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[SOLVED] SMD timing capacitor options?

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d123

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Hi.

I was looking into SMD capacitors (mainly thinking of timing in the range of seconds, not milliseconds), and from what I see the widest selection are:

Aluminium Electrolytic (dry)

Tantalum

Ceramic

All the good dielectrics seem to have a low capacitance range, or are very expensive indeed.

I really prefer PET/polyester for a lot of reasons (shame polystyrene are only small values due to size), not thrilled about Tantalum, much less ceramic for timing applications, so am a bit stuck on which way to go now moving from through-hole to more SMD-based circuits.

I have been reading that Polyphenylene Sulphide, Niobium Capacitors, PEN Film are all good choices.

Anyway, do I basically have to settle for Tantalum over the Aluminium Electrolytic dry types for timing applications? Recommendations welcome, thanks.
 

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Can't be answered without talking about timing accuracy first. Precise analog timing in a second range can be only achieved with film capacitors. If you don't worry about some ten percent variation over temperature and between production lots, electrolytic and high permittivity ceramic capacitors can be used. But since decades, precise timers are preferably made digitally, at least as analog timers with frequency divider/counter.
 
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Thanks.

With each tiny step forward I seem to be slowly being backed into a corner of having to learn how to use a PIC.

I doubt, given what I do I'd ever need more than an accuracy of - let's say - 95% (ideally and over optimistically 98%) to put a number on it. Timing accuracy will depend on each circuit in question, I'm just trying to understand SMD options (because I was disappointed to see so few PET/MKT types available at 10uF), so your answer is very enlightening - SMD film capacitors appear to be few and far between, and 10uF was (comparatively speaking) very expensive - hard to justify that for a little hobby circuit, even for decent timing accuracy.

One part of the circuit I'm about to start in a few days only needs to have about 10 seconds on time, but precision isn't important for that; not sure if the other part with the dual slope ADC needs a precise timing capacitor though, even if it's only a 7136, but as that's in the pF range shouldn't be a problem to find.

I've tried to keep timing capacitors to about 10uF and use higher value resistors due to something I read regarding leakage, maybe that doesn't apply so much to PET/polyester film anyway.

Electrolytic are pretty dreadful - like a bad watch spring (I'm so sad: I actually timed on-circuit a 555 astable pulse using different types of capacitor to see if there were any differences...polyester/metal film won by a long shot on accurate repeatability), not sure if the newer SMD ones are as "better than the through-hole version" as I have read they are. And I read that ceramic is not a good choice for timing in general as they have temperature variations and the piezoelectric effect (I think that's the name) causes unwanted oscillations that alter timing.

If for the time being I won't be using PICs or similar, are SMD ceramic and aluminium electrolytic really so improved upon their through-hole version counterparts?

And would you recommend the Tantalum SMD capacitors over the previous two types? - Tantalum gets quite varied reviews in dielectric comparison charts and articles on the subject of capacitor selection by function.

Your answer explains to a degree (besides size and some dielectrics being "young" technologies) the low capacitance range over many of the more appealing dielectric types I've never tried such as PEN film or Niobium.

Once again, thanks.
 

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I agree that a small processor, e.g. 6- or 8-pin PIC is a reasonable solution using modern components. They achieve about 1% accuracy with the calibrated internal oscillator. Digital timer isn't however restricted to processor solutions, there are also some dedicated timer ICs using a higher frequent (= smaller capacitor) RC oscillator and a frequency divider, also standard CMOS ICs like CD4060 (oscillator + divider) can be used.

Dual slope converters don't require absolute accuracy, but low loss capacitors like PP film.
 
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I see what you mean, I've used the 4060 for a couple of things. Good to know about dual slope, I think for now I'll just mix through-hole and SMD as I go learning about and trying the latter and where certain components I already have are appropriate, not ideal but also a way to use up components that will otherwise gather dust.

BTW - changed the PMOS for PNPs, as you suggested, and at last testing of a few things (before adding the rotary switches) the power supply was working as okay as I'd expect it to, thanks, it was a lot easier calculating the resistors and especially avoided a big mistake with the VGS huge voltage drop at higher current I wasn't aware of.

I'll look into dedicated timer ICs (other than beloved 555) when next thing is finished, as I'll definitely need one.
 

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plastic film give the longest RC time constants at the expense of low dielectric constant and thus low leakage current required from all stray sources. but 22M Ohm may be suitable max.

With digital timers, why bother with long analog time delays? Ever considered a CD4040/4060?
 
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