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Measuring High Voltage DC while drawing low currents (nanoamps)

Discussion in 'General Electronics' started by Andrew, Aug 10, 2004.

  1. Andrew

    Andrew Guest

    I'm trying to measure DC voltages in the range of 100-1000V, however a
    normal multimeter with a 10 megaohm impedance won't work because it
    draws too much current (my device is very resistive and runs only
    10-50 microamps). I have even tried a high voltage probe and it still
    draws too much current. Does anyone know of a device (or how to make a
    device) to measure high voltages (100-1000V) while drawing only
    nanoamps of current?

    I have also tried putting a 2.5 gigaohm resistor in series with the
    multimeter and that doesn't give accurate voltage measurements.

    Thank you in advance for your help and ideas.

    Andrew
     
  2. Hold onto that 2.5 Gohm resistor, and connect it in series with a
    picoammeter. i = E/R = 1E3/2.5E9 = 4E-7 amps

    Or, in plain English: this rig would read 400 picoamps. per volt

    Brian W
     
  3. Isn't a resistor in the Gohm range the same as an open circuit?
    I've measured non-zero current flowing through a supposedly
    open mechanical switch.

    Andrew may wish to measure the current through the Gohm resistor
    and multimeter series combination and compute the voltage.
     
  4. Well, your heart is in the right place:

    the average DVM reads to 0.1 microampere,
    so on a 1000 volt source it would read 0.4 microamp.
    plus or minus 0.1

    That's not great resolution!

    Brian W
     
  5. John Larkin

    John Larkin Guest

    Some of the old Fluke differential voltmeters used a null detector
    working against a precise 1000 volt supply, essentially infinite input
    impedance at null. They're a nuisance to operate (must be manually
    nulled) but they're very precise and very cheap on ebay.

    Or: charge a known value capacitor for a while, say 0.1 uF. Then
    parallel it with another (discharged) cap of, say 100 uF. Both must be
    good-quality film caps. Ignore the small spark when paralleled. Use a
    high-impedance (fet opamp + dvm) circuit to measure the resulting
    voltage, then multiply by 1001.

    John
     
  6. Aaron

    Aaron Guest

    I did as you suggested and the output is still a sinewave. I have the scope
    set to DC Coupling 5 volts per division trigering on the upward slope at 5Ms
    /div.

    Is the scope bad?
     
  7. Trek Inc. Instruments

    Electrostatic Volt meters. These will be accurate within a few
    percent. This is for what you want. Be prepaired to lay out some cost
    for any of these.

    http://www.trekinc.com/products/esvm.asp

    Jerry G.
    ========



    (Andrew) wrote in message
     
  8. Bob Stephens

    Bob Stephens Guest

    You can try lifting the third prong on the scope's AC mains plug.
     
  9. Since the scope is showing SOMETHING, it is clearly working enough to
    show your sinewave. Bob Stephens suggests defeating the ground pin of
    the AC cord. This can be unsafe for the unsuspecting though it can be
    very useful. BTW, I carry a ground lifter in the pouch on top of my
    Tek scope for this very thing.

    I suggest checking the ground lead on the scope probe. They frequently
    get broken and without the ground reference, your scope would display
    what you describe.

    gg
     
  10. Robert Baer

    Robert Baer Guest

    1) 1 1Gohm or a 10Gohm resistor is very definitely *NOT* an open
    circuit. In certain cases, such values can act like a short circuit.
    2) Perhaps the current you "measured" came from somewhere else; the
    actual switch is OK. The body may be bad, or there may be alternate
    paths that you did not recognize.
    3) The lowest scale of any multimeter that i have seen is 33 microamps
    full scale; with 50 microamps being "typical" of the decent quality
    multimeters.
    4) A 1Gohm resistor will load a 1000V supply at 1uA; a significant
    percentage of the specified load (10-50uA).
    5) Putting a 1K ohm resistor in series with the supply (and load),
    will gove a voltage drop of 1mV at 1uA; within the capability of some
    DVMs - and certainly not unduly reducing the available voltage for the
    load. A 1Meg resistor would develop 1V at 1uA, which may be tolerable
    (1% worst case for specified voltage range.
     
  11. Robert Baer

    Robert Baer Guest

    1) He mentioned a multimeter, not a DVM. A multimeter gives limited
    readability and accuracy.
    2) The "average" 3 1/2 digit DVM can read down to 20nA full scale, as
    long as one can tolerate the added 10Meg series resistance; the 200V
    scale gives a 20uA full scale sensitivity (with an intolerable voltage
    drop).
    If a DVM is to be used, then use a shunt is recommended, so that the
    IR drop is minimized to 200mV full scale.
     
  12. Yes, he mentioned multimeter. Taking the Radio Shack digital
    multimeter (Cat# 22-811) as an example, it can read to 0.1
    microampere as I suggested. That gives very limited resolution with
    a 2.5 Gohm series resistor, as I suggested.
    What multimeter, or DVM did you have in mind?
     
  13. You are thinking of a moving coil movement (called a D'Arsonval
    meter to engineers of a certain vintage.) These days, multimeters are
    digital displays, in general.

    I take it you are thinking of a way of measuring the load current of
    an HV supply. The question refers to measuring the terminal voltage
    of a high impedance source, doesn't it?

    Brian W.
     
  14. Robert Baer

    Robert Baer Guest

    The majority of DVMs have a 200mV scale and 10 meg input resistance.
    Parallel with 10K shunt (which adds negligible error), and one has
    (2E-1 volts)/(1E4 ohms) = (2E-5 amp) or 0.2 microamp FS sensitivity;
    change the shunt as needed - just remember the error increases as the
    shunt value increases (unless one compensates{ ie using 101K instead of
    100K).
     
  15. Robert Baer

    Robert Baer Guest

    The term "multimeter" is generally reserved for analog (D'Arsonval)
    meters, and the term "DVM" stands for "Digital Volt Meter".
     
  16. Robert Baer

    Robert Baer Guest

    In the special case where one knows the value of the load, then the
    indirect method of measuring load current and calculating the supply
    (load) voltage from that, is acceptable.
     
  17. There is the indirect way. Use two voltmeters, and a reference power supply.
    One volt meter measures the output of the adjustable power supply, the
    other voltmeter measures the difference between the reference voltage
    and the item being tested. The difference voltmeter can have a high value
    resistor in series if you want. You adjust the reference power supply until
    the difference voltmeter reads zero and draws zero current (thus not
    loading the item being tested. Then use the second meter to read the
    reference voltage. This is useful if the voltage to be monitored is not
    changing. The procedure could be automated with operational amplifiers.

    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
     
  18. On Thu, 12 Aug 2004 18:49:12 GMT, Robert Baer
    Hmmm....2E-5 amps is 20 microamps FSD.

    Brian W
     
  19. On Thu, 12 Aug 2004 18:52:58 GMT, Robert Baer
    Hmmm...I would agree that perhaps 30 years ago, people thought of
    multimeters as having moving coil movements.
    Try a google search on "multimeter" now to get a feel for the current
    understanding.

    Brian W
     
  20. Robert Baer

    Robert Baer Guest

    Sorry; i goofed. Thanks for the correction.
     
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