Series Double Triode Voltage Regulator
You can see the circuit diagram of a series double triode voltage regulator below .
Triodes T1 and T2 are used as direct coupled feedback amplifier in which output voltage variations are returned as feedback to oppose the input changes. The glow tube VR maintains the cathode of triode T2 at constant potential w.r.t. ground. The triode T2 functions as a control tube and obtains bias from the potentiometer R3. The resistances R3 and R4 are range limiting resistors. The capacitor across VR tube helps to minimise the tendency of the circuit to generate audio frequency oscillations.
Working of Series Double Triode Voltage Regulator
The unregulated d.c. supply is fed to the voltage regulator. The circuit will produce an output voltage (Vout) which is independent of changes in input voltage and of changes in the load over a wide range. With a decrease in load or increase in the input voltage, there would be tendency for the voltage across the resistive network R2, R3 and R4 to rise. The result is that voltage on the grid of triode T2 becomes less negative. The triode T2 then conducts more current and a greater current flows through R1 which causes a greater voltage drop across this resistor. The increase in voltage across R1 will raise the negative potential on the grid of triode T1. This increases the resistance of T1 and hence the voltage across it. The rise in voltage across T1 tends to decrease the output voltage. The reverse would be true should the load increase or input voltage decrease.
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