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 Post subject: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 2:20 am 
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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.


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 7:59 am 
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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


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 1:02 pm 
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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.


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 2:11 pm 
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Usually Lurking wrote:
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

They are about 3/8" in diameter.


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 2:42 pm 
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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.

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 6:31 pm 
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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.

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Sat 28, 2012 9:34 pm 
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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


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Wed 26, 2017 12:10 pm 
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I know this is an old thread but I to am trying to determine the breakdown voltage for RMC .005 Z5U capacitors. I see that they are considered inexpensive to manufacture.

I just looked up the current price of .005uf 1kv Cera-Mite ceramic disc capacitors and they are $1.19 each.

http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkey562R5GAD50

That seems like high-way robbery to me for something which isn't of the best quality.

The circuit these are used in is a 1960s transistor amplifier. These are connected to the output transistors (one each) across the Base and Collector with a 27ohm 1/2w resistor in parallel. The DC input on this little 8 watt amp is around 35-40vdc. I suppose a .005uf polyester 200v would be much more stable?

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Wed 26, 2017 12:36 pm 
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Without a picture, it's hard to guess what the voltage rating of your caps is. RMC was a popular brand years ago and made lots and lots of different kinds of caps. Usually Lurking has already given you a good clue, if you look at the thickness and diameter of your caps, the voltage rating should be comparable to others of the same uF value for which the voltage ratings are known. There were not a whole lot of voltage ratings made, BTW. There were some very tiny 25-volt and 50-volt ceramics made for transistor radios, but most ceramic discs are either 100-V or 500-V. Beyond that they made 1-kV, 1.5-kV, 2-kV, and 3-kV caps but those are really big.

The fact that Z5U ceramic capacitors are inexpensive to manufacture does not mean that they are low quality or inferior to other types. Z5U ceramic has one of the highest dielectric resistances known to man, so the leakage current through them is nil for all intents and purposes. The capacitance does tend to vary with temperature more than other kinds of caps, but it's not necessary to have precision capacitance values in most bypass and coupling applications while extremely high leakage resistance may be a real benefit. Ceramic caps have higher Q's than film types, less dissipation, and much lower inductance. Not to mention the fact that they'd be smaller and less likely to pick up or radiate stray fields.

One characteristic of ceramic disc capacitors is, they can act like piezoelectric crystals and "ring" or buzz at certain frequencies. This is not a problem if the voltage across the cap is mostly DC, but if you try to put 30-VAC across a 100-volt cap, you might hear it. The mechanical action also does some really weird things to the capacitance! In cases where that might be a problem, manufacturers sometimes used caps with higher voltage ratings, which are less likely to evidence this effect. This is why you'll sometimes see 500-volt disc caps used in places where the DC voltages are much lower.

It's not altogether clear that any of these factors would matter in your application, but many vintage 1960s semiconductors are getting hard--if not impossible--to find replacements for, so I would not take a chance on maybe turning an amplifier into an oscillator and roasting it by changing parts around from what was there originally. Just track down some Z5U ceramic caps of the same size and value as the originals and it should be fine.

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Wed 26, 2017 1:06 pm 
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I have some of the .005uf 1KV Vishay / Cera-Mite in stock. I guess I will then use those. I understand perfectly that some of these parts are hard to come by..... A great term one of the original Seeburg Jukebox techs uses is "it's made from un-obtanium".

I do service on newer equipment too and have even seen .047uf 25vdc ceramic capacitors in Italian manufactured digital sampled church organs. I also have some of the bigger 2KV and 4KV types for old vacuum tube Hammond repairs.

I had read on some other board that the Z5U was considered junk.

Sometimes you can actually find a search twice. Here is where I was apparently reading snake oil:

https://electronics.stackexchange.com/questions/67743/how-can-i-find-the-voltage-rating-for-a-particular-ceramic-capacitor

This from the third comment down in the above linked thread:

"NP0 isn't necessary unless it is in a critical circuit, but Z5U is pretty junk. X7R is much better at not much more cost. (xxU and xxV caps lose something like 80% of their capacitance when running near rated voltage! The things should be illegal to rate at 0V)"

Thanks for the clarification :mrgreen:

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Thu 27, 2017 3:59 am 
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A 0.01uF cap isn't likely going to be used in a tuned circuit (if we are talking about antique radios). So Z5U will be fine 99% of the time. Being ceramic, they will likely out live any paper and possibly even out live most poly capacitors. That's why most military quality radios switched to ceramic caps.
As far as microphonics, don't concern yourself with it unless you use these in super high gain circuits where very high impact vibrations are likely to occur. I've done laboratory testing of ceramic capacitor for microphonics and found that you need to really smack them with high impact stresses to get any response at all. The microphonics concern seems to go around on the web over and over and over. Heck, you will even find lots of them used in input circuits of high power guitar amplifiers where you would expect microphonic problems.


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Thu 27, 2017 4:03 am 
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tom.seeger wrote:
A 0.01uF cap isn't likely going to be used in a tuned circuit (when we are talking about antique radios). So Z5U will be fine 99% of the time. Being ceramic, they will likely out live any paper and possibly even out live most poly capacitors. That's why most military quality radios switched to ceramic caps.
As far as microphonics, don't concern yourself with it unless you use these in super high gain circuits where very high impact vibrations are likely to occur. I've done laboratory testing of ceramic capacitor for microphonics and found that you need to really smack them with high impact stresses to get any response at all. The microphonics concern seems to go around on the web over and over and over. Heck, you will even find lots of them used in input circuits of high power guitar amplifiers where you would expect microphonic problems.


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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Thu 27, 2017 8:27 am 
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That sounds great thanks for the additional information.

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: Apr Thu 27, 2017 9:43 am 
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Quote:
Sometimes you can actually find a search twice. Here is where I was apparently reading snake oil:

https://electronics.stackexchange.com/q ... -capacitor

This from the third comment down in the above linked thread:

"NP0 isn't necessary unless it is in a critical circuit, but Z5U is pretty junk. X7R is much better at not much more cost. (xxU and xxV caps lose something like 80% of their capacitance when running near rated voltage! The things should be illegal to rate at 0V)"

I didn't see anything wrong with what was in your link, but I did see a note that some posts had been removed. As far as Z5U capacitors go, they do change value drastically with just about anything you do to them: temperature, voltage, frequency, you name it. If you want proof, consult a manufacturer's data sheet and look at the performance curves. This is why you don't find them in tolerances tighter than +/- 20%; they don't hold still long enough for anything better.

As for microphonics, I have seen them respond to the air pressure changes caused by the air conditioner cycling on and off.

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 Post subject: Re: ceramic disk capacitor voltage rating?
PostPosted: May Thu 04, 2017 7:45 am 
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I'll try to pull these two threads together in some way.....

Unless I had not previously stated, the ceramic bias capacitors in this case are used in a 8 watt output audio tape player circuit, for a 3M Cantata 700 model 94bg background music system of yesteryear.

The .0047 200v film capacitors I have in stock are physically smaller than the original .005uf ceramic originals if that was a concern (in the case of oscillation within a Radio).

The other thread for 3M branded Germanium transistors has been solved. There is an image showing the transistors, and what I believe are the bias capacitors and resistors.

http://antiqueradios.com/forums/viewtopic.php?f=13&t=319927

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