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ceramic disc capacitors stressed?

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    #1

    ceramic disc capacitors stressed?

    I'm wondering if people have worked with ceramic disc capacitors at rated voltage. Normally for a electrolytic you'd leave some breathing space, even a 16V cap for 12V could be sufficient. That's about a 33% buffer. And you'd want even more for cars due to additional spikes (even 25V).

    Now I was wondering about a ceramic disc capacitor. This capacitor I saw is a 270pF 1KV capacitor connected across a 900V source... That's only a 11% buffer... anyone run a cap this close and what kind of lifetimes/failures do you see? For 900V what kind of voltage rating would one suggest to ensure longevity of the cap?
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    #2
    Re: ceramic disc capacitors stressed?

    given the fact that a ceramic can be +-60% i would not put less than 3KV on that circuit.

    i'v seen lots of 1KV caps heat up and fail in 380v circuits!

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      #3
      Re: ceramic disc capacitors stressed?

      Hmm... is that +/- 60% on voltage or capacitance, I thought it was the latter.

      Anyway my debug effort this turned out to be a red herring. The capacitor is fine (how, I am still shocked) and I was simply loading down the 900V wire due to the input impedance of my multimeter. 20Mohm and 100Mohm make a big difference when paralleled with a 9Mohm resistor voltage divider...

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        #4
        Re: ceramic disc capacitors stressed?

        capacitance, but it's still going to need a big headroom

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          #5
          Re: ceramic disc capacitors stressed?

          Some time ago, I got an easy to use capacitance meter which measures quickly, and when I decided to check my (used Y-class and HV) ceramic capacitor collections, a number of my units were found to have a significant loss of capacitance out of rated tolerance.
          My first choice in quality Japanese electrolytics is Nippon Chemi-Con, which has been in business since 1931... the quality of electronics is dependent on the quality of the electrolytics.

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            #6
            Re: ceramic disc capacitors stressed?

            I suspect that neither Y or HV caps are really used for timing, and only used for filtering. One would hope that capacitance is slightly overdesigned to help against loss over time. But I'd expect that working voltage should have more headroom than the design that I saw...

            This 1KV capacitor across a 900V voltage (focus grid) is in a Tektronix scope. I'm still quite surprised they decided to cut it that close.

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              #7
              Re: ceramic disc capacitors stressed?

              Different ceramic dielectrics have different temperature and voltage coefficients. Z5U and Y5V can "lose" 80% of their nominal capacitance at hot or cold temps and with an applied DC voltage close to the rated voltage. X7R and X8R "only" lose around 25% of their rated capacitance in similar conditions. If accuracy is critical, use CGO (NPO) or X7R with an applied DC voltage <20% of rated.
              PeteS in CA

              Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
              ****************************
              To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
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                #8
                Re: ceramic disc capacitors stressed?

                Welcome back PeteS, thought you abandoned us

                Fortunately this is a filter cap on a focus grid so the value isn't critical. I was just worried about the lifetime of the capacitor since it's operating so close to the voltage rating, and leakage being the worry.

                I suppose leakage would self destruct the capacitor, so I guess we'll know when it happens.

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