tempus fugit said:
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)?
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
voltage.
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.