Voltmeter

VOLTMETERS Test Instruments & Measuring Devices V = I_mR_s + I_mR_m I_mR_s = V - I_mR_m R_s = (14-1) V I_m R_m where V = voltage range desired I_m = meter current R_m = meter resistance R_s = multiplier resistance or series resistance Example: A 2 mA meter movement with internal resistance of 25 ohms is to be constructed as a voltmeter. What value must the series resistance be to measure full scale voltage of 100 volts? Solution: R_s V I_m R_m Since R_m is negligibly low, then: R_s V I_m 100 2x10³ R_s 50kW When a voltmeter is connected in a circuit, the voltmeter will draw current from that circuit. This current causes a voltage drop across the resistance of the meter, which is subtracted from the voltage being measured by the meter. This reduction in voltage is known as the loading effect and can have a serious effect on measurement accuracy, especially for low current circuits. ES-14 Page 6 Rev. 0