How To Expand The Range Of A Voltmeter?

Gas filled cabinet Series additional resistor to expand the range of voltmeter. When using magnetoelectric, electromagnetic or electrodynamic measuring mechanism (i.e. meter head) to directly measure voltage, due to its structural and technical characteristics, the voltage allowed to be measured is very small, only about tens of millivolts, which cannot meet the needs of actual measurement. In order to be able to measure higher voltages and not affect the measurement results due to its small internal resistance, the magnetoelectric, electromagnetic and electrodynamic voltmeters used in actual applications use the method of connecting additional resistors in series with the measuring mechanism to expand the range. The connection method of additional resistors is shown in Figure 4-11. In the figure, the internal resistance of the measuring mechanism is Rc, and the additional resistor is Rs. According to Ohm's law, it can be obtained:
I=U/(Rs+Rc)
Select a suitable additional resistor Rs according to the voltage to be measured. The current passing through the measuring mechanism can be limited to the allowable range. At the same time, as long as the additional resistor Rs remains unchanged, the current I is proportional to the voltage U between the two points to be measured. The pointer deflection angle a still reflects the magnitude of the voltage at both ends of the measurement.