1000V 0.5uF electrolytic cap replacement ? What voltage rating  - Page 1
Forum section: electronics / repair

author: lordvader88
author_rank: Frequent Contributor
timestamp: November 13, 2017, 04:15:51 pm
content: 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

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2017, 05:31:13 pm
content: 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.

author: Cyberdragon
author_rank: Super Contributor
timestamp: November 13, 2017, 05:33:20 pm
content: Quote from: lordvader88 on November 13, 2017, 04:15:51 pmNo, 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

author: flowib
author_rank: Contributor
timestamp: November 13, 2017, 09:45:28 pm
content: 470nF 1000V is still a relatively common value, Wima makes them. there are also axial types from Kemet.if the original circuit calls for .5µ 1KV a modern replacement will do just fine.

author: Cerebus
author_rank: Super Contributor
timestamp: November 14, 2017, 12:42:21 am
content: Quote from: lordvader88 on November 13, 2017, 04:15:51 pmIf you do end up using two (or more) in series please remember that capacitors in seriesdon'tadd 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.]

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2017, 08:37:23 pm
content: 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.