I just ran across some older ceramic disk caps in my dad's stuff. They are .01uF, and are marked:

RMC
.01
20%
Z5U

Any idea what the breakdown voltage of these caps would be? I'd like to be able to use them to replace paper/foil caps in my Silvertone 1970A.

Rich,

You will have to go by the physical size of the caps. Try to find an old RMC or Mallory catalog that lists these, and match yours up by the diameter.

Z5U discs were made with voltage ratings at least as low as 10V, so you do want to be reasonably sure your parts are good for enough voltage.

The Z5U tempco is about the least stable of the ceramic formulations; these have poor temperature and voltage coefficients. They are microphonic, too. But they are small, and inexpensive to make.

Ted

I believe those are 1 kV. I have some of another brand that are marked 1 kV. I just put an RMC on the leakage tester and it didn't object to 750V.

They are about 3/8" in diameter.

At one time you could safely assume that unmarked disk ceramics were good for either 500 or 1000 volts, since those were the commonly supplied ratings. The ones you have could be anything but are most likely 500 or 1000 volts and should be good for bypass use.

If you want to experiment by raising the voltage applied to one until it breaks down, then use the rest at about 1/2 that voltage.

When comparing disk capacitors of the same type and manufacturer, it's the thickness that gives you an idea of how much voltage they can withstand.

If you break one open and look at the actual ceramic (not including the outer coating), if it is about the thickness of a credit card or less, you're talking perhaps 50 or 100 volts. Thickness of a dime would put you in the 200-volt to 500-volt range. The thickness of a couple of pennies stacked together would give you 750 volts to 1-kV. Three or four pennies stacked would be 5-kV or more.

It's not a good idea to use a disk capacitor on higher voltages than it was designed for. Under such conditions they have a nasty tendency to work perfectly well for a while, then fail dramatically. Also, as was already mentioned, Z5U ceramic does not make for a very stable capacitor; the capacitance can change considerably--like 20% or more--with temperature and age. But it is one of the most inexpensive materials that can be used, so it is quite common. Best to use them in bypass applications where changes in capacity have little or no impact on circuit performance.

Rich,

I looked for an old RMC catalog without any luck. Someone may have this online, but where?

The old Sprague ceramic disc line is still made by Vishay Cera-Mite, and the datasheets are on the Vishay site.

Looking at these currently available discs, I would expect your parts to be rated between 100 and 500VDC.

The Vishay .01uF 100V Z5U parts are specified as 7.5 mm diameter.

Their .01uF 500V Z5U parts are specified as 11mm diameter.

Mouser lists 500V Chinese parts as small as 5mm dia.

Your 3/8 inch diameter is about 9.5mm.

And yes, the breakdown voltage is determined by the thickness of the ceramic, but that is harder to deal-with. As the ceramic becomes thicker, the area of the electrodes must be increased to get the same capacitance, so the diameter does reflect the voltage rating, at a given capacitance value.

It should be noted that very old parts are likely to have lower ratings than present-day production units of the same physical size. Sometimes this is due to improvements in materials and manufacturing techniques, sometimes through "improved" specifications.

I expect your parts were made long ago by Hoosiers in Circle City, so perhaps they tend to the larger size end for their ratings.

Ted