Is this new capacitor ok?

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  • aliasdck
    Senior Member
    • Sep 2009
    • 100
    #1

    Is this new capacitor ok?

    I am in the process of recapping a power supply for a large 3 phase UPS. I just had one question concerning one of the caps I received from digikey.

    I have attached a picture of the cap. On the left is the original cap... there is nothing wrong with it, but I have been asked to replace every cap on the power supply since a couple of the smaller secondary caps failed. The cap in the picture is on the primary side rated 220uF 450V.

    The cap on the right is the replacement I ordered from digikey. As far as I can tell, the cap I ordered was nearly identical in specifications... just newer and a few mm shorter. Ripple rating was only a tad bit better in the new cap.

    What concerns me is the esr reading of the new cap. The old capacitor reads .21 ohms on my blue esr meter while the new cap reads over double at around .53. Is this an indication of a problem with the new capacitor? Could it just have been sitting on the shelf unused for too long?

    Normally I wouldn't be too concerned since this cap is only filtering 120hz on the primary side of the power supply (as opposed to the 2khz test frequency of the esr meter) but considering the critical nature of this UPS I thought I had better get a second opinion...
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    • aliasdck
      Senior Member
      • Sep 2009
      • 100
      #2
      Re: Is this new capacitor ok?

      There is no maximum esr rating for the new cap in the KMQ datasheet however there is a maximum ESR rating for the old cap in the KMH Series datasheet.

      The maximum ESR for the KMH Series capacitor is 1.13 ohms @ 20 C @ 120hz. Since I am testing both caps with an esr meter that is using a test pulse of 2khz, I don't think I have much to worry about since both readings are well below this.

      I was just concerned since the new caps esr was double the old cap but now I suspect it may just be due to different design/makeup (The KMQ series is said to be the "downsized" version of the KMH series). The reading seems negligable anyways when comparing it to the maximum esr reading on the old datasheet.

      Any thoughts?

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