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In Reply to: RE: Installing a current meter.. posted by DAK on June 18, 2017 at 13:24:13
Using one as an amp meter would mean that there would be or should be a shunt resistor across the meter of an ohm or so. Using a a meter as a volt meter that goes full scale at 0.2 volts means that you have to determine the correct resistor network to measure across.
Measure the resistance of the meter. It is likely that your volt meter will drive the meter to full scale or it has an internal shunt
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Do you think it also needs a shunt resistor
Is this a panel meter you're discussing? If so, I would expect a 200mA meter not to need an external shunt. The only way to be certain is to rig up a variable power supply and fixed resistor for a test. For example, connect a 100 ohm resistor in series with the meter, then slowly begin applying voltage. If the needle goes to full scale quickly, it needs a shunt. Otherwise, it should read half scale (100mA) when you reach 10V.
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Buy Chinese. Bury freedom.
I have a ton of old panel meters. Most labeled "mA" that have a low scale (500mA or less) are 200mV DC meters without an internal shunt.
Amp meters that are above that always have an external shunt. Far cheaper to test than to assume. :-)
"Most labeled "mA" that have a low scale (500mA or less) are 200mV DC meters without an internal shunt."
If I understand, you're saying the meters are mostly 0.2V FS, and it's the difference in coil resistance that provides different FS current readings. That would mean a 200mA DC panel meter is likely to have a coil resistance of one ohm. Is that correct?
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Buy Chinese. Bury freedom.
Most old panel meters need an external resistor network to function correctly. My advice is to test it carefully before putting it in a circuit and destroying it. But yes, if the coil resistance is 1 ohm then it could be used directly as a low current ammeter.
When you measured it, did the needle deflect? A volt meter will put 0.1 to 0.2 volts or so out when measuring resistance. Be careful with a meter as you can easily fry it by putting too much voltage or current on it if there is no internal or external shunt or voltage divider.
If your meter is set up as an ammeter, it will have a shunt resistor that the volt meter measures across. So, if the scale is 0.2A or 200mA, a 1 ohm shunt (internal or external) will make the meter see 0.2v at full scale. Good ol' Ohms Law.
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