# identifying a panel meter

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

1. ### tempus fugitGuest

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?

Thanks

2. ### Randy DayGuest

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
meter.

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
meter:
R
o------/\/\/\/\---o----o
| | |
--- / \ /
- (_/_) \ Shunt
| \_/ /
| | |
o-----------------0----o
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

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...

BE SURE THE CURRENT LIMITING RESISTORS YOU USE
ARE OF SUFFICIENT WATTAGE TO HANDLE THE CURRENT!!!

3. ### Don KellyGuest

The meter may already have internal resistors as it is likely to be a
voltmeter.

4. ### Randy DayGuest

Don Kelly wrote:

[snip]
I hadn't thought of that. Good point.

5. ### tempus fugitGuest

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)?

Thanks

6. ### Michael BlackGuest

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.
Michael

7. ### Roger JohanssonGuest

I think you are talking about an audio volume meter, if this is the same

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.

8. ### Roger JohanssonGuest

Ooops, should have disconnected the circuit before measuring the
resistance in the pot.

9. ### tempus fugitGuest

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.

THanks