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Freaky Amazing DMM?!

Discussion in 'Electronic Equipment' started by Paul, Jan 15, 2009.

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  1. Paul

    Paul Guest

    I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over*
    100Mohm impedance in 400.0mV mode. So I charged a 4.7uF Mylar
    capacitor to 36mV DC, and then placed the AM-240 (while in 400mV DC
    mode) across the Mylar cap. After 25 minutes and 40 seconds it was
    35..2mV. That comes to 14Gohms. So I thought it may be due to bias
    current or offset voltage, and reversed the DMM polarity. Same
    results. So then I charged the Mylar to 200mV. Same results.

    Then, I measured the parallel resistance of my 4.7uF Mylar cap by
    charging it to 184.8mV, disconnected the AM-240, and 1050 seconds
    later connected the AM-240 and measured 177.1mV. That comes to
    5.25Gohms, which is what I would expect from this capacitor. Actually,
    for months I've been telling people my guesstimate for this cap is

    Anyhow, what kind of circuit are they using in this AM-240? It
    appears as if it *resists* change! When it is disconnected it tends
    to somewhat maintain the DC voltage, regardless of polarity. IOW, lets
    say it's measuring the DC voltage on the Mylar cap, and it's 180mV.
    Then one of the leads is removed. The AM-240 DC voltage decreases a
    bit, not too much, but it slows down, and tends to hang around, say
    160mV. If I reverse the polarity, to -180mV, the same thing happens
    except it hangs around at -160mV. If the AM-240 was measuring say
    35mV, and then disconnected, it tends to hand around at oh 20mV to

    Very interesting DMM. Not sure to like or dislike this.

    Thanks for any info.
  2. Nothing new there, many DMM's have selectable "high impedance" or "HI-Z"
    modes on the mV range. e.g. the Fluke 87.
    The input impedance is so high that the small input capacitance maintains a
    charge when you disconnect.
    A bit disconcerting when your meter doesn't read zero, but it comes good
    when you connect a source.

  3. Paul

    Paul Guest

    BTW, here's a bootstrapped input amp circuit -->

    Thanks Clifford.

  4. Paul

    Paul Guest

    I don't see it listed on the spec sheet, but I've seen a lot of other
    similar Amprobe meters that are around 30pF input.

  5. Eeyore

    Eeyore Guest

    Looks like you bought a good 'un ! There are some bargains out there if
    you search.

  6. Max65

    Max65 Guest

    Probably the input of the AM240 has a non linear devices (maybe just
    the input protection diodes).
    If I'm right, when you connect a capacitor the non linear device works
    like a RF detector that charges the capacitor to a voltage lesser than
    its forward voltage level. The multimeter wires work as the antenna in
    this case.
    Try to twist the multimeter wires together and see what happen.

    Have fun.
  7. Paul

    Paul Guest

    The AM240 is not charging the cap per say. So far it has merely tried
    to *maintain* the caps charge. So far, regardless of the caps charge,
    or its polarity, the AM240 has tried to maintain the charge.

    I've never seen a DMM do this before. Maybe it's possessed. ;-)

  8. Paul

    Paul Guest

    Yeah, Mylars are good, but even they are imperfect. I've spent
    probably too much time testing for dielectric absorption over the
    years. Although Mylars have hardly no dielectric absorption. I guess
    there are caps better than Mylars in terms of dielectric absorption.
    An air gap cap, but how big would it have to be to make 4.7uF?

  9. James Arthur

    James Arthur Guest

    Sure, mylars leak, and the meter has some bias current too,
    however small it might be. If you hit the magic voltage
    the two might even cancel.

    Tom Bruhns posted some remarkable polypropylene cap leakage
    measurements to SED a few years back. Garden-variety 0.1uF
    caps had 50-year time constants.

    James Arthur
  10. Paul

    Paul Guest

    In the first link he mentions 50 years for a 0.1uF cap. That comes to
    1.6e+16 ohms! Here's the datasheet of my Mylar capacitor or very
    similar-- don't know the manufacturer of my 4.7uF Mylar -->

    The spec shows an insulation resistance of >=10 Gohms (20 °C, 100 VDC,
    60 s), and >=2 Gohms (20 °C, 500 VDC, 60 s).

    Insulation resistance -->

    I guess it's possible to make a cap with 1.6e+16 ohms, but I would
    tend to first believe it's due to either dielectric absorption or
    small signal rectification of electromagnetic signals. As you know,
    any two atoms forms a junction. There are a lot of impurities in
    capacitors, thus forming diodes, albeit poor diodes. It's not really
    possible to have all of the poor diodes counter act each other out,
    which is probably why a cap, even a good cap can produce a DC voltage
    in a good electromagnetic field. In the post he says that he did not
    place the experiment in a shield. From my experience that's normally
    unnecessary, unless you're near a wifi or radio station, but you never
    know. It's difficult to say, but my best guess, and it's just a guess,
    that he was seeing dielectric absorption, and perhaps a bit of DC
    voltage produced by rectifying electromagnetic signals.

    My quick and dirty test of my 4.7uF Mylar showed 5.25Gohms parallel
    resistance, but I didn't spend much time logging the data. It's
    probably a lot higher given time to relax. It's possible the
    insulation resistance would have increased over time.

  11. James Arthur

    James Arthur Guest

    I'd say clean the cap carefully with alcohol, soak it
    at voltage for a few days, then measure leakage.

    But if you think it's getting charge from the air, a
    shielded box would easily answer the question.

    Meanwhile, either way you got a nice deal on a DMM
    with a very high input impedance.

    Ain't nuthin' wrong with that.

    James Arthur
  12. krw

    krw Guest

    A craftsman never blames tools for his failures. Hackers, on the
    other hand...

  13. Apples and oranges.

    "A craftsman" works the medium he ends up selling directly and
    irrecoverably with the tool or tools involved so his 'toolwork' is his
    work product, and one can never blame the tool for the finished product,
    only the tool pusher.

    "A hacker" doesn't use a 'tool', they hack. When they fail, it is
    because they didn't hack long enough or correctly enough. They *ARE* the
    tool that failed, and ALL failures are their fault.
  14. krw

    krw Guest

    That is certainly true (though perhaps your heirs are the ones who
    will find you), but doesn't modify my statement. In this case, the
    tool *can* be used as long as the one using it knows what he's
  15. krw

    krw Guest

    No, your incompetence didn't pass by anyone.
    It is not a limitation of the tool, only the user. Lower the
    impedance if it's too high.
    I'll take your word that you don't do "craftsman like" work. You
    clearly don't know how.
    *I* know what I'm doing. You've clearly demonstrated that you
  16. Rich Grise

    Rich Grise Guest

    Then pick the right voltmeter for the job, duh.

    Hope This Helps!
  17. YD

    YD Guest

    What's wrong with using a hi Z voltmeter in a live panel? I do it more
    often than I really care for, and never have a problem.

    - YD.

    File corruption detected. Select option:
    1 - Call the cops
    2 - Call the press
    3 - Bribe it

    Remove HAT if replying by mail.
  18. It's not the right tool for the job.
    A meter which gives you a false reading with bad (or no) contact is a
    BAD idea in dangerous high voltage/high energy circuits.
    That is why proper meters with low impedance (Kohms) modes exist for
    the job, e.g. Fluke 113, 110 series, 289 etc.
    Oh dear...

  19. krw

    krw Guest

    There isn't anything wrong with it. Analog meters are dead. High
    impedance digital meters are only a problem if there is a loose nut
    inside the panel, holding the leads.
  20. Then I suppose Fluke and others make all those purpose designed Low-Z
    electrical meters for loose nuts then?

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