Capacitor rating

[ppppenguin]

I've found a bag of capacitors. Individually packaged, then packaged again in bags of 5. I'm opening them up for ease of storage and decided to test some. They are nominally 1n or 1000p, but what voltage? Tried with my 500V bench PSU. No leakage. Tried with my 5kV low current PSU. No leakage. My guess is that the official rating is 1kV or so and I'd trust them in just about any domestic set in any role.

Does anyone recognise the part?

[ppppenguin]

You can tell I'm having a sort out during lockdown All sorts of bits have been coming out of the woodwork. Coming towards the end of the capacitors now. Rather to my surprise these 10n ceramic discs also withstood 5kV. Only 20 or 30 of them in the bag. To be kept and marked as useful for valve kit.

I've also found about 10 squillion 47n 25V disc ceramics, same again of small 0.1" pitch 100n low voltage ceramics. No marked voltage but wouldn't push those beyond 20V.

Then there are small numbers of various chunky case mica caps and others too.

All the decent components going spare will be made into goody bags for sale at Harpenden and/or Retrotech to raise a few quid for the Museum. I've got a growing box of R/C/L, transistors, diodes, chips

[BusyBee]

A lot of my older stock is pretty much the same as yours as I have some capacitors which may be as your first, but not in packets, so I don't know details. I do have a number of Erie ones like your second, which I find useful and reliable but I have not tried them at high voltage. Interestingly I did come across some others I have - 330pf but slightly smaller in diameter than your Erie ones, which have 5kV rating printed on them. They are fatter than most.

Tracy

[ppppenguin]

I very much doubt if either the 1n or 10n discs are rated at several kV. More that they have a very wide safety margin over rated voltage. There may be metal migration or other odd effects if they are run at very high voltages. What they are almost certainly not good for is high frequency or pulsed high power. Such as in transmitters or TV LOP stages.

The 1n caps look ideal for heater chain decoupling in TVs. These can often fail so mass repalcement is the right thing to do. I've got a number of small 1n 750V polyester(?) which I've used but these ceramics look better.

[Kalee20]

I'm thinking too, that some ceramic dielectrics have a significant variation of dielectric constant with applied voltage. Capacitance drops off.

So, they may not break down at 5kV, but it could be that capacitance falls by 10% at 1kV, and this might determine voltage rating (quite apart from metal migration).

Just a thought...

[ppppenguin]

Not sure I want to attempt to measure capacitance as I wind up the voltage to several kV I suppose I could find some bigger caps rated at several kV and put them in series. Then hope my DMM or Peak capacitance meter doesn't get the wrong sort of ouch as things charge and discharge. Zeners in inverse series perhaps?

Anyway, if the 1n and 10n caps can withtand 5kV I'm sure they'll be fine for any normal radio or TV decoupling jobs. But I'll keep them away from LOP pulse roles.

One day I'll acquire or build a higher voltage PSU. An old Brandenburg 25kV job would be nice. Otherwise I've got some old diode split LOP transformers from computer monitors which could be turned into simple variable EHT supplies. I'd build in a metering resistor which would double as a safety discharge. Maybe one day.

This sort of thing, but made easier with a diode split LOPT: http://www.iaeng.org/publication/WCECS20...06-209.pdf

[Refugee]

Those diode split LOPTs have a focus chain with an earth tag that can be used for feedback to an SMPSU chip.
I have got the beginning of one in a case that once contained an ethernet router.

[ppppenguin]

I'd be perfectly happy with an open loop design. A knob to set the voltage and a meter to measure it. If I was testing capcacitors I'd prefer the voltage to collapse under load. If working with CRTs the current drain is low enough that manual control should be fine.