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Old 23rd Sep 2021, 6:30 am   #1
Radio1950
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Default Ceramic Capacitors - Change of Value With Time

Ceramic Capacitors - Change of Value With Time

Am I the only one to have not read about this effect with certain ceramic capacitors?

I recently went hunting in my old bins for some 2% tolerance ceramic capacitors for a test box, and found some disc ceramics that were 50-60 years old at least.
On test, most had a value about 30% lower than marked.
That had me interested.
I looked for this effect on the internet and found quite a few articles on this apparently well known effect, and perhaps its cure.

Well, just in case someone else, like me, had never read much about it, here are two of many articles.
https://content.kemet.com/datasheets/F9000_K.pdf
https://www.johansondielectrics.com/...ng-made-simple

Makes one think about the caps in old HF coms receivers etc, sitting around for a long time before restoration.
Did those tuned circuits with fixed ceramic caps really need a retune?
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Old 23rd Sep 2021, 7:29 am   #2
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Default Re: Ceramic Capacitors - Change of Value With Time

It's known about, but only in the more careful parts of the industry.

NP0 and C0G are codes for temperature coefficients, but they tend to get used as if they were specific types of ceramic dielectric. But to get these low tempcos the ceramic can't contain barium titanate, so they don't show the ageing phenomenon, and their capacitance isn't a function of applied voltage. They are linear, and an excellent choice for tuned circuits in radio applications.

Then there are 'the other' dielectrics. What in days gone by were referred to as 'High-K' ceramics. This means an unusually high dielectric constant, achieved with ferromagnetic ceramics and that brings with it ageing effects, voltage dependence of capacitance, non-linearity. A whole host of problems. But they do pack a lot of capacitance/voltage into a small volume. The blessed things are microphonic as well!

So these high dielectric constant capacitors have some pretty evil habits. This makes them really only suitable for use as decoupling devices and DC blocks if you are careful. Use in tuned circuits is right out. Timing circuits with them are dodgy and don't even think about integrators etc. I first ran into trouble with them in loop time constant circuits for the loop amplifiers of phase locked loops. I first noticed the microphony coupling rumble in from fan motor bearings making phase noise sidebands. Then I came across the non-linearity making a loop unstable at large oscillations, while still stable at low levels. It was metastable. You could make the loop unstable and see it oscillate or you could stop it and it stayed in either condition. The last thing I suspected was a capacitor foible.... Oh, well, you live and learn.

There has been a recent trend towards very high density ceramics as surface mount alternatives to electrolytic capacitors, to get round their nasty ageing and leaking effects. Few people are aware that the ceramics have their own ageing issues if left cold and uncharged.

NP0/C0G parts are available up to around 2200pf and down below 1pf. They're OK. Histab in old speak.

A 2% tolerance just means the part was selected, no guarantee of stability, that's a different parameter! Usually such tight tolerances are found on the low tempco types. Tight tolerance on the less stable materials is a waste of time, bu it was done. People needed larger values then NP0 could provide for timing circuits and audio.... but the non-linearity and ageing were overlooked.

High-K ceramics have their uses, but you have to think carefully about using them.

David
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Old 23rd Sep 2021, 8:28 am   #3
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Default Re: Ceramic Capacitors - Change of Value With Time

David has said it all!

I first came across the tempco problem in a 555 timer circuit, years ago. I needed a 2.2μF timing capacitor, it was a surface-mount assembly. Having recently been visited by a rep from a well-known capacitor company, extolling the virtues of their products, I chose one of their small Z5U ceramic capacitors.

It worked fine till the temperature reached 60°C. Then the timing period reduced.

Changing to a tantalum type fixed the problem.

This was purely the temperature dependency... the loss of capacitance with time, never had a chance to kick in. But it was a lesson in reading the data sheet carefully.

There is a corresponding phenomenon with regard to high-permeability ferrite materials (also a ceramic!): a choke wound on a ferrite core will gradually lose its inductance. It is generally only significant in toroids. Other configurations, especially when an air gap is included, have the effect diluted.

A Google search for 'ferrite disaccommodation' will come up with some hits.
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Old 23rd Sep 2021, 9:04 am   #4
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Default Re: Ceramic Capacitors - Change of Value With Time

When reading a datasheet, sometimes you have to pay great attention to just what they don't say. Not all own up to all the foibles.



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Old 23rd Sep 2021, 3:16 pm   #5
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Default Re: Ceramic Capacitors - Change of Value With Time

This has gone way off topic so the Scrabble posts all deleted .

Cheers

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Old 23rd Sep 2021, 6:07 pm   #6
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Default Re: Ceramic Capacitors - Change of Value With Time

I used to work for a ceramic capacitor maker, they all reduce capacitance over time (some very small x to the power of 10 per hour, x depends upon material). The standard was 1000 hours after the last heat cycle (this resets them) to minimise real world problems. As they were tested well before 1000 hours a correction factor was made to the measured value.
 
Old 23rd Sep 2021, 6:10 pm   #7
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Default Re: Ceramic Capacitors - Change of Value With Time

Thinking about it (it was over 20 years ago) it is 10 to the power of the very small x per unit of time. That formula would lead to no capacitance at all after an infinite time, obviously the plates would still give something, the formula was good for a few thousand years, we didn't bother with that slight error.
 
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