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Approximate AC to DC capacitor voltage rating equivalence? Other ideas?

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  • Approximate AC to DC capacitor voltage rating equivalence? Other ideas?

    Hi everyone, long time no see.

    I have a bunch of GE-built AC capacitors, rated at 475 V AC. I suppose their dielectric will withstand the full voltage swing, +475 to -475? Are these things "implicitly" rated for 950 VDC?

    They look sturdy as heck and are very large for just 60 uF capacitance, I'm not an expert on large AC mains installations but I assume these critters have thermal/pressure compensation too. Also I suppose the AC applications where they usually go require a couple KVAR, which would make a tube amplifier a very light load.

    Is it "relatively safe" to power it up with continuous 900 VDC? I take full responsibility if anything blows up, I really am asking for your unconventional wisdom here

    Thanks in advance.
    Valvulados

  • #2
    Hi jmaf, long time no see!

    Simple answer, the DC voltage rating is equal to the peak AC, not the peak-peak. If a capacitor has an AC voltage rating, it'll be RMS for a sine wave. So the DC rating of your caps would be 1.4 x 475 = 665V DC.

    More complicated answer: The effective DC voltage rating may be somewhat higher than 665V because AC is better at breaking down insulation than DC, so a capacitor rated for AC use is overbuilt a little. Maybe it could even withstand a DC voltage equal to the peak-peak AC, which is called the "corona offset voltage" by capacitor makers.

    But I wouldn't count on it, I'd use the 665V rating.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      Originally posted by Steve Conner View Post
      Hi jmaf, long time no see!

      Simple answer, the DC voltage rating is equal to the peak AC, not the peak-peak. The AC voltage rating is RMS, so 1.4 x 475 = 665V DC.

      More complicated answer: The effective DC voltage rating may be somewhat higher than 665V because AC is better at breaking down insulation than DC, so a capacitor rated for AC use is overbuilt a little. But I wouldn't count on it, I'd use the 665V rating.
      Thanks for that, Steve. Then 665 VDC is more than enough for my 40 Watt amps(500 VDC unloaded), so I guess I can play around with those caps.

      Long time indeed! Work has kept me away from the hot bottles, but it's always great to pass by and see everyone. Glad to "see" you
      Valvulados

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