Voltage on the top of Capacitor

Discussion in 'General Electronics Chat' started by mikec, Nov 4, 2008.

  1. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    Hi Guys

    New to the forum, and am looking for some help.

    I am trying to fix an SMPS in my Plasma TV which has just gone out of warranty. On tracing the voltage from the Bridge rectifier I noticed that there is 120V on the tops of the Smoothing Electrolytic Capacitors. Is this right? I always thought that the cans on these babies were insulated.

    Thanks for any help

    Mike C
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    It depends on the applicaton. They might be using some kind of multiplier circuit in there. See if there's an "intentional" looking insulator or wafer between the capacitor can and the chassis.

    eric
     
  3. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    Cheers Eric

    I cant see anything under the caps, of which there are two off. The voltage is AC by the way

    Here is a pic of the the board for those that like a bit of circuit porn :D

    http://farm4.static.flickr.com/3024/2990282924_c106c4f57f_o.jpg

    The caps in question are the two brown ones labeled up as 450V.

    The output of the bridge rectifier is showing 350V DC by the way with an input of 250V AC, so looks like some sort of doubling action in here.

    Mike C
     
  4. AG3Y

    New Member

    Nov 4, 2008
    8
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    What are you using as your ground reference Mike? I would think it should be that wide trace going around the outside edge of the circuit board. If you are picking up AC on those caps, I have doubts that your meter is hooked to the proper ground.

    Good luck, Jim
     
  5. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    hmm

    I have to admit I am using the chassis as ground, but am going to try it on the board ground.

    Mike C
     
  6. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    ok
    just checked it against the board ground, and the same about 125V AC on the tops of the Caps
     
  7. AG3Y

    New Member

    Nov 4, 2008
    8
    0
    I'm still betting that you haven't found the right ground, yet. Over on the right side are a bunch of headers. Can you see which pins might be the common ground there ? I would suspect several would be tied to the same point, and those would most likely be the ground for the board.

    Jim
     
  8. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    Jim

    On the top header pins 3 and 6 are labeled as ground in the manual. I ve just tried using this and same result. I've also use the holes labeled Z1401 which has a ground tag same result.

    Can you explain why a wrong ground would show such a high voltage?

    Mike C
     
  9. AG3Y

    New Member

    Nov 4, 2008
    8
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    I suspect you are measuring something that is a result of a ground loop sort of situation. Wiki has an article on the construction of electrolytic capacitors that may be of interest to you, if you have not read it before.

    To answer one of your questions directly, the aluminum can of an electrolytic capacitor is generally NOT insulated from the negative lead, even in the radial style ( both leads sticking out the bottom ) so whatever voltage the negative lead is connected to will show up on the case itself!

    It would be very difficult to diagnose what kind of circuit you might have , but it is almost certainly a switching type of power supply, where neither the negative, nor the positive leads of the large capacitors are at a real ground potential !


    I notice all over those heat sinks the triangle with the lightning bolt in the center. Obviously this means that some fairly high voltage with respect to ground is present on those heat sinks. That is a lot of metal exposed with those high voltages, so it behooves you to be careful.

    Switching power supplies are the very devil themselves to try to troubleshoot, so I would do everything I could to get a schematic of the thing in order to have some chance of figuring out what is going on inside the unit.

    It is also possible that there is some form of feedback or voltage sensing going to the outside circuitry, which would prevent the supply from working properly unless it was hooked up to the load.

    Many factors at work against you here. You really need to have someone close by who has an understanding of these units, in order to give you more of a chance of getting it to work again.

    Sorry I can't be of more help. If you can find a circuit diagram, and either show it to us here on the forum, or give us a URL to where you found it, it will be quite difficult to offer any more advice.

    Good luck, and please inform us of any progress.

    Jim
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Significant AC on the smoothing capacitors of any power supply implies a rectifier fault. I would not expect 120 volts of ripple, unless something was drawing hellishly excessive current.

    Fixing SMPS is normally uneconomic/ impractical because of the difficulty of obtaining crucial components, so most repairers just swap out the module.
     
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Switch off the main's supply from the board and check with your continuity function on your multimeter if the top of the capacitors is connected to ground or to the positive line? Does it output only a positive voltage or a negative voltage? Are these capacitors connected just after the rectifier?

    As for the 350VDC you read at the output of the rectifier is correct because the peak value of a 250Vrms AC supply is about 353V. You read this peak value because the capacitor after the rectifier are actually a peak detector circuit and with no load or a very light load the ripple voltage is small and you read almost the peak value of the input 250VAC voltage.
     
  12. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    Thanks Guys

    Getting a circuit diagram has been near impossible, even though I have the service manual, its the one component that they seem to have left off. I assume because its probably third party to Hitachi.

    Here is a circuit diagram I have constructed from tracing the PCB by eye, so may have errors, and there are of course some components that I have had to Guess.

    http://farm4.static.flickr.com/3196/3004584483_ba60c6f987_o.jpg

    I will do a continuity check later tonight on tops of the caps.

    Mike C
     
  13. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    If your circuit is approximately correct then its not strange to measure a voltage at the top of these capacitors because their negative lead is not connected directly to the negative side of the output of the rectifier.
     
  14. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    Just did a continuity check, and there seems to be a capacitance between the case and the negative leg, ie you see the meter ramp up to full impedance as the cap charges.

    other than that no continuity.

    I'm going to make an assumption this is normal, and that its picking up the ac from somewhere further down the circuit

    Mike C
     
  15. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Maybe the top of the capacitor is connected to the positive lead of the capacitor, check it. However, its normal to have some voltage at the top of the capacitor, if the top is connected to a capacitor lead, because none of the capacitor leads is connected to zero volts.
     
  16. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    AG3Y

    Looks like you were correct. Having put the ground lead of my DMM on the negative rail of the of the circuit ie the negative side of the bridge rectifier I now see zero volts on the top of the cap. I am also now seeing the correct voltage coming out of the Power Factor Corrector of the SMPS.

    Here's the circuit diagram for reference.

    http://farm4.static.flickr.com/3016/3010671611_147e8570eb_o.png

    I'm transfering this discussion to a new thread that I have started called problem with SMPS.

    Thanks for your help guys.

    Mike C
     
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