Capacitor problem?Stray voltage?

Thread Starter

ISB123

Joined May 21, 2014
1,238
While I was repairing a PC PSU(230V) I accidentally touched the metal top of mains smoothing capacitor.Well I received a small shock similar to placing a 9V battery on your tongue. I measured 130V at top of the cap, Is this normal?
 

Hypatia's Protege

Joined Mar 1, 2015
3,203
While I was repairing a PC PSU(230V) I accidentally touched the metal top of mains smoothing capacitor.Well I received a small shock similar to placing a 9V battery on your tongue. I measured 130V at top of the cap, Is this normal?
What was the measurement relative to? --- FWIW it sounds as if you've a 'ground fault' somewhere -- Either in the cap itself or the assembly to which the case of the cap is mounted...

Best regards
HP
 

Thread Starter

ISB123

Joined May 21, 2014
1,238
What was the measurement relative to? --- FWIW it sounds as if you've a 'ground fault' somewhere -- Either in the cap itself or the assembly to which the case of the cap is mounted...

Best regards
HP
Black probe was placed on neutral line off the 3pin plug and red probe was on top of the cap.

There are two caps only one is showing 130V across it.So I guess that one cap has malfunctioned and is causing problems for the PSU.
 
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Hypatia's Protege

Joined Mar 1, 2015
3,203
Black probe was placed on neutral line off the 3pin plug and red probe was on top of the cap.
IMNSHO enough current to 'feel' represents excessive leakage! -- How is the Cap mounted? Is its metallic case electrically connected to the PSU assembly at any point? -- If not, the problem is isolated to the capacitor...

Best regards
HP:)
 

Thread Starter

ISB123

Joined May 21, 2014
1,238
It's standard 200V aluminum electrolytic cap. It's not connected to anything but the PCB trace.Only low voltage side of PSU is grounded.
 

Hypatia's Protege

Joined Mar 1, 2015
3,203
It's standard 200V aluminum electrolytic cap. It's not connected to anything but the PCB trace.Only low voltage side of PSU is grounded.
If it's a standard electrolytic it is polarized and, hence, not a 'mains filter' but, rather a post-rectifier filter, decoupling filter, etc...

Owing to the nature of their construction, certain electrolytic caps effectively bond the 'can' to their negative leads (via 1MΩ to 15MΩ measured 'leakage' resistance)... Depending upon the Cap's electrical location with respect to the output lines, the PSU may be compliant (inasmuch as the 'hot spot{s}' are inaccessible while the unit is assembled) -- That said, I strongly doubt the described condition is 'normal'! --- At very least please verify that potentials on all output leads (including 'ground returns') are at negligible AC potentials with respect to chassis and mains ground!

Best regards and good luck!
HP:)
 

Hypatia's Protege

Joined Mar 1, 2015
3,203
Addendum:

Note that the described condition may be normal provided the Cap in question precedes the SMPS circuitry proper -- An image may be helpful:)

Best regards
GP
 
That is not a line-isolated node --- So... I feel the condition may appropriately be regarded as 'normal':cool: --- Even so, I advise performance of the 'safety checks' outlined in post #6 -- To wit:
please verify that potentials on all output leads (including 'ground returns') are at negligible AC potentials with respect to chassis and mains ground!
Best regards
HP:)
 

Thread Starter

ISB123

Joined May 21, 2014
1,238
That is not a line-isolated node --- So... I feel the condition may appropriately be regarded as 'normal':cool: --- Even so, I advise performance of the 'safety checks' outlined in post #6 -- To wit:


Best regards
HP:)
I tested ground and its is working as intended.I guess that this fenomene is normal,now just to figure out whats wrong with the psu. It only shuts down when 12V line is under load.
 
I tested ground and its is working as intended.I guess that this fenomene is normal,now just to figure out whats wrong with the psu. It only shuts down when 12V line is under load.
Many PSUs of that 'caste' 'stall' on overload -- Are you certain the difficulty is with the PSU (as opposed to the load)? -- If 'yes' -- At approximately what current level does the unit shut down?

Best regards
HP:)
 

Thread Starter

ISB123

Joined May 21, 2014
1,238
Many PSUs of that 'caste' 'stall' on overload -- Are you certain the difficulty is with the PSU (as opposed to the load)? -- If 'yes' -- At approximately what current level does the unit shut down?

Best regards
HP:)
Yes the problem is in the PSU.I'm running a DC motor that draws around 5A max.
I can use the motor on 5V line without problems but as soon I plug it into the 12V line psu shuts down.Generally if I try to draw more than 10W from 12V it just shuts down. Sometimes I have to flick the PSU switch few time before PSU turns on and this only occurs if 12V is loaded or it was loaded.I guess the problem is in some bad cap but I just cant find any which look bad, already replaced cap on 12V line and it hasn't solved the issue.

PS

60W load on 12V line drops the voltage to 10.9V which is only 0.4V short of under voltage protection.I guess I'll have to check some more caps at 12V.
 
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Yes the problem is in the PSU.I'm running a DC motor that draws around 5A max.
I can use the motor on 5V line without problems but as soon I plug it into the 12V line psu shuts down.Generally if I try to draw more than 10W from 12V it just shuts down. Sometimes I have to flick the PSU switch few time before PSU turns on and this only occurs if 12V is loaded or it was loaded.I guess the problem is in some bad cap but I just cant find any which look bad, already replaced cap on 12V line and it hasn't solved the issue.
Inasmuch as electric motors exhibit significant starting currents I suggest a purely resistive test load...

as soon I plug it into the 12V line psu shuts down
Please be advised that, for a given load, the current drain will be 2.4X at 12V...

I guess the problem is in some bad cap
Quite likely... --- For starters please check C20:)

Best regards
HP:cool:
 

Thread Starter

ISB123

Joined May 21, 2014
1,238
I think that you are right that C20 is causing the problem. After replacing C20 with another cap which turns out to be a bad one too(took it from a spare PSU)voltage dropped to 10V.
 
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