Electronic – What does the multimeter dial do internally


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Everyone who has ever handled a multimeter is familiar with these dials. The position of the dial indicates the maximum range of the quantity that can be measured.

But why are we required to adjust the maximum range ourselves? What happens internally in the multimeter when the dial is adjusted, say, from 20V to 200V? If we have the dial on 20V, and the voltage measured is 50V, why can't the meter provide a measurement? I don't have much knowledge on the internal workings of a multimeter, but I understand that voltage is measured by letting an infinitesimal amount of current through the meter and measuring the magnetic field (something along these lines). But why can't the meters adjust their range themselves?

EDIT: I know there are autoranging meters, but I'm interested in knowing why others have to be adjusted manually.

Best Answer

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This image (source) ought to tell you all you need to know about how it works. There are wiper contacts on the dial, shown at the bottom, that mate with pads on the meter's PCB.

These pads are connected to different taps of a voltage divider to divide the voltage, or to pass current through a current shunt.

Internally, the meter can only measure voltages from, say, -0.2V to +0.2V. The range switch changes the voltage divider to prescale the input voltage to be within that range, and on most meters will also send a signal to the LCD to tell it where to put the decimal point.

As for why you have to do it yourself instead of the meter doing it for you: Nothing more and nothing less than price. A meter that auto-ranges is more expensive than one that doesn't due to the need for additional hardware to detect when it's over-range and perform the switching.