SERIES CONTROL OF UJT TRIGGERING CIRCUIT.
THEORY:
The UJT is the simplest triggering device for thyristor. It is also simple to synchronise the UJT relaxation oscillator using zener clipped synchronisation.by using the RC changing time of the capacitor. The delay angle can be controlled.
For automatic control of the output in a thyristor regulator the delay angle in a thyristor regulator should be varied using control of input voltage.The feedback voltage is applied to the input which will vary the delay angle to control the output. A transistor connected in series in the charging circuit controls the charging current & hence the delay angle.when the input to base of PNP series transistor is low, the conduction will be more & the delay angle will be less.if the control input is more the conduction in the transistor (PNP series) will be less causing more delay angle. The refrence voltage in case of series control circuit is the zener (supply) voltage.minus the base emitter voltage drop of series transistor.
APPLICATION: -
Voltage Regulator: the output voltage is applied through potential devider to control the input (base of series transistor) to get regulation. . The reference voltage is supply voltage & hence divider ratio is low.
Current Regulator: A CT input can be used & can be supplied to the base of series transistor after rectification & filtering.
ADVANTAGES:
� Less components & hence higher reliability.
� inherent simplicity & hence easy to manufacture
� No inversion is required if control input is at base of series transistor.
LIMITATIONS:
� it cannot trigger high current rating SCR
� Reference is base emitter drop less than the supply voltage. The variation in the base emitter junction causes power regulation of the output. the regulation is however better than the shunt control as,
(a) Voltage ratio of refrence to output is not very low.
(b) To supply voltage, zener reference provides good regulation.