Lecture 5 Current/Voltage Measurement Resistance Measurement

Current/Voltage Measurement

Circuit Model for ideal ammeter/voltmeter An ideal ammeter has an equivalent resistance of 0 Ohm. An ideal voltmeter has an infinite equivalent resistance.

d’Arsonval meter When current flows in the coil, it creates a torque on the coil, causing it to rotate and move a pointer across a calibrated scale.

Commercial Rating Rating: 50 mV and 1mA Interpretation: When the coil is carry 1 mA, the voltage drop across the coil is 50 mV and the pointer is deflected to its full-scale position.

A DC Ammeter Circuit R A limits the amount of current in the coil.

Example 3.5 (a) A 50 mV, 1 mA d’Arsoval movement is to be used in an ammeter with a full-scale reading of 10 mA. Determine R A. How much resistance is added to the circuit when the 10 mA ammeter is inserted to measure current?

A DC Voltmeter Circuit R V limits the voltage drop across the meter’s coil.

Example 3.6 A 50 mV, 1 mA d’Arsoval movement is to be used in a voltmeter in which the full-scale reading is 150 V. Determine R V. How much resistance does the 150 V meter insert into the circuit?

Resistance Measurement R 1,R 2, and R 3 are known resistors. R x is the unknown resistor.

Determine R x Adjust the variable resistor R 3 until there is no current in the galvanometer. Calculate the unknown resistor from the simple expression: – R x =(R 2 /R 1 )R 3