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 Post subject: Ceramic Caps - how to determine working volts
PostPosted: Aug Tue 28, 2007 7:48 pm 
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Location: Mt. Prospect, IL USA
Hello all,

After over a decade of working on radios, I never saved any information about codes for working voltage of ceramic caps. I just got a grab bag full of them, and only about a third have the working voltage marked. Some have codes: Y5P, Z5U, Z5F, Y5V, or Z5P. Do these codes tell anything about working voltage?

Also, I thought I read that ceramic caps won't work well in RF circuits. Are there some definite no-no's regarding the use of ceramic caps in antique radios? As a rule, I buy mylar caps, but found some nice cheap grab bags and couldn't resist.

Thanks to all with pointers...


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 Post subject:
PostPosted: Aug Tue 28, 2007 9:18 pm 
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I suppose if I had a bunch I'd hipot a few and use 'em up to 20-25% of the breakdown voltage...

The codes you listed are temperature characteristics - none of those are good enough for use in RF circuits. For RF use NPO or Nxxx (N330, N750, etc) if you need temperature compensation. The Y and Z ceramic used for high value caps (Hi-K) has TERRIBLE temperature and voltage stability - these should be used only for bypassing.


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 Post subject: Re: Ceramic Caps - how to determine working volts
PostPosted: Aug Wed 29, 2007 8:33 pm 
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Chuck Braun wrote:
I thought I read that ceramic caps won't work well in RF circuits. Are there some definite no-no's regarding the use of ceramic caps in antique radios?

Hi Chuck,

Quite the contrary... ceramic caps work very well at RF. In fact you'll find nothing but ceramics in most VHF and UHF radios, where other types won't work at all.

Their usage in antique radios would be a question of aesthetics and authenticity rather than engineering.

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 Post subject:
PostPosted: Aug Wed 29, 2007 9:20 pm 
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Well, I found many .0047 and .0068 uF ceramics in the bag, 2KV, and I'd use them mostly for bypassing and AF coupling or tone circuits. I could imagine a ceramic being bad in a tuned circuit, if it changes value with temperature.

Since I've already destroyed the originality of my Zenith with NTE mylar caps and heat-shrink tubing, I guess the ceramics will do. Still, it's a puzzle as to why no voltage ratings are given. Maybe it's just because this is the solid-state age, and they were all meant for low-voltage circuits once the tube age (not for us) ended...


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 Post subject:
PostPosted: Aug Wed 29, 2007 9:34 pm 
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I'm going to guess that if the voltage rating was high enough to use in just about any tube circuit, the cap would be big enough to put the value & voltage on it, rather than just the tempco.


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 Post subject:
PostPosted: Aug Wed 29, 2007 10:55 pm 
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Location: Melrose, NY
Chuck,

Radio Daze published a really neat summary of color codes
some time ago, that covers _molded_ mica caps. Not sure if
it's still available, but I could fax/scan a copy if you want it.

But it sounds like you're talking about _disk_ ceramics.

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Das will spielen. -Nietzsche


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 Post subject:
PostPosted: Aug Thu 30, 2007 9:02 pm 
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Just Googled up this info:

Ceramic—The disc is a common package for ceramic capacitors, but monolithic, glass-encapsulation and feedthrough
packages are also available. Ceramics also come in axial-lead packages that resemble film resistors. Such capacitors consist of two
thin sheets of metal (silver, nickel, etc) separated by a dielectric of titanium dioxide or similar ceramic material.
Designers have long used ceramic capacitors for RF tuning and bypass applications, in part due to their low series inductance.
Ceramic capacitors with wire leads are useful at frequencies ranging from dc to the low VHF region. A temperature range of –55 to
+125°C is typical, with working-voltage ratings up to several kilovolts. Ceramics are available with both positive and negative
temperature coefficients.

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 Post subject:
PostPosted: Aug Thu 30, 2007 9:22 pm 
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You really can't be certain of the voltage rating on unmarked disc ceramic caps. Used to be back in the 50's-early 60's that only HV types or LV types were marked, and all unmarked ones could be presumed to be either 500 or 1000 volts. I still don't know how they expected you to differentiate between those two ratings.

But since that time, there are jillions of unmarked ones, some of which have ratings below 100 volts.

When scrapping out junk, I make it a point to put a dot of paint, marker, or nail polish on disc ceramic caps that came out of tube-type equipment since those are going to be good for 500 volts or more. The ones I don't mark are undetermined as to voltage, so I use them in very LV applications like cathode or AVC bypasses, or coupling caps between volume control and grid of 1st AF where they are never going to fail.

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 Post subject: ceramic disk caps....
PostPosted: Dec Tue 25, 2007 10:23 pm 
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Location: Beaverton, Oregon
Chuck Braun wrote:
Well, I found many .0047 and .0068 uF ceramics in the bag, 2KV, and I'd use them mostly for bypassing and AF coupling or tone circuits. I could imagine a ceramic being bad in a tuned circuit, if it changes value with temperature.

Since I've already destroyed the originality of my Zenith with NTE mylar caps and heat-shrink tubing, I guess the ceramics will do. Still, it's a puzzle as to why no voltage ratings are given. Maybe it's just because this is the solid-state age, and they were all meant for low-voltage circuits once the tube age (not for us) ended...



So you’re not to use ceramic disk caps to replace the old wax caps except when they are used in bypass circuit, AF coupling or tone circuits but not in a tuned circuit?
YUKERS, I've used ceramic disk caps to replace ALL my wax caps in my radio...Was that a no, no?


