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Have meter that reads volts. Would like to...

Discussion in 'Electronic Basics' started by Don Bruder, Mar 15, 2005.

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  1. Don Bruder

    Don Bruder Guest

    Heya folks, got a cheap car voltmeter - it reads in (duh!) volts. 6-20
    of 'em, to be exact. Nothing particularly fancy or ultra-accurate, just
    "a voltmeter" that'll be handy for keeping a general eyeball on the
    charge state of the system.

    I know that a meter that reads volts and a meter that reads amps are
    often constructed around the same meter movement, as is evident from the
    analog VOM I've got that can read all three values of Ohm's law without
    having to change the movement - just the setting of a selector.

    Problem is, I've never managed to wrap my head around the "how-to"
    involved in changing from one to the other. Every explanation of the
    idea I've encountered leaves me scratching my head.

    So what I'm looking for here is a way to set it up so that I end up with
    a dual-purpose "When left alone, it displays volts, but when <something
    - button push, switch toggle, etc> is done, it displays amps" meter. I
    COULD go down to the parts store and buy its amp-reading mate, but I've
    got this minor problem called "lack of place to put another one". If I
    can make this one "convertable", I'd like to go that route.

    What do I need to do to make a voltmeter read either volts or amps on
    user demand? Obviously, I'm going to need to create/calibrate an "amps"
    scale for it, but that's pure trivia - Part of the "trim" when it gets
    done. I need the "nuts and bolts" at this stage.
  2. When you open the instrument and inside you will find a meter
    movement mechanism and a resistor in series with it, or two series
    resistors which you can switch between, if there are two different
    voltage scales.

    For a meter mechanism to be used as a voltmeter you need a series
    resistor which can absorb most of the voltage, because the mechanism
    itself can only handle something like 50-300mV.

    If you want to use that mechanism as an ammeter you need a parallell
    resistor which can handle most of the current, because that mechanism can
    only handle something like 1mA by itself.

    Inside an analog multimeter you will find several series resistors for
    the voltage scales, and several parallell resistors for the amps ranges,
    all switched in by a multiposition switch.

    The easiest way to find out what series resistor you need for a certain
    voltage range is to connect a highohm pot in series and set it to its
    full resistance. Then connect a suitable test voltage in series with the
    pot and the mechanism. Adjust slowly towards lower resistance until the
    meter shows the value you want for that voltage.

    Measure the pot and replace with a fixed resistor. Or with a fixed
    resistor and a small trimmer, if you want to be able to adjust it later.

    For current you take a low value resistor, like a piece of wire, or a bar
    of metal. Send a suitable current through it. Put the two connections
    from the mechanism together and touch the wire with them. Pull the two
    connections slowly apart until you see the meter show the value you want
    it to show for that current, solder the wires there and cut off excess
    length of the shunt wire/metal bar.

    Or learn to calculate what resistors you need :)
    Use another instrument to measure the voltage and current of the meter
    mechanism when it shows full scale.

    If it has a voltage of 100mV and a current of 1mA you know that the inner
    resistance of the meter is 0.1/0.001=100Ohm

    If you want a voltmeter for 100Volt you must add series resistance which
    makes the total resistance a thousand times higher, because 100V is a
    thousand times greater than the 100mV of the meter.

    100Ohm * 1000 = 100kOhm. So add a 100k resistor in series.
    (that will become a total of 100100 Ohm, a little too much, but a
    difference of one tenth of a percent doesn't matter)
  3. PeteS

    PeteS Guest

    All moving coil meters (that's what's in your analogue VOM) ultimately
    measure current. for current ranges, we shunt (parallel) the path
    around the meter and for voltage we add resistors in series to set the
    current for the meter. Although this type of movement is common, moving
    iron meters are often used for your battery / charge indicator (in fact
    it probably is as it ranges from 6-20V).

    The first thing to figure out is which type of movement it is, and then
    follow one of the various guides to designing custom ranges.
    (There are a profusion of such guides on the web - try these search
    moving coil meter design
    moving iron meter design

    A couple of things

    1. Moving coil meters usually have a sensitivity of 20k / volt - i.e. a
    full scale current of 50uA.
    2. Moving iron meters are robust (which is why they tend to be used in
    the automotive industry)
    3. Moving iron meters are slower to respond, but can give an average of
    a fluctuating line.

    So, once you've figures out the movement type, come back and someone
    will no doubt give you a full guide on making the necessary
    modifications for your little project.


  4. Don Bruder wrote:

    Meters that need only a small voltage to read full scale and also need
    only a small current to read full scale are well suited to be made
    into a volt-amp meter. The trick is to arrange with external
    resistors to have the full scale requirement to be met with the
    desired full scale measurement.

    Lets say you have a magnetic movement that hits full scale at 1
    milliamp of current and has 10 ohms of internal resistance. Ohm's law
    relates voltage, resistance and current (Resistance = volts / amps) so
    when 1 milliamp is passing through that internal 10 ohms, there will
    be a 0.01 volt drop across the meter.

    To arrange for this meter to be a volt meter that reads full scale at
    20 volts you need to add more resistance in series so that only at
    that voltage will the 1 milliamp full scale current pass through the
    meter. Since that occurs with 20,000 ohms (ohms law again), total and
    the meter already has 10 ohms, you would need to add another 19,990
    ohms in series. (Resistor in series simply add up)

    If, instead you wanted to make the meter read full scale for 10
    amperes, you would need to parallel it with a big, low value
    resistance, such that the parallel combination dropped the full scale
    voltage of 0.01 volt when 10 amps passed through both the resistor
    (mostly through the resistor) and the meter's 10 ohms. 0.01 volt/10
    amps = 0.001 ohm. When working with parallel resistors, it is easier
    to convert them to conductances (1/resistance), since conductances in
    parallel simply add up. Then convert the effective conductance back
    to resistance with a 1/conductance calculation.
    So 1/10 ohms + 1/shunt resistance (that carries most of the 10 amps) =
    1/ total resistance
    So the shunt resistance needs to be about... well crap, it is so close
    to .001 ohm ohm that the meter's 10 ohms makes almost no difference.

    You leave the .001 ohm shunt in series with the current, permanently,
    and switch the meter to either be connected across the to be measured
    voltage with a 19,990 ohm resistor in series, or connected directly
    across the shunt resistor. The shunt will lower the voltage to
    whatever load is in series with it by 0.01 volt when 10 amperes is
    passing through.

    Now back to your meter. If it already has the equivalent of the
    19,990 ohm resistor already inside the case, you will not be able to
    use it without doing surgery on it, since this resistor cannot be in
    series when you make current measurements. You need ot start with a
    simple meter movement with both a low current full scale requirement
    (so the voltage dropping series resistor will not get so hot) and a
    resistance (equivalent ot saying that it has a low full scale voltage
    requirement) because that full scale voltage is what the shunt must
    drop top measure current.

    By the way, the shunt resistor may be able to be a piece or wire that
    is already involved in getting current ot the load. If you can pick
    two spots on it that have the desired full scale drop you can just
    connect two small wires to those points and take them back to the
    meter. The difficulty comes in knowing where those two points are.
  5. Andrew Holme

    Andrew Holme Guest


    If this is for a car, due to the extremely heavy current, instead of
    splicing series resistance into the battery lead, you might want to
    consider a clamp-on magnetic current sensor.
  6. Surfing the web a little I found this tutorial about how to make shunts
    to construct an Amp meter.

    This circuit is just one of many on this site:

    The circuits on this site are beautiful examples of easy to read
    schematics suitable for beginners. There are tutorials about IC's like the
    555 and 741 and other components like capacitors, mosfets, relays, etc..
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