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Capacitors keep blowing

in 912 / 914 Technical Questions 9 years ago

  • Re: Capacitors keep blowing

    by » 9 years ago

    Mark,

    I would check the voltage rating and the temp rating. However if it was simply because you were using a lower temp rated capacitor it is extremely doubtful that it would fail in just a few minutes. When You operate a cap beyond its designed operational temp it will reduce the life, but shouldn't be reduced down to a few minutes.

    I think it is more likely that the voltage has exceeded the caps rating. Voltage doesn't flow through the cap, it goes in and comes back out the same wire like inflating and deflating a balloon. If electricity went through a cap then it would be called a resistor. If a cap is operated above its maximum voltage rating the insulation inside can no longer keep the electricity from flowing through it. Think of a porous balloon. This is what can cause a sudden failure.

    Bad connections or grounds before the cap will not not cause the cap to fail, unless an anomaly causes the system voltage to malfunction and spike abnormally high (beyond the caps rating). A bad connection will reduce the flow of electricity. This does not magically make more voltage and overload the cap.

    It might be useful to see if they were both using the same caps. Higher quality caps (meaning more expensive) seem to be a little more robust. As you read above, it should be rated above 105c for long life in a hotter environment.

    Kevin K.
    Sonex 3300
      
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  • Re: Capacitors keep blowing

    by » 9 years ago

    Capacitors can be damaged by:
    1. Over voltage
    2. Over temperature
    3. Reverse polarity
    4. High ripple current


    No.s 1 & 3 are the most likely reasons for rapid failures. You might need an oscilloscope to look at the waveform coming off the regulator.

    I suspect that if your voltage is too high then the regulator is probably at fault.
      
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  • Re: Capacitors keep blowing

    by » 9 years ago

    It was stated earlier in this topic that some users are installing larger capacitors like 33,000uF rather than 22,000uF. This has an advantage of reducing the ripple voltage on the regulated output, but nothing comes for free. This improvement comes at the expense of a higher inrush current which would be proportional to the percentage increase in capacitance. i.e. 50% higher for this example.

    The problem relates to the size of the fuse in the main circuit. If it is scaled by the aircraft designer for 22,000uF, then be aware that you are now working in the "headroom" or over-load allowance region of the fuse. If you do not know what current overload was allowed in the electrical design, you may be getting close to tripping the main fuse due to the higher inrush currents.
      
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  • Re: Capacitors keep blowing

    by » 9 years ago

    I blew a capacitor.
    Replaced it with a 22,000uF but upped the voltage to 40V.(from the previous 25V)
    The vent should be placed at the top.
    I'm told they will breath due to differing atmospheric pressures (altitude).
    So far, so good after about 60hrs.
    I will note that it failed the flight after a fast, steep descent.
    It blew the electrolyte out the vent and over the firewall.
      
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  • Re: Capacitors keep blowing

    by » 9 years ago

    Rhett, there are a number of reasons for the electrolytic caps to vent or "blow".

    During operation of an aluminum electrolytic capacitor with non-solid electrolyte, gas pressure normally increases. The gas is mostly hydrogen. Excess pressure is avoided by permeation of the gas through the capacitor’s seal.

    Reasons for venting include, overvoltage, reverse voltage, AC voltage or capacitor failure. The excess pressure can cause the capacitor to explode. To reduce the risk of explosion aluminum electrolytic capacitors are usually equipped with pressure-relief vents as described. These safety vents are intended to rupture and release the gas pressure at which time the capacitor will have visual damage and should be replaced.

    It sounds like your example may have been due to rapid pressure changes to the capacitor or over voltage and hence I assume the reason for the higher voltage rating on the replacement.

    Any electroylte fluid should be well washed off hands and surfaces.
    Thank you said by: MARK JACKSON
      
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