What if a capacitor doesn't have it's capacitance?

Discussion in 'General Electronics Chat' started by WrinkledCheese, Jun 2, 2010.

  1. WrinkledCheese

    Thread Starter New Member

    May 30, 2010
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    What do you do if you need to replace a burnt out ceramic capacitor that doesn't have the capacitance marked? The only markings on the capacitor is 8D and 3KV. So I know it's 3,000 volts, but I don't know what the capacitance is and I can't find anything that tells me what 8D means...I'm guessing it's a part number.
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
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    Unfortunately, you don't have much information about your burnt ceramic cap. You might just have to choose a save value based on the size of your cap.
     
  3. WrinkledCheese

    Thread Starter New Member

    May 30, 2010
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    I went to the local industrial electronics shop and ... there are no assumptions that can be made of the size. They ranged from mF to pF for the exact same size capacitors. Although they were 1KV and not 3KV. It's a blue ceramic disc type capacitor similar to the one in this picture. http://imgs.tootoo.com/f9/e5/f9e5e083a0122870ba057da718274897.jpg

    There isn't a way to tell without the markings?

    If there is another capacitor with the 8D and the 3KV, is it possible it could be different...I'm guessing yes since it doesn't say what the capacitance is on it. I don't suppose I would be able to tell by removing the blue plastic coating?

    P.S. It's not physically burnt, but when I use a continuity tester on it, it doesn't have a solid connection all the way through. The other capacitors do. So I say it's burnt out, blown up....no good.

    I'm trying to fix a computer monitor there are three cables that go from the power supply to the actual monitor LCD matrix component. One of the capacitors seemed loose, as in it wiggled. The solders seemed solid. This is the same capacitor that's showing 0 continuity on my multimeter.
     
    Last edited: Jun 2, 2010
  4. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
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    I must admit that choosing via size would be difficult. You would have no luck trying to remove the blue plastic coating, the markings would be evident outside. You could try a capacitance meter, but I'm dubious about whether it will give you an adequate reading or not.
     
  5. davebee

    Well-Known Member

    Oct 22, 2008
    539
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    I'm a little confused - shouldn't a healthy capacitor show no continuity?

    Are we talking about DC continuity?

    On a large capacitor you may see a short spike on a continuity meter as the capacitor charges, but after that, you should see zero continuity, shouldn't you?
     
  6. WrinkledCheese

    Thread Starter New Member

    May 30, 2010
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    I will have to get back to you on this one, I think I'm doing everything all wrong...

    [EDIT]
    Isn't a capacitor just a non-conductive material able to store energy - in conjunction with conductive materials to charge it with - so while the charged medium is charged, there would be energy flowing from one end of the capacitor to the other creating a continuity circuit, otherwise...how would a capacitor work in a circuit? There would be no direct contact between the terminals, but energy inserted into point A would come out of point B provided there is a draw and charge. This is just my logical explanation of my understanding, PLEASE, correct me if I'm wrong...P.S. I'm wrong.

    I will revamp my findings process and check back with you after work.
     
    Last edited: Jun 2, 2010
  7. SgtWookie

    Expert

    Jul 17, 2007
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    A capacitor blocks DC, but passes the effects of AC.

    You really need a capacitance tester to check very small values of capacitance; either that or make an LC or RC circuit with a known inductor or resistor, and look at the waveform on an O-scope.
     
  8. WrinkledCheese

    Thread Starter New Member

    May 30, 2010
    11
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    The only capacitance meter I can find around here that goes above 20μF is $250 which is a Fluke multimeter. A little expensive for a casual hobby. I found some on e-bay but it will take about 2 weeks - transfer funds to pay pal and shipping - to get them so I will have to go that route.

    If I can get this monitor working, I will call it even with all the bad luck I've been having the past month.
     
    Last edited: Jun 2, 2010
  9. WrinkledCheese

    Thread Starter New Member

    May 30, 2010
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    I found a site that has a part # 8D for a 8pF ceramic disc capacitor with a 1KV rating. The 3KV version's part# is 8D3KV. I hope this is a good sign. I'm going to test the one that is still working, I hope, on the board and then test the one I have removed and hope that this is the issue. Otherwise I will have to test the whole board.

    If a capacitor is dead how will I tell if it's not properly marked? I suppose the only way is to hope there is another capacitor with the same markings and functioning and to test it.

    I ordered a capacitance meter from eBay for $21.32 CAD. Will arrive in the next few weeks, have to wait for PayPal funds transfer. I will have an 8pF 3KV ceramic capacitor ordered since they don't have any in stock so I'm not waiting for the other thing while one arrived. Once I do all this I will update this thread...

    You guys are probably tired of my questions by now :p I've downloaded all volumes on the website and I will be printing them for reading in my leisure time without being stuck to a computer screen.

    ...I have too many hobbies. If there is such a thing.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    If you want even more in-depth info, search for the Navy NEETS series.
    It's available on Davidson U's website for downloading in .pdf form.

    No sense in printing it out, as it would cost a lot of money and burn up lots of paper.
     
  11. cyberfish

    New Member

    Jun 3, 2010
    10
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    For measuring capacitance, if you don't need too much accuracy, you can just use a 555 in oscillator mode. From the resulting period/frequency (which can be easily measured by a microcontroller, or oscilloscope), you can calculate the capacitance.
     
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