Wide range of voltmeter multitester

The voltage between the terminals of an analog voltmeter is 100mV when the needle is at full scale.

In order to extend the measurement range of the voltmeter are used to trim a resistor in series. See the figure.

The Rx resistor in series with the voltmeter internal resistance (Rm), is a voltage divider

The voltage in each of the resistors is proporsional to the values ​​they have. Then:


Vx / Rx = Vm / Rm

The total resistance of the measuring instrument is: Rt = Rm + Rx, then Rx = Rt - Rm.

Where: Rt = Vt / Im

With what you get: Rx = (Vt / Im) - Rm

Calculation to extend the measurement range of our instrument from 100mV to 1 Volt

With:
- Vt = 1V,
- Im = 50uA, and
- Rm = 2K

Substituting these values ​​into the formula above:

Rx = (Vt / Im) - Rm

Rx = (1/50x10 -6) - 2x10 3 = 18K

Then the 18K resistor is needed

This type of resistors in series (to vary the range of voltage ) and parallel / SHUNT (to vary the current range) are those with the multitesters typical analog to vary the measurement range.