identifying a panel meter

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

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!!!

meter.

to

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

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

[snip]

I hadn't thought of that. Good point.

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

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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

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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

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.

--
Roger J.

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

--
Roger J.