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Autocorrecting multimeter?

Discussion in 'Electronic Basics' started by LabMonkey, Nov 15, 2005.

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  1. LabMonkey

    LabMonkey Guest

    Dear Newgroup,

    I came across an interesting phenomenon:

    In the lab we placed a "100M" Ohm resistor (whose resistance we
    couldn't measure directly with the multimeter) in series with a 10MOhm
    resistor (which we could measure). Building a basic voltage divider (3V
    battery) one should get 0.3V on the 10M resistor.

    Now the internal resistance of both multimeters we have is 10M (Fluke
    175, Amprobe 18-A), so we are placing two 10M in parallel. Doing the
    basics: the equivalent resistor combination is then 5M (sorry if that
    was insulting :) which would cause a 0.15V drop to be measured.

    Instead, on the Amprobe 18-A I read the 0.3V, while on the Fluke I get
    0.15V. So apparently the Amprobe somehow realizes it is changing the
    voltage and corrects for it.

    Has anyone ever come across this before?

    Any ideas on how the meter does this?

  2. eehinjor

    eehinjor Guest

    I have done such experiment.this depends on the precesion you want,and
    the multimeter's internal resistance.
  3. Dan Hollands

    Dan Hollands Guest

    I doubt there is any autocorrecting going on - I suspect the input impedance
    is just much higher than speced - may be spec is really "greater then 10M"


    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  4. LabMonkey

    LabMonkey Guest

    Once we know the resistance of the "100M" resistor we then place it in
    series with the multimeter and the battery. In essence we are replacing
    the known 10M resistor with the unknown internal resistance of the

    For both multimeters we are getting a votlage reading of 0.3V - which
    means 10M internal resistance.

    As for the precision of the reading, both multimeters give us three
    digits after the decimal point. So it isn't a problem of rounding.

    Any ideas of how to verify the internal resistance of the meters
    another way?

  5. John Fields

    John Fields Guest

  6. John Fields

    John Fields Guest



    E1R2 3V * 10M
    E2 = ------- = ------------ = 0.2727... V
    R1+R2 100M + 10M



    E1R2 3V * 5M
    E2 = ------- = ------------ ~ 0.1428V
    R1+R2 100M + 5M

    Dunno, but I haven't.

    Nope, but if it was me I'd have it do something like this:

    Vin+<---+ |
    | |
    [R1] [VR1]
    | |
    | |
    [R2] [R4]
    | |

    Where R1 and R2 are 5 megohms each, giving me the 10 megohms input
    resistance I need for the meter, but upset by R3 + R4 being in
    parallel with R2. After connecting to Vin+ and Vin- and and
    detecting current in R3 and R4 I'd turn on VR1 (a FET, probably)
    and ramp it through its resistance range until the current in R3
    went to zero (or until the currents in VR1 and R4 were equal). At
    that point the bridge would be balanced, the input resistance would
    be R1 + R2, and I'd measure the current in R4 (or VR1) and

    E2 = I2R4 = E1
  7. LabMonkey

    LabMonkey Guest

    Thanks for all your help. I will try and find the parts to build the
    circuit and compare it with the capabilities of the meter.
  8. John Fields

    John Fields Guest

  9. John Larkin

    John Larkin Guest

    Sounds like the Amprobe actually has near-infinite resistance on its
    low range; that's not unheard of. It does *not* correct for source

    Put the Fluke and the Amprobe in series, measure the battery, and tell
    us what they report.

  10. Jasen Betts

    Jasen Betts Guest

    What figures do you get if connect both voltmeters in series across the 3V

    I get the feeling that atleast one of the meters isnt 10M resistance.

    for measuring the "100M" resistor how about connecting it in series with the
    meter and a 1M resistor and measuring how much current it passes when a
    known voltage is applied?

  11. LabMonkey

    LabMonkey Guest

    You were right (although it is a little trickier :)

    Placing a 3V battery in series with the Fluke and Amprobe meter I get
    1.5V on both IF I have the Amprobe meter on a range other than mV. If
    instead I have the Amprobe on the mV range the votlage it displays is
    naturally "Ol" while on the Fluke I read 0.095V.

    Doing the math I get 300M internal resistance for the Amprobe on the mV
    range and 10M on any other range.

    Oh the joy of undocumented features.

    Thanks for all the input from all of you!
  12. LabMonkey

    LabMonkey Guest

    Actually I spoke too soon.

    To answer your first question: when having the Amprobe meter on the
    Volt range both meters read 1.5V which is what you get when the
    internal resistance are the same. If instead I place the Amprobe meter
    on the mV range it reads Ol and the other meter reads 0.095V instead of
    1.5V. This indicates that on the mV range the Amprobe meter uses a
    different resistance than on the Volt range. From the initial battery
    voltage, the votlage drop on the Fluke meter (which was in series) and
    the internal resistance of the Fluke (10M) I got 0.3GOhms.

    I will try out your suggestion once I find a 1MOhm resistor.

    Now onto the latest problem. When I place the Amprobe meter in series
    with the 100M resistor and apply the 3V battery to both I get 0.3V drop
    displayed on the meter when it is in the Volt range. If I place it in
    the mV range I yet again read Ol. But having an electrometer I used
    that to measure the votlage drop on the 100M resistor, which is 0.2V.
    Solving again the simple votlage divider equation I now get out an
    internal resistance of 1.4GOhms for the Amprobe meter (instead of the

    I will do it again tomorrow just to verify the result.
  13. John Fields

    John Fields Guest

    Try this:

    3V E1
    | |
    [Rx] [Rm]
    | |

    With Rm (your meter) on the millivolt range, adjust Rx until you get
    some arbitrary voltage reading (E2) on the meter. Record the value
    of Rx and E2.

    Now solve for the parallel combination of Rx and Rm:

    E2 R1
    Rt = -------

    Now, knowing Rx, solve for Rm:

    Rt R1
    Rm = -------
  14. John Fields

    John Fields Guest


    Rx Rt
    Rm = -------
  15. John  Larkin

    John Larkin Guest

    The Amprobe may be "infinite" resistance with some protection diodes,
    so things may not be linear.

    Connect a good 1 uF film capacitor across the Amprobe input, set to mV
    range, apply 100 mV or whatever, disconnect source, and watch it
    discharge. Then you can compute internal resistance.

  16. LabMonkey

    LabMonkey Guest

    Ran into some problems:

    the meter has an auto off feature which I can't turn off (at least I
    couldn't find anything in the instruction manual). So after 10 minutes
    it turns itself off. So with the capacitor experiment this only rules
    out internal resistance with time constants less than 10 minutes. If
    the internal resistance were 300M and the capacitor was 0.9mF, then the
    time constant would be 4.5 minutes - which is not what I saw. After ten
    minutes it had decayed to 0.200mV instead of 0.230mV.

    As for the other experiment, the electrometer I use to measure the
    voltage on the 100M resistor has some problems. I consistently got out
    that the experimental equivalent parallel resistance was greater than
    Rx, which naturally can't be.

    I will try and find an electrometer which is more reliable, but as it
    is the weekend I will not be able to get my hands on anything until

    Thanks for all of the help :)

  17. LabMonkey

    LabMonkey Guest

    I found out how to trick the meter in staying on: quickly press the
    "hold" button twice and it resets the clock for the 10 minute

    Only problem is, when I did that I didn't get an internal resistance of
    1.4G. The time constant would have been about 20 minutes and I had it
    running for 4.5 hours with it only decreasing to 160mV from 257mV.

    So either the capacitors were doing something funny, or the internal
    resistance some how changes between even higher resistors when on the
    "mV" range.

    I am still trying to get a hold of an electrometer
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