Low leakage capacitors

,000 M=A5=D8

=A5=D8" ?

">I'm looking for a cap that would contain as much charge as possible

Aluminum Electrolytic (as long as it doesn't get too warm.)

George Herold

,000 M=A5=D8

=A5=D8" ?

The "need not..." probably means "once our tests tell us we've reached a "high enough" value, testing stops.

One of the SED regulars did (is still doing?) a long-running test of this insulation-resistance question. Sorry, I don't recall who it was, but if you search the SED archives you may find what he learned. My dim recollection was that the better capacitors were actually better than their specs suggested (probably the better films).

Good luck!

Polycarbonate & polypropylene are awfully good. You'll lose more through fingerprints on the casing than leakage through the dielectric.

Tom Bruhns filled some up and measured their discharge over many months (on that single filling), posting the results to s.e.d.

HTH, James Arthur

Hi,

I'm trying to find the lowest leakage capacitor, somewhere in the range of 0.01uF to 1uF, *might* be able to live with

0.001uF, for DC use. I'm not sure if PTFE (teflon) is the best choice, but here's one

-->

It says, "Insulation Resistance: 1000 M¥Ø*¥ìF Need not exceed 100,000 M¥Ø at 25 ¨¬C" What exactly do they mean by "Need not exceed

100,000 M¥Ø" ?

Also I heard polystyrene is good.

I've performed various RC time constant tests on caps by placing a voltage source on the cap for a period of time, removing the voltage source and time it, then measure the DC voltage after a certain time period, and calculate the parallel resistance. It appears that the longer the voltage source is held on the cap results in more parallel resistance-- possibly dielectric absorption.

So this makes it more difficult to find the best type of caps for this use. IOW, perhaps one cap might have less leakage after 1 minute, but after 10 hours perhaps another type of cap might have less leakage.

I'm looking for a cap that would contain as much charge as possible for about one day. The voltage levels are around 1 volt.

It may not be true that one type of dialectric is necessarily better than another. The exact composition of each type can vary considerably, in terms of purity, structure, water content, etc. etc. Also manufacturing quality can make a big difference. You may be better off looking for caps specifically designed for low leakage/capacitance ratio, regardless of what they're made of.

Have you looked at C0G ceramics?

Ian

That's why it's best not to post in HTML...

James Arthur posted the lead, here's the thread.

Newsgroups: sci.electronics.design From: snipped-for-privacy@aol.com (Tom Bruhns) Date: 3 Jan 2003 15:46:02 -0800 Local: Fri, Jan 3 2003 6:46 pm Subject: Low leakage parts

Some of you may recall some postings I've made over the past couple of years about the self-discharge rate of polyester and polypropylene caps. The time constants I saw were on the order of a few years for the polyester and over 50 years for the polyprops.

A month or so ago, someone asked about making a simple toggle circuit for turning 12V lights on and off, and I posted a couple solutions. One of them was a "this is really simple, but it probably won't work very well" circuit using just a capacitor to hold the voltage on the gate of a power mosfet. Well, I built that ckt, using an 0.01uF cap across the gate-source, and toggled it "on", so the cap was charged to about 12V. Then I disconnected the power and went on a holiday trip. Just came back, and it's still in the "on" state, three weeks later. Sooo...I'd say that modern power mosfets also have pretty low gate leakage current. To hold the voltage above the nom. 3V required to turn the mosfet on, the average leakage must have been less than

50fA, assuming 21 days, 0.01uF and 9V delta (and no arithmetic errors). (It was cool, about 18C, and likely wouldn't do quite so well inside a car with the windows rolled up in Phoenix in the summer...)

Cheers, Tom

No html there, what appeared to be missing was a charset header.

Hi,

I'm trying to find the lowest leakage capacitor, somewhere in the range of 0.01uF to 1uF, *might* be able to live with 0.001uF, for DC use. I'm not sure if PTFE (teflon) is the best choice, but here's one

-->

It says, "Insulation Resistance: 1000 M¥Ø*¥ìF Need not exceed 100,000 M¥Ø at 25 ¨¬C" What exactly do they mean by "Need not exceed 100,000 M¥Ø" ?

Also I heard polystyrene is good.

I've performed various RC time constant tests on caps by placing a voltage source on the cap for a period of time, removing the voltage source and time it, then measure the DC voltage after a certain time period, and calculate the parallel resistance. It appears that the longer the voltage source is held on the cap results in more parallel resistance-- possibly dielectric absorption.

So this makes it more difficult to find the best type of caps for this use. IOW, perhaps one cap might have less leakage after 1 minute, but after 10 hours perhaps another type of cap might have less leakage.

I'm looking for a cap that would contain as much charge as possible for about one day. The voltage levels are around 1 volt.

Any help is greatly appreciated! Paul

Alex: To store "as much charge as possible" at 1V only probably a supercap of several Farades will be the best.

As far as dielectric absorbtion is concerned... I have a sample of a Russian teflon capacitor 0.1uF/600V. I tried to measure it on an RLC bridge. On all four test frequencies from 100Hz to 100kHz the bridge read a pure capacitor -- just could not detect any series resistance or parallel conductance! For comparison a polyester general purpose "greencap" does show losses on the RLC meter and a paper capacitor would look absolutely dreadful.

Regards, Alex