows pretty brightly when touched on the barrel connector.
I pried open the power supply and found one of the worst PCBs inside it (attached images).

Can anyone point out where the problem is (as to why am I getting the leakage)?

Note : I can see that R8 is damaged.
Edit : R8 has no connectivity (and shows as open circuit). Is this the problem?

Thank you.
« Last Edit: January 21, 2022, 05:01:49 pm by 4kruby »
 

Offline rob77

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Re: Identifying problem with a cheap SMPS.
« Reply #1 on: January 21, 2022, 05:08:09 pm »
R8 is a fusible resistor, and seems like someone by-passed it after it was blown (the red wire soldered after the R8) - mains input should go to the 2 points on the PCB. also seems the capacitor across the primary and secondary is not a proper Y class .. i would throw away the whole thing.
 

Offline 4kruby

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Re: Identifying problem with a cheap SMPS.
« Reply #2 on: January 21, 2022, 05:22:57 pm »
Thanks. I am not going to use it anyway as it gave me a shock when I first used it months ago.

It's a new SMPS and i opened it for the first time. I think the manufacturers itself did this.

which is the Y capacitor are you referring to? is it C5?

 

Offline Gyro

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Re: Identifying problem with a cheap SMPS.
« Reply #3 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).

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.

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?).
« Last Edit: January 21, 2022, 05:50:23 pm by Gyro »
Regards, Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline HumbleDeer

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Re: Identifying problem with a cheap SMPS.
« Reply #4 on: January 21, 2022, 05:54:16 pm »
I agree with Gyro above;

SMPS without an earthing terminal usually have output ground tied to mains neutral with a Y cap, for interference suppression reasons, compensating for an "invisible" capacitor inside the transformer. The cap provides a return path essentially, so it doesn't have to try and find it through your equipment.

Some cheap supplies don't use a Y cap, and instead just use a high voltage capacitor. This is incredibly dangerous.

If it is a proper Y cap, not damaged, and the SMPS is otherwise not damaged, then the remaining reason could be a high capacitance. Higher capacitance has more potential for suppression, but also leaks more AC current into the Output negative.

One note against Gyro's comment:
5. Very poor soldering - solder splashes where there really shouldn't be solder splashes.
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
 

Online magic

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Re: Identifying problem with a cheap SMPS.
« Reply #5 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.
 

Offline 4kruby

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Re: Identifying problem with a cheap SMPS.
« Reply #6 on: January 21, 2022, 06:06:01 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).
I think you are right. That capacitor is connected between DC output positive with the rectifier diodes. it is rated 470, 1KV

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.

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?

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?).
No one cares. TII.

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.
I can check that. Thanks. Just one or two learning before I destroy this thing.
« Last Edit: January 21, 2022, 06:13:24 pm by 4kruby »
 

Offline Gyro

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Re: Identifying problem with a cheap SMPS.
« Reply #7 on: January 21, 2022, 06:22:06 pm »
I think you are right. That capacitor is connected between DC output positive with the rectifier diodes. it is rated 470, 1KV

Definitely not Y rated (or safe) then.

Quote
No one cares. TII.

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.



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

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.
« Last Edit: January 21, 2022, 06:27:06 pm by Gyro »
Regards, Chris

"Victor Meldrew, the Crimson Avenger!"
 
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Offline 4kruby

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Re: Identifying problem with a cheap SMPS.
« Reply #8 on: January 21, 2022, 06:53:21 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.

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.
 

Offline Gyro

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Re: Identifying problem with a cheap SMPS.
« Reply #9 on: January 21, 2022, 07:11:17 pm »
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...

« Last Edit: January 21, 2022, 07:16:45 pm by Gyro »
Regards, Chris

"Victor Meldrew, the Crimson Avenger!"
 
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