SHUNT CONTROL OF UJT TRIGGERING CIRCUIT.
THEORY:
The UJT is the simplest thyristor-triggering device. A relaxation oscillator is usedfor triggering of SCR.Normally zener clamped synchronization is used for triggering circuit. The frequency of pulses, controls delay angle. Higher the frequency, lower will be the value of delay angle & higher will be the conduction angle. The thirstier regulator controls the output by the automatic control of delay angle, so it is necessary to vary the delay angle, so it is controlled by voltage feedback.A shunt controlled UJT triggering circuit provides this function..a transistor is connected across the charging of capacitor .it provides a shunt path for the charging current of the capacitor .when no input is applied to the base of shunt control transistor ,the delay angle is minimised at zero point . The