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identifying a panel meter

Discussion in 'Electronic Basics' started by tempus fugit, Nov 1, 2004.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I'm thinking about building a project that requires a 0-100mA panel meter.
    Being the packrat that I am, I have a panel meter scrapped from an old
    stereo that is for signal stength. Can I use this meter? Is there a way to
    test it to see if it would be usable for current reading, and what it's
    range might be?

  2. Randy Day

    Randy Day Guest

    Sure. Get a voltage source (9v battery or
    power supply) and some resistors varying
    from 100k down to 100 ohms or so. You might
    want the low-ohm resistors to be higher-
    wattage types, just in case.

    Starting with the highest, put each in
    series with the meter & voltage source and
    observe the deflection (if any). As you
    step down through the resistors, the meter
    should deflect further and further.

    When you find a resistance value that deflects
    the meter close to its maximum, simply take
    your V and R values and plug them into I=V/R
    to get the approximate max current for the

    I *wouldn't* recommend using a potentiometer
    for this; if the meter is a medium- to high-
    current type, you'd probably fry the pot.

    Oh yeah, even if the meter is a 10ma, for
    example, you can probably still use it in
    your project; just create a shunt resistor
    out of wire and put it in parallel with the
    | | |
    --- / \ /
    - (_/_) \ Shunt
    | \_/ /
    | | |
    (created by AACircuit v1.28 beta 10/06/04

    You can try calculating the proper value for
    the shunt resistance based on the gauge/length
    of wire you use, or you can trial and error it
    using the same method above...

  3. Don Kelly

    Don Kelly Guest

    The meter may already have internal resistors as it is likely to be a
  4. Randy Day

    Randy Day Guest

    Don Kelly wrote:

    I hadn't thought of that. Good point.
  5. tempus fugit

    tempus fugit Guest

    So, is there a difference between a volt and current meter (i.e., if it is a
    voltmeter, could I use it to measure current for this project)?


  6. A voltmeter is an ampmeter with a series resistor to convert the voltage
    to current.

    If this was a cheap tuning meter from a stereo chances are pretty good
    that it's a fairly sensitive meter (in the hundreds of uA range) and
    expects an external resistor for measuring voltage. If you want to measure
    current, in your case you'll very likely need a shunt resistor in order
    to measure the higher currents you want.
  7. I think you are talking about an audio volume meter, if this is the same
    thread I read the other day.

    Such meters seldom have any internal resistors, they contain just a
    movable coil in a magnetic field.
    (If there actually is a resistor inside the meter you can open it and
    remove it, if it doesn't fit into your plans. You can also glue a new
    scale inside it, if needed)

    That coil has some resistance, maybe 300 Ohm, and the meter gives full
    deflection for 350mV or so.

    So it can be seen both as a current meter and a voltage meter.

    If you want to use it as a voltmeter for higher voltages you need to put
    a resistor in series with it. If you want to use it as a current-meter
    for higher currents you need to put a resistor in parallell with it.

    Or I can explain it like this:

    Such a moving coil instrument can not handle much current or much

    If you want to use it as a voltmeter you need a resistor, 1k-100k, in
    series with it, which can take most of the voltage, so the meter itself
    only need to handle the voltage it is made for, 350mV or so.

    If you want to use it as a current meter you need to put a resistor,
    0.1-100Ohm, in parallell with it, to take most of the current, so the
    meter itself only uses the current it is made for, like 0.1-10mA.

    The first thing you should do is to find out what voltage and current the
    meter itself needs for full deflection. You can measure the inner
    resistance of the meter.

    When you know the inner resistance and the maximum voltage of the meter
    you can calculate what resistor you need to connect in parallell or in
    series to get the current or voltage meter you need.

    When you test such a moving coil meter you can safely use a pot, 10-200k
    in series with it. The current for maximum deflection is very low,
    typically 1mA.

    Put a 200k pot in series with the meter and a 3-9Volt battery, start at
    maximum resistance in the pot. Lower the pot slowly until the meter shows
    full deflection. Measure the voltage over the meter, measure the current
    through it. Measure the pot too. Then disconnect this circuit.
    Use a resistance masuring DMM to test the resistance in the meter.

    If you used a 9Volt battery, and you measured the pot when the meter
    shows full deflection you know what resistor to use in series to create a
    voltmeter for 9Volt.
  8. Ooops, should have disconnected the circuit before measuring the
    resistance in the pot.
  9. tempus fugit

    tempus fugit Guest

    Perfect - I knew it had to be something like that, but I wasn't exactly sure
    what the inner workings of a meter were (except for the coil and magnet).

    I'll mess around with it and see what I can come up with.

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