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Meter impedance

Discussion in 'Electronic Basics' started by James Harris, Oct 9, 2003.

  1. James Harris

    James Harris Guest

    Beginner question: was trying to check voltages on a microphone
    amplifier board last night but had more than a sneaking
    suspicion that my meter was more of a load than the circuit.
    Transistor base bias currents were set in a resistor ladder that
    had over 1.5MOhms between 9v supply and ground. Query is, Would
    I have been better to use a digital voltmeter due to its higher
    impedance? - They do normally have higher imedances, don't they?
    TIA
    James
     
  2. John Fields

    John Fields Guest

    ---
    You haven't said anything about the impedance of _your_ meter, but
    most DMM's look like 10M ohms shunted by some 10's of picoFarads, so if
    you're using a moving coil meter on a low voltage range it's not likely
    that it'll be that high.

    An easy way to find out what your meter looks like on any given voltage
    range is to measure a voltage and then, without changing the voltage
    range, to make the meter part of a voltage divider, like this:


    +V
    |
    [POT]
    |
    [METER]
    |
    GND

    Assuming the voltage you measured was +V, adjust the pot until the
    meter reading is 1/2 of that, then disconnect the pot from +V and
    measure its resistance. The resistance you measure will be the
    impedance (well, the resistance, actually) of the meter on that range.
    Easy, huh?-)
     
  3. Spajky

    Spajky Guest

    yes, ... yes ...


    -- Regards, SPAJKY
    & visit site - http://www.spajky.vze.com
    Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!"
    E-mail AntiSpam: remove ##
     
  4. John Fortier

    John Fortier Guest

    Older analog multimeters generally had input impedances in the tens of
    killohm range, which made them pretty useless when measuring high impedance
    sources.

    Digital meters have far higher input impedances and if the meter in question
    has a JFET or MOSFET input, you can forget about loading effects.

    I can well remember measuring the anode voltage of an amplifier tube at just
    over thirty volts and receiving a significant belt from it when I tried to
    pull the anode cap off! In fact, my experiences with transmitters have
    generally been less than happy (ref; welding my elbow to a tuning coil!)

    John
     
  5. Steve

    Steve Guest


    Seems you already know the answer! Yes, DMM would be better. My
    analogue meter has a 20kOhm/VDC sensitivity, by DMM has 10Mohm on all
    ranges. Quite a difference when you're looking at 1.5Mohms!

    10Mohm in parallel with 1.5Mohm and you get around 1.3Mohm. Looking
    at a base bias voltage you'd be in a low range, say 2.5V, making for
    50kohm meter impedance. Put this in parallel with 1.5Mohm and you get
    around 48.4kohm!!!

    Be aware that not ALL DMMs have 10Mohm impedance, the cheaper ones
    tend to be more like 1Mohm. You can get a fairly decent one for
    AUD$40-$60.

    niftydog
     
  6. JeffM

    JeffM Guest

    They do normally have higher imedances, don't they?

    As a rule, yes.

    I really hate it when folks specify "DVM" instead of "high impedence meter"
    --or even better, "meter with 10Mohm or higher input impedence".

    There's nothing magical about "digital";
    anyone can build a DVM with a 1Mohm input (or even 1kohm) if he decides to.
     
  7. That isn't quite right. I'm SURE somone will correct me if I'm in
    error on this. A 20,000 ohm/volt meter will present a 60K load set to
    3 volt scale. Set it to 100 volt scale and its 2 meg. So, on a 1000
    volt setting its 20 meg. The load of the digital meter is 10-11 meg
    all the time so it would load less on low voltage settings but more on
    high voltage. My old analog was 50K ohm/volt.

    As for your jolt, 30 volts was the DC component. What was the AC? I
    trust you only made that mistake once (like I did).
    GG
     
  8. John Fortier

    John Fortier Guest

    I was using a rugged old AVO, the military version, and the measured
    voltages were well known to be inaccurate when measuring high impedance
    source voltages. What the actual internal resistance was, I must admit I
    have no idea, but I wasn't the only one to fall foul of this particular
    error.

    After this and the elbow welding incident, I became extremely cautious. As
    to what the AC component was, the transmitter in question was capable of 2
    KW VHF, so I think the answer there is HIGH!

    John
     
  9. James Harris

    James Harris Guest

    I appreciate all the help you guys have given. I'm now convinced
    to look out for a digital meter - and will be checking the
    impedance spec carefully!
    Thanks, James

    he decides to.
     
  10. James Harris

    James Harris Guest

    John,

    Not sure if this is very accurate but it's close enough. On
    10VDC range, I read meter resistance as 150K, and on 2.5VDC
    range resistance as 310K. Am convinced and have bought a DMM
    with a 10MOhm impedance on DCV.

    Thanks,
    James

     
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