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Old 7th May 2009, 01:59 PM   #1
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Default Capacitors and maximum voltage ratings

Hi There,


I've bought a few Audyn MKP capacitors for my tube amplifier project with a maximum rating of 400Vdc

Yesterday i started wondering if i made a mistake buying these capacitors. The powersupply is 280-0-280 at 300mA whith a full wave rectifier. Calculating the maximum voltage results at -=395 for the first capacitor, which is just below the maximum rating.

However when i simulate the power supply in duncans amps PSU designer the voltage initially surges to around 450V for a very very short time.

Would this damage the capacitors? and should i look after a type which has higher ratings?


Thanks!

Hilbert
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Old 7th May 2009, 02:47 PM   #2
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Hilbert,

Caps. frequently can withstand a turn on surge that's in excess of the WVDC. Still, you are very much on the ragged edge.

If you are employing SS rectification, adding a well sized inrush current limiting thermistor could be the difference between a reliable unit and parts failure. A data sheet for GE/Thermometrics parts is here.
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Old 7th May 2009, 02:50 PM   #3
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If 280X1.41=395V, how is it possible to get voltage higher than 395V? Is it simply the unloaded voltage of the transformer secondary x 1.41 at startup?
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Old 7th May 2009, 02:53 PM   #4
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Quote:
Originally posted by boywonder
If 280X1.41=395V, how is it possible to get voltage higher than 395V?

Dude,

You are overlooking loading/regulation effects in the power trafo. The 280-0-280 rating is for a loaded down condition. The voltage will be greater, when "juice" is 1st applied.
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Old 7th May 2009, 03:06 PM   #5
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Yes, the initial surge is around 450V which is 1/8th above the maximum specification, its only less than 1/10 of a second after which it drops to around 395V.

I'm not sure wether this will damage the capacitor ( i think its able to handle it but you can never be too sure ).
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Old 8th May 2009, 02:36 AM   #6
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I'm certainly not as learned as most of you, but her's my 2 cents...if you take into consideration the tolerances of caps, you could be over the top even at the 395 watt working volts, not? I think I'd go for 500V caps...no sense tempting failure???
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Old 8th May 2009, 02:47 AM   #7
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Quote:
how is it possible to get voltage higher than 395V?
It is easy to get much more if there is a choke somewhere. Is there a choke in this power supply? Otherwise, I wouldn't expect even a momentary surge past steady state values unless tubes are cold and power supply is unloaded initially.
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Old 8th May 2009, 04:14 AM   #8
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Quote:
Originally posted by bereanbill
I'm certainly not as learned as most of you, but her's my 2 cents...if you take into consideration the tolerances of caps, you could be over the top even at the 395 watt working volts, not? I think I'd go for 500V caps...no sense tempting failure???
Cap tolerance has more to do with capacitance. The voltage rating has to do with the dielectric strength of the dielectric in the cap (polypropylene, teflon, paper, etc.) Materials with higher dielectric strength and thicker films can withstand more volts before breaking down/arcing over.

There is some margin in dielectric ratings for caps (the Panasonic TS-HA's, HB's etc have a momentary voltage rating that's 50V higher than the nominal rating IIRC), but start-up over voltage is sustained for seconds, considerably more than the rated time for overvoltage, I would imagine.

hilbert: Consider Panasonic TS-HB's for the PS electrolytics, they are rated at 450V working voltage, and have excellent ripple ratings.
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Old 8th May 2009, 05:27 AM   #9
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Default Ratings

Your Caps unfortunately will not do for this application....can you return them??
You should be using caps that will see no more than 2/3s' of its WVDC rating..a 500 WVDC will still be too close. A 500 will work...but not for very long.......maybe a couple hundred hours running.
A 630 WVDC should be used.....this stretches lifetimes to tens of thousands of hours, similar to tube lifetimes.
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Old 8th May 2009, 06:28 AM   #10
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No I can't return them anymore, i have these for too long now. I might use them for another project, maybe a preamplifier when time comes. Too bad, i'll look around for electrolythic caps, 630V MKP capacitors are too expensive for what i want to build.

I have the book "building valve amplifiers" from Morgan Jones, he wrote something about combining an elco with a small MKP cap to get a cap which acts almost the same as a MKP cap from the same size. I'll dive into the book when i have the time again.
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