Question

a) Sketch a diagram of an automatic voltage regulator of the zenor bridge type used in the control unit of a 3 phase ac generator.

b) Describe how the regulator  sketched operates, making particular reference to the operation of the zenor diodes.

Answer.

Sketch

b) In the sketch of the AVR of the zenor bridge type, resistor R1 provides bas current for transistor TR1, transistor TR2 is an error amplifier comparing the voltage on variable resistor VR1 with the zenor voltage ZD.

If the output voltage rises, voltage on TR2 base starts to rise causing TR2 to draw more current and this reduces the base voltage on TR1 and brings the voltage back to the correct level.

If the output voltage falls, TR2 draws less current and TR1 base voltage rises to compensate.

Zenor diode ZD1 is reversed biased, so when the voltage is above 12volts, the zenor goes into breakdown condition until the voltage level decreases below 12volts, then acts as a normal diode, reverse biasing. Ie. Zenor passes current in reverse direction at zenor breakdown voltage.