The RMS voltmeter

Thermocouple: convert heat into output dc voltage

2rmsV KV1

Disadvantages: The thermocouple behavior is non linear. To

overcome of the disadvantage of nonlinear behavior of the measuring thermocouple, a balancing thermocouple is added in the feedback circuit. The two couple elements form a part of a bridge in the input circuit of dc amplifier.

The unknown ac input voltage is amplified and applied to the heating element of the measuring thermocouple. The application of heat produces an output voltage that upsets the balance of the bridge. The unbalance voltage is amplified by the dc amplifier and fedback to the heating element of the balancing couple. Bridge balancing is established when the feedback current delivers sufficient heat to the balancing couple so that the voltage outputs of both couples are the same, at this point the dc current in the heating element of the balancing couple is equal to the ac current in the input couple. Therefore this dc current is directly proportional to the rms of the input voltage and is indicated in the output circuit.

At balance

rmso

2o

2rms

21

VV HenceV K V K

VV

Construction of Thermocouple

The thermocouple is a junction of two dissimilar metals on which an emf is generated when there is a difference in temperature between the hot and cold junctions EC and ED are the two dissimilar metals

Hot junction