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Capacitance question

Discussion in 'General Electronics Discussion' started by welchs101, Nov 14, 2009.

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  1. welchs101

    welchs101

    7
    0
    Feb 2, 2009
    I am working on PCB board and i solved the problem. But i have a few questions. I have 4 100uF 25V caps in parallel. They are not bulging and they measure "ok" using my hand-held fluke meter. I also measured the "resistance" of each cap and they again seem "ok" measuring in the MegOhm range.

    I replaced all 4 caps with TWO 220uF 63V caps. I did this just to see if my problem went away. AND IT DID. So here are my questions......i hope someone can help answer them.

    1) All 4 caps seem to measure ok with my fluke meter. But obviously one or all were in fact bad. I guess my fluke meter is not a good measurment tool for caps........but why?

    2) My next question is this. The original circuit had 4 100uF caps in parallel. I replaced all 4 with TWO 220uF caps just to see if this fixed my problem. My question is should i order additional 100uF caps and put them in or leave it as is with the two 220uF caps. Is their an advantage either way? Not sure.....can someone help. My guess is i will have to order additional 100uF caps which is ok.....but just want to understand why.

    thanks for your help.
     
  2. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    You say nothing about what kind of circuit this is, but I guess your problem was ESR-related. With all else being equal; going up on the voltage rating will decrease the ESR. Different makes & models of caps have vastly differing ESR values. Computer MB's for instance needs the best ultra-low ESR caps you can get your hands on.
    Like a battery; a capacitor has a specific internal series resistance, shorting it will only bring about a certain peak current. This resistance needs specific measuring instruments to be revealed. A DMM isn't made to measure/present this value.
    With HF and switch-mode gear it's often recommended to put a plastic or ceramic capacitor in parallell with the electrolytics to "fill in the weak spots in their properties".
    So I suggest you leave the two 220uF caps as they are and put in a couple of 0.1-0.47uF plastic or ceramic caps, but if it's a switch-mode PSU or a MB I suggest you look around for ultra-low ESR's.
    The bigger size can you can fit in the better it is.
     
  3. welchs101

    welchs101

    7
    0
    Feb 2, 2009
    Just checking to see if anyone has responded and sadly no one has. Are my questions confusing, do i need to re-state them? If so just let me know.

    I am really perplexed. thanks again.
     
  4. welchs101

    welchs101

    7
    0
    Feb 2, 2009
    The caps are part of an inverter board circuit from an LCD computer monitor.

    I read your answer and to be honest i dont understand it. I measured the caps capacitance and resitance using a fluke DMM (as you stated) and they appeared to be good. I remove the seemingly good caps with two known good caps (never used before) and boom the inverter is working as it should.

    also, not sure if i should buy 4 new 100uf caps to replace the two 220uF caps i put in initiallly on a temporary basis.
     
  5. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Backlight inverter or what?
    In switchmode circuits the capacitance value is of much less importance than the Equivalent Series Resistance and the ripple current rating. The ESR will usually rise before the capacitance starts to drop as the capacitor ages.
    A capacitor also has an Equivalent Parallell Resistance but this is rarely specified.
    The resistance you measured however is a low-voltage dc leakage resistance that has little to do with the stabilizing action of the capacitor, although if it passes dc current it's damaged and will blow up if used.

    The answer to your last sentence depends on the data of the two caps you put in. Read my previous reply.

    You seem like a very impatient guy. You should do some research on Google with the hints I have given you, or if you want others to do it for you - present details about the parts.
     
  6. neon

    neon

    1,325
    0
    Oct 21, 2006
    You cannot check a capacitor with a VOM it will only tell if shorted but not how bad. and if good how good. at 1.5v it may look great at 20 volts leaks like a sieve. You may check these caps only with a bridge for a null.
     
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