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Low leakage capacitors

Discussion in 'Electronic Design' started by Paul, May 1, 2009.

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  1. Paul

    Paul Guest

    (Sorry, forgot to post to other groups.)

    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
    -->

    http://www.cde.com/catalogs/MCM-MIN.pdf

    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
     
  2. Guest


    ">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
     
  3. cassiope

    cassiope Guest

    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!
     
  4. James Arthur

    James Arthur Guest

    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
     
  5. Ian Iveson

    Ian Iveson Guest

    Paul wrote:

    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
    -->

    http://www.cde.com/catalogs/MCM-MIN.pdf

    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
     
  6. Ian Iveson

    Ian Iveson Guest

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

    Greegor Guest

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

    http://groups.google.com/group/sci.electronics.design/browse_frm/thread/c1e9a32fc3c6a53e

    Newsgroups: sci.electronics.design
    From: (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
     
  8. Jasen Betts

    Jasen Betts Guest

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

    Alex Guest

    (Sorry, forgot to post to other groups.)

    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
    -->

    http://www.cde.com/catalogs/MCM-MIN.pdf

    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
     
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