Author Topic: RC filter and electrolytic capacitor leakage  (Read 7753 times)

blackhole

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RC filter and electrolytic capacitor leakage
« on: October 07, 2010, 02:27:06 AM »
Hi, all!

I have one question regarding electrolytic capacitor leakage.
Let's say you have a voltage reference (IC or CCS+LED).
Since they are quite noisy (we want nV) and use RC filter to lower the noise, time constant < 1Hz.
This is then buffered by opamp.
To make such filter I usually use 1k resistor and 1mF capacitor -  electrolytic, of course. However, they
there is a concern regarding capacitor leakage. OK, this means we have some DC error (let's assume we have taken care of opamp DC errors, leakage, etc.).
If we do not care about the absolute value but we care only "medium term" (cca 1Hz and above) stability (usually case in audio),  this is OK. But is this all? Are there any other side effects? Hmm...

blackhole

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Re: RC filter and electrolytic capacitor leakage
« Reply #1 on: October 07, 2010, 05:03:48 AM »
I did more research: leakage leads to 1/f noise. How much does bootstrapping (as W. Jung) does help?
I guess I have to try it :)

Gen. Dreedle

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Re: RC filter and electrolytic capacitor leakage
« Reply #2 on: October 07, 2010, 10:55:53 PM »
Never tried bootstrapping. I guess, in theory, higher Z means less leakage. Higher Z also means more noise pick-up. (Won't say what commercial design I have poked my nose into, that had that problem.)

My solution is to use a low leakage 'lytic. They are out there. Usually only in through-hole construction.

High z is not a concern for me, as I almost always end up making a voltage divider. Suppose I could put the divider before the RC filter, instead of making them the same function. Haven't seen where it hurts me, so I have been taking the easy way out.
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MystereoN

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Re: RC filter and electrolytic capacitor leakage
« Reply #3 on: October 08, 2010, 12:37:36 AM »
For EU guys those KL's and RLB's are available only overseas. Damn spoiled Americans  :-X ;D
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blackhole

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Re: RC filter and electrolytic capacitor leakage
« Reply #4 on: October 08, 2010, 01:37:59 AM »
I've done some simulation (not real measurements - since I'm where they could do black holes - not really, but media thinks so). If I use bootstrapping I could reduce leakage by factor 10 (since I've used 1k and 10k resistors). However, I've checked KL's specs and they are of that order of magnitude lower. So, I have the feeling it's not worth doing it.
Hopefully, I'll do some real measurements soon.

Now I only need to get those KL's (1000uF, 6.3V). You are right MystereoN - not easy to find them in EU.

MystereoN

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Re: RC filter and electrolytic capacitor leakage
« Reply #5 on: October 08, 2010, 02:58:01 AM »
Drop PM to me if you can find good reliable source. I need lower values and higher voltage.
http://www.tadiranbatteries.de/eng/downloads/Both_Eng.pdf
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Gen. Dreedle

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Re: RC filter and electrolytic capacitor leakage
« Reply #6 on: October 08, 2010, 09:23:23 AM »
Can't always get exactly what you want, here. 1000 uF @ 10 V..........yep.

Come up with a big enough order, and I can get them.
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seagreen

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Re: RC filter and electrolytic capacitor leakage
« Reply #7 on: October 09, 2010, 08:13:33 AM »
Is this a group buy???
No double blind testing was used in the making of this post.

Gen. Dreedle

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Re: RC filter and electrolytic capacitor leakage
« Reply #8 on: October 09, 2010, 11:25:43 AM »
Hell no! Just an offer to help. The only reason to make the order "large enough" is to spread out the cost of mailing a handful of caps. Probably cost a buck more to mail 100, as 3.

Guys like us are spoiled. We can call Mouser, and have parts at our door the following morning. At $5-6, for shipping. And that is for a good handful of stuff.

Somehow, I think a lot of members can't do that.

Don't forget they have to appease Mr. VAT.
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
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seagreen

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Re: RC filter and electrolytic capacitor leakage
« Reply #9 on: October 09, 2010, 01:15:42 PM »
Well, somebody had to play the role of group buy cop.

Other places in the world have better access to other parts, so it kind of evens out.  Being able to source the best of both is a good deal.
No double blind testing was used in the making of this post.

MystereoN

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Re: RC filter and electrolytic capacitor leakage
« Reply #10 on: October 09, 2010, 01:18:01 PM »
Only a joke about spoiled Americans  ;D

No problem to order from Mouser, but shipping charge 40,00 for orders bellow 150,00  >:( for me. If I use my reserve adress in Austria, shipping charge is only 20,00  ;D

Another funny part: some interesting parts have following notice
International Trade in Arms Regulation - Restricted Product  Export outside of the USA is prohibited.
No problem with the same part ordered from manufacturer, they arrived without restrictions  ;D
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Gen. Dreedle

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Re: RC filter and electrolytic capacitor leakage
« Reply #11 on: October 10, 2010, 01:03:24 PM »
I can do a flat rate, Priority Mail box, for under $15. Only problem with that service is there is no tracking available.
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"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
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MystereoN

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Re: RC filter and electrolytic capacitor leakage
« Reply #12 on: October 11, 2010, 03:17:38 PM »
General, thank you for your kindly offer. Just feel a little uncomfortable torturing other people with my problems.

Placed an order with delivery to Austria. If the first shipment get lost, I will ask you for help with the second one  ;)
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MystereoN

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Re: RC filter and electrolytic capacitor leakage
« Reply #13 on: October 14, 2010, 03:30:44 AM »
Wow, those FedEx people are fast. Order placed on October 11, goods delivered on October 14. Incoterms DDP used :D
White color letter on black sleeve? To me looks more like light gray on black  ;D
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MystereoN

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Re: RC filter and electrolytic capacitor leakage
« Reply #14 on: October 17, 2010, 04:12:31 AM »
speaking about RC filters....with COOKBOOK is much easier to cook ;) And I like classic meal  ;D

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Never argue with a fool,
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