Author Topic: 1000V 0.5uF electrolytic cap replacement ? What voltage rating  (Read 766 times)

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Offline lordvader88

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1000V 0.5uF electrolytic cap replacement ? What voltage rating
« on: November 13, 2017, 04:15:51 pm »
Its a great big wax coated tin can style from the 1960's, for the HV in an old RCA vacuum tube oscilloscope. I'm sure I heard that the old elec.cap's could survive higher surge voltages, but modern cap's don't survive nearly as well. And so I need higher than 1kV rating.

So what would be a choice ? Not a common size, but I can use some in series, maybe 2x 630V 0.27uF. Looking on Mouser
« Last Edit: November 13, 2017, 04:36:51 pm by lordvader88 »
 

Offline Kleinstein

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Re: 1000V 0.5uF electrolytic cap replacement ? What voltage rating
« Reply #1 on: November 13, 2017, 05:31:13 pm »
For such a high voltage, I would consider film caps instead of electrolytic. I remember that I saw 1600 V types at times when TVs used CRTs. You can still get something like WIMA FKP1 series caps for 1250 V (DC).

The old way coated style are likely also not electrolytic, more like paper insulated.
 

Offline Cyberdragon

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Re: 1000V 0.5uF electrolytic cap replacement ? What voltage rating
« Reply #2 on: November 13, 2017, 05:33:20 pm »
Its a great big wax coated tin can style from the 1960's, for the HV in an old RCA vacuum tube oscilloscope. I'm sure I heard that the old elec.cap's could survive higher surge voltages, but modern cap's don't survive nearly as well. And so I need higher than 1kV rating.

So what would be a choice ? Not a common size, but I can use some in series, maybe 2x 630V 0.27uF. Looking on Mouser

No, it's not electrolytic. It's either paper or mylar (most likely paper). You could add a higher voltage if you really want, but all that will do is give you a higher ESR and be less effective.

The correct choice would be the right part.

https://www.mouser.com/Search/m_ProductDetail.aspx?Cornell-Dubilier%2f474MSR102K%2f&qs=3tP%252bN51vMXdq%2fDvyfEYBKQ%3d%3d&gclid=EAIaIQobChMIkIjZoYm81wIVhLrACh1PagF5EAQYASABEgLHevD_BwE
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Offline flowib

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Re: 1000V 0.5uF electrolytic cap replacement ? What voltage rating
« Reply #3 on: November 13, 2017, 09:45:28 pm »
470nF 1000V is still a relatively common value, Wima makes them. there are also axial types from Kemet.

if the original circuit calls for .5ยต 1KV a modern replacement will do just fine.
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Online Cerebus

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Re: 1000V 0.5uF electrolytic cap replacement ? What voltage rating
« Reply #4 on: November 14, 2017, 12:42:21 am »
So what would be a choice ? Not a common size, but I can use some in series, maybe 2x 630V 0.27uF. Looking on Mouser

If you do end up using two (or more) in series please remember that capacitors in series don't add together like that. 270nF in series with 270nF is 135 nF, not the 500 nF you're looking for. Capacitors in parallel add, capacitors in series produce the reciprocal of the sum of the reciprocals of the individual caps.

[Insert standard warning about working on high voltage or mains circuits that gets given to anybody who has just made a fundamental error which suggests that perhaps they don't have the experience to work on potentially fatal voltages or energies.]
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Online TimFox

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Re: 1000V 0.5uF electrolytic cap replacement ? What voltage rating
« Reply #5 on: November 15, 2017, 08:37:23 pm »
Also, if you wire two high-voltage capacitors in series, their leakage currents, though small, will be different and the equilibrium voltage across the two devices will differ.  Therefore, one voltage will be higher than the other and may exceed the rated voltage.  If this is necessary, you must add a high-value resistor across each of the two capacitors to ensure that the DC voltage divides equally.  The current through each resistor must be substantially larger than the leakage of the capacitor (and stray leakage across its body, etc.).  This was common for electrolytic capacitors, which have relatively high leakage current.  For good film capacitors, the leakage current should be very small and therefore a large resistor can be used.
 


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