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Capacitor Substitution

Discussion in 'Power Electronics' started by H2814D, Aug 24, 2019.

  1. H2814D


    Nov 4, 2017
    Hi all. I have tried to find a definitive answer to my question on my own, however, I am not confident in what I have found. This was my last, but obviously, most trusted attempt.

    I realize substituting capacitors with higher voltage and temperature ratings is acceptable, if you cannot find the exact spec replacements, but I am trying to replace a cap that I cannot find the same uF rating for. I've tried the common online supply stores (Digikey, Mouser, Arrow) and ebay and amazon. No luck.

    It is a 950uF/200V snap in cap. 2" long, 1" diameter, 10mm lead spacing. There are tons of 1000uF's with the correct dimensions, and those would be very close, considering there is often a 10-20% tolerance with these things. So my question is, would I be able to get away with a 1000uF (or other suggested option) cap in place of this, or does that alter the eventual DC output voltage of the unit? There are two of them on the board and the application is for an RV power supply from 110vac to 12vdc, so it both powers the 12 volt electronics in the RV and acts as a house battery charger when plugged into AC. Note that the circuit board does list the cap values under the removed caps. 950uF/200V.

    Pics of the board and the cap are attached. If it matters, the mains are on the right side of that board and the DC output on the left (white and blue wires).

    Thanks in advance for what I'm sure will be another interesting education. :)

    Attached Files:

  2. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir H2814D . . . .

    It says . . . .
    Note that the circuit board does list the cap values under the removed caps. 950uF/200V.
    Then DO ALSO NOTE +- 20% so, you are just fine, just be sure of the +105 degree temp range due to its potential rough temperature environs that it may be subjected to.
    PLUS I would be changing the pair simultaneously. . . . but only IF ?

    In " reading " the board, I am seeing AC power coming in the right bottom corner and then getting an X capacitor, then a contrawound line inductor pair and yet another X cap and a line fuse and then over to the right to a heat sink mounted FULL WAVE RECTIFIER block. Its rectified DC comes back to those two black capacitors for filtering. They DO operate at a VERY non taxing S...L...O...W up and down sinusoidal 50/60 cycles.
    So, typically, they are QUITE trouble free, as being compared to the other caps that handle the decades or hundreds of kilohertz of switching cycles of HARD pulses / square waves operating frequencies .
    That with gradual time, just heat up and eventually dehydrate those caps electrolyte . . . in an exponentiall manner.

    Now, relevant to your passing judgement, do you visually see any physical fault or electrolytic leakage being associated with those two big boys ?

    HOWEVER ! . . . . NOW . . .do look back at the fans direction , and don't you see an intermediate size E-cap that . . . looks like to me . . .that it has a domed cap pooching upwards at its triangular imbedded stress striations.
    If so, that might be your units REAL problem . . .? ? ?

    Aside observation / comment . . . .

    Located betwixt the cooling fan and the central MAIN power transformer . . . . . lies one HELL(x100) of a ferrite based inductor !
    Also, as you have already found, its dang nigh impossible to find that S.H.I. ( t . t . y . ) brand of capacitors.

    73's de Edd . . . . .

    Every morning is the dawn of a new error.

    Last edited: Aug 24, 2019
  3. dave9


    Mar 5, 2017
    Yes 1000uF, 200V or higher rated would be fine. Why are you replacing them? These are at AC line frequency and generally have an easy life, rarely fail unless there was a huge surge that came in, and a surge that big would probably take out the switching transistor(s) if not the bridge rectifier too, before the caps.

    I agree with 73's suggestion that the other E-cap may be a problem. Depending on what value it is, I might replace it with one of these or similar:
  4. H2814D


    Nov 4, 2017
    Sir 73's de Edd. Thank you for your reply. I am confident, with your reply, that I can substitute the 1000uF for the 950uF. Replacing both of them at the same time was my intention. One of them had a slight bulge and was leaking.

    Your observation about that other cap is absolutely correct and it was the first one I noticed and the first one I pulled off the board. A 2700uF 25V. In addition to the obvious bulge, the covering appears to have melted downward toward the base/leads. I've never seen that happen before, but the heat in Las Vegas can do things you wouldn't believe to electronics. I attached a pic.

    Once again, thank you for your response.

    Attached Files:

  5. H2814D


    Nov 4, 2017
    Thank you, too, dave9. I'll be ordering shortly. :)
  6. 73's de Edd

    73's de Edd

    Aug 21, 2015
    LAS WEGAS . . . . you're in LAS WEGAS . . . .thats what my nephew there, thought when he encountered this . . . .
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