The Shop > Electronics & IC Programing

DIY Ampermeter

<< < (2/2)

one_rod:
Thanks for the explanation chaps.

As I suspected;  "missing something simple"  :loco:

We buy a lot of ammeters and voltmeters at work. Never thought of them as being the same equipment with a different label. 




one_rod.

latheman:
Never thought of them as being the same equipment with a different label.


er, not quite. Moving coil meters, to give them their correct name, all work on current in that the movement is proportional to the current flowing through the device. (Actually, the current induces a magnetic field in the moving coil which acts as a limited movement motor.) Anyway, because the coil has resistance, it follows that for any current flow, there will be a voltage developed across the coil: typically a coil may measure 2000 ohms, and have a maximum current capacity of 50 micro amps, hence the maximum voltage will be 0.1v for a full scale deflection. Therefore, provided these maxima are not exceeded, the meter will read both current and voltage simultaneously with the scale being written to suit. Usually, for a meter to read voltage, they are specified as being so many ohms per volt, eg for this example, 20 000 ohms per volt and this does give an easy way to calculate the resistance of the particular range being used.

In order to read current in excess of the, eg 50 micro amps, it is usual to place a low value shunt in parallel with the meter. If therefore it was required to read 1amp, the meter would take 50 microamp whilst the shunt would take 1 amp less the 50 microamp, hence the value of the shunt would be 0.099995 ohms, a lot less than that of the meter. Note that the voltage across the combination would remain the same, eg 0.1V which might, or might not be significant.

In order to read voltage in excess of, eg 0.1v, it is necessary to use series resistors to limit the the current flowing through the measuring circuit, so for a, eg 10v range, the total resistance would be 200 000 ohms, of which 2000 ohms would be in the meter with 198 000 ohms in the external circuit. If you wanted to measure say 1v, then the series resistance becomes 18 000 ohms. Note that the maximum current through the combination remains 50 micro amp which might be significant in a high impedance circuit.

In brief then, to measure amps, you use a parallel low value resistance, whilst to measure volts you use a series high value resistor.

Hope this helps,

Peter G. Shaw

Navigation

[0] Message Index

[*] Previous page