do power filtering caps need to be electrolytic

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do power filtering caps need to be electrolytic

Postby JustinFun » 17 Apr 2013, 15:49

Hi everyone,

I've recently got into designing and building all-smd versions of classic pedals - for the challenge, and because i am fed up with drilling pcbs! Once I've got some layouts I'm really happy with I'll post them in the 'ready to build' section.

I'm still using through-hole 47u electrolytics for power filtering though - two more holes to drill! Unacceptable. I know you can get smd electros but they're ungainly and expensive, so I was wondering ... can I use a 47uf smd ceramic for the same purpose (like this one http://uk.farnell.com/murata/grm32er61c476ke15l/cap-ceramic-47uf-16v-x5r-1210/dp/1828819RL) or is there some inherent property of a polarised electrolytic cap that makes it the only or best choice for this purpose?

Sorry if this is a dumb question, I'm not an EE, trying to pick theory up as I go along.
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Re: do power filtering caps need to be electrolytic

Postby teleK » 17 Apr 2013, 17:26

supposing you can find other caps that big, i see no reason not to.
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Re: do power filtering caps need to be electrolytic

Postby cpm » 17 Apr 2013, 20:37

somebody posted this about ceramic caps:

http://my.execpc.com/~endlr/ceramic.html

Says about the X7R class capacitors:
These capacitors don´t have the good high frequency characteristics of Class 1 ceramics, they all show a drop in capacitance and a rise in dissipation factor as they go above 1 MHz. However, some high-voltage/high current RF capacitors are made of the better Class 2 ceramics, for use in the 1-40 MHz range. High-K ceramic capacitors can show significant piezoelectric effects; if you tap them they will produce a voltage spike. This is caused by the barium titanate, the main material in high K ceramics. The higher the K, the stronger this affect. This can cause problems with circuitry dealing with low-level signals, making them vulnerable to mechanical shock. This is another reason to use Class 1 ceramics when designing analog circuitry. Class 2, and higher, ceramics do have an advantage in size, however. In fact, high-K ceramics can compete with electrolytic capacitors in many applications where low ESR is more important than bulk capacitance.


*big drift through the voltage range: not a significant issue for power filtering application
*worse performance in the MHz: not an issue; if those high frequencies make a problem, you've got a more complex scenario than a plain filter cap.
*vulnerable to mechanical noise: may be an issue. I've found some high gain circuits which would reproduce all kind of mechanical noise applied to the enclosure, switch, etc; However the board was rigidly attachet to the footswitch (bad combination)

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Re: do power filtering caps need to be electrolytic

Postby cpm » 17 Apr 2013, 20:48

By the way, i too would like to get a better understanding on what kind of caps are better for each situation, in a sensible and economical way.
I'd like to go surface components as much as possible, and i guess there are some applications that would require some "kind" of caps to perform optimally, and other would make for any shitty and cheap smd cap without any problem...

I mean, audio path would require better specced caps, what kind? Which would be appropiate as big coupling caps, in the 1uF-10uF range ?
or non critical, not audio, digital interface, etc... i guess here whatever may fit

Also i'd like to get some words on using small inductors or chokes on power filtering, as a complement to filter caps...
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Re: do power filtering caps need to be electrolytic

Postby apollomusicservice » 17 Apr 2013, 22:21

ESR (equivalent series resistance) and its ESL (equivalent series inductance) affect the way the capacitor performs in circuits.
Electrolytic caps - usually made with "internal coil" have high inductance and high ESR and its suitable for keeping and maintain stability of DC supply.
Wide tolerance make them as bad choice for audio filters.
Non-electrolytic caps - low ESR, higher ripple current, non-linear capacitance which means that they will change capacitance at the end of voltage range.
For coupling audio signals the best cap is no cap. :block:

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