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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 18th Nov 2018, 10:48 pm   #1
ricard
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Default Why ceramic caps in radios

Radios (thinking vintage here of course, both valved and transistorized) are often sprinkled with ceramic caps. Is there any technical reason why ceramics are better than e.g. polyester, or is it just a question of price, in particular when the disadvantages of ceramics such as their lack of temperature stability could be tolerated?
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Old 18th Nov 2018, 11:28 pm   #2
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Default Re: Why ceramic caps in radios

Polyester wasn't much used in RF circuitry. Capacitors for tuned circuits were Mica, polystyrene or ceramic.

Mica and polystyrene tied you to their temperature coefficients, but a variety of ceramics gave a choice of temperature coefficients and could also be used to correct the tempcos of other parts.

Polyester capacitors would have been useable in the places where paper ones were often used.

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Old 19th Nov 2018, 2:18 pm   #3
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Default Re: Why ceramic caps in radios

Ceramics could be quite small, and by the 1960s smallness of domestic items was seen as a virtue. In uncritical situations their nonlinearity is not a problem.
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Old 19th Nov 2018, 2:25 pm   #4
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Default Re: Why ceramic caps in radios

There was a period in the early 60s when ceramics were the most cost effective technology. Philips used them a lot after they dropped the notorious 'black pitch' caps and before they switched to plastic film.
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Old 19th Nov 2018, 2:59 pm   #5
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Default Re: Why ceramic caps in radios

Ceramics have minimal self inductance, partly due to their compact dimensions, which is important in RF circuitry. Their capacitance can be said to have a high 'Q' factor.

Most have poor temperature coefficients which relegates them to decoupling applications only, they aren't much good for tuned circuits.

They are also microphonic, which means they can't be used in audio circuits of any quality.

There are different types of ceramic material used to make capacitors. X7R is a favourite for values around 100nF, C0G can be used for lower ones. Things like Y5V allow very high values to be made for the size, but the other characteristics are usually not as good as the others. It is important to replace like with like in precision applications, a tatty 'mouse ear' disc from the bottom of the spares box won't always do!
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Old 19th Nov 2018, 9:24 pm   #6
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Default Re: Why ceramic caps in radios

You have to choose ceramic dielectrics rather carefully.

C0G and NP0 materials have very low temperature coefficients and are not microphonic. You get them up to about 2200 pF in reasonable sized packages. Above that you start to pay with worse tempcos. Y5V and similar materials vie to replace small electrolytics, but they are microphonic and drifty.

I shove 300W pulses through a single 100pF capacitor which is a 1.4mm cube. THe ceramic is porcelain.

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