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 Post subject: Re: ceramic disk caps....
PostPosted: Dec Tue 25, 2007 10:58 pm 
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Maximum1 wrote:

So you’re not to use ceramic disk caps to replace the old wax caps except when they are used in bypass circuit, AF coupling or tone circuits but not in a tuned circuit?
YUKERS, I've used ceramic disk caps to replace ALL my wax caps in my radio...Was that a no, no?


The no-no is when it comes to caps in a frequency determining or tuned circuit. They wouldn't have been paper in the first place...usually mica.

I use a lot of the little tiny low value disc ceramic caps <500pf for projects and general horsing around. No tempco or voltage markings. If I had 'found' them in my parts drawer I would think they are for transistors radio. Those tiny things are rated at 500 volts! but then you see older discs from the 50s/60s as big as a dime only rated at 100 volts. Not much way to tell without knowing their provenance.


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 Post subject:
PostPosted: Dec Tue 25, 2007 11:00 pm 
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The use of disc ceramic caps in audio circuits might create problems under some circumstances. Disc ceramics can exhibit piezoelectric effects when subject to stress or shock. This results in generation of small voltages that could appear in the audio output as noise.

Since this problem is limited to frequencies in the audio spectrum, it does not affect use of disc ceramics at IF or RF frequencies well above 100 Mc.

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 Post subject:
PostPosted: Dec Wed 26, 2007 3:14 am 
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Leigh wrote:
The use of disc ceramic caps in audio circuits might create problems under some circumstances. Disc ceramics can exhibit piezoelectric effects when subject to stress or shock. This results in generation of small voltages that could appear in the audio output as noise.



Ive seen that statement basically word for word on a few forums recently. Can anyone provide the original source? Or actual documentation?

Since many consumer and commercial radios and amps have been using ceramic caps in audio circuits for decades Im rather skeptical of any theoretical problem. Sounds like something from an audiophool forum.

Ive been using ceramic in audio circuits for decades and can report no noise, squeals, oscillations, etc.

Carl


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 Post subject:
PostPosted: Dec Wed 26, 2007 3:34 am 
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Burnt Fingers wrote:
Ive seen that statement basically word for word on a few forums recently. Can anyone provide the original source? Or actual documentation?
Since many consumer and commercial radios and amps have been using ceramic caps in audio circuits for decades Im rather skeptical of any theoretical problem. Sounds like something from an audiophool forum.

Carl,

If you accuse me of audiophoolery, I'm gonna come have a talk with you, boy!

I posted a link to the original article quite some time ago.

One reference is a paper entitled "Piezoelectric Effects Ceramic Chip Capacitors" on the Kemet website: http://www.kemet.com/kemet/web/homepage ... 0Noise.pdf
This was written by John D. Prymak, Applications Manager for Kemet. Kemet is one of the largest manufacturers of ceramic capacitors in the world.

The first paragraph reads:
"Most dielectrics of ceramic capacitors exhibit a characteristic identified as piezoelectric effects that can cause unexpected signals in certain circuits. In some cases, the piezoelectric effect may result in the appearance of electrical noise, while in other cases an acoustic sound may be heard, emanating from the capacitor itself."

The phenomenon is fact, easily duplicated by anybody with test equipment and the knowledge to use it. As to theoretical problems... some people understand analytic techniques, and some don't.

And what is the reference to "squeals, oscillations, etc."? I don't recall ever mentioning such. It appears you made that up for your own purposes.

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Last edited by Leigh on Dec Wed 26, 2007 4:58 am, edited 5 times in total.

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 Post subject:
PostPosted: Dec Wed 26, 2007 3:59 am 
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No doubt thats the Walter Jung/Richard Marsh article that dates back to about 1980. Its all over the web complete with graphs and scope traces, etc. Indeed the effects of a ceramic coupling cap are measurable and of some significance in hi-end audio work. But as you say, there have been 10s if not 100s of millions of nice pieces of equipment made over the years using ceramic coupling caps without issue. Given that our old radios have numbers like 5 and 10% distortion its not something we need to worry about.

Its a great article by the way and worth the read. The original site has been reconfigured but its on web.archive.org without the photos and graphs. Maybe some of the other sites have all that as well.

http://web.archive.org/web/200308071226 ... ickcap.htm


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 Post subject:
PostPosted: Dec Wed 26, 2007 5:12 am 
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Well, I found the article that mentions mechanical sensitivity of ceramic caps. It's on the Planet Analog website http://www.planetanalog.com/story/OEG20000912S0057

This is the article I remember, entitled "Silence of the Amps", from September 2000. It was written by By Jim Williams, Staff Scientist and Todd Owen, Applications Manager, Linear Technology Corporation.

Quoting one paragraph from Appendix B (without permission):
"Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor the stress can be induced by vibrations in the system or thermal transients. The resulting voltages produced can cause appreciable amounts of noise, especially when a ceramic capacitor is used for noise bypassing. A ceramic capacitor produced Figure B4s trace in response to light tapping from a pencil."

Unfortunately, the photos don't show up for me. Don't know why not. Look at the caption for photo B4: "A Ceramic Capacitor Responds to Light Pencil Tapping. Piezoelectric Based Response Approaches 80 micro V(P-P)." Perhaps someone can get the photo so show up. For the photo and text, scroll down to "Ceramic Capacitors" in Appendix B.

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Click "Grebe Stuff" for Synchrophase info


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