Balanced Bridge Voltmeters

Such voltmeters can be used for measurement of both dc and ac voltages and are very popular. These voltmeters are available in both versions, vacuum tube type and transistorized type and are described below: In this instrument two identical triodes T1 and T2 are used. Plate voltage is applied to both of the triodes through two, identical resistors
Balanced Bridge Voltmeters - schematic

RL and a variable resistance RQ. Resistance R0 is so adjusted that plate voltages of both of the triodes become equal when there is no voltage applied on the grid of triode T1. So, in this condition, meter connec ­ted between the plates of the two triodes reads zero. The bias battery ECC biases the cathodes of both of the triodes positive and so the bias of the grid of both of the triodes becomes negative with respect to their respective cathodes. When a positive dc voltage is applied to the grid of triode T1, potential of grid with respect to cathode increases and so the plate current increases. Due to increase in plate current of triode T1 the voltage drop across the resistor RL connected in series with triode T1 increases and, therefore, plate voltage of triode T1 falls. The increased current also flows through the resistor Rk which is common to both of the tubes T1 and T2. An increase in current through resistor Rk causes increase in voltage drop across it and, therefore, cathode of triode T2 becomes more positive. It results in increase of -negative grid bias of tube T2 and therefore, plate current of triode T2 is reduced. With this voltage drop across resistor RL connected with plate of triode T2 falls and, therefore, plate voltage of triode T2 increases. Thus with the application of dc voltage on the grid of triode T1 voltages of the two triode plates do not remain same and a current flows through the PMMC meter,...

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