Steve
Society Members
vintageradio.co.nz
Posts: 734
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Post by Steve on Dec 25, 2021 7:26:55 GMT 12
James I think it's more about the designed failure mode of safety caps vs. normal caps, rather than the voltage rating.
Y caps (phase to ground or neutral to ground) are designed to positively fail open, so that when they do, they don't pass current and potentially liven the chassis. When they fail you might get an increase in interference, but that's not dangerous, just frustrating.
X caps (cross-the-line, phase to neutral) do the opposite, failing short to trigger a safety device like a fuse or circuit breaker.
800V is fine for normal operation, but a partial lightning strike or other spike event could very easily damage a normal cap allowing leakage current to flow.
My understanding is that you should use XY safety caps or just leave them out entirely. I'm not sure of the legislation around that.
Also disclaimer: this post should be used as a springboard to further research and not a complete or necessarily correct explanation of anything. May contain traces of nuts, assembled on a line that also processes soy, wheat and transistors, no free steak knives.
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Post by Peter Walsham on Dec 25, 2021 7:38:16 GMT 12
Good morning James
Ah, the dreaded modulation hum. I haven't come across that problem in in a NZ domestic made valve radio receiver (those fitted with power transformers anyway), but often have it crop up in American AC/DC sets (and my AM transmitter). I understand It was a particularly troublesome problem in sets that had solid state rectifiers - but I have not done any recent experiments/research on this. Having said that, I can't think of any NZ made radios that have had capacitors connected to the mains side of the power transformer. Anyhow, the 800 Volt capacitors you're using would be 100% perfect for the job.
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Post by Philconut on Dec 25, 2021 15:52:57 GMT 12
The peak to peak voltage of the 230V sinewave is 2 X SQRT2 (1.414) X 230V = 650V so an 800V rated cap will be fine.
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Post by Peter Walsham on Dec 25, 2021 16:35:24 GMT 12
John - James was talking about him using 800 Volt AC capacitors - not 800 Volt DC capacitors.
Steve - Safety capacitors are unlikely to be available in 0.0047uF values? Safety caps are more designed to be used as a series impedance in low current power supplies, rather than mains RF bypass or interference suppression capacitors. However, you're dead right about 'Y' caps. being designed to fail 'open' and 'X' rated caps to fail short circuit - although I have lost count of the many hundreds I have replaced that have gone low in value rather than failing outright.
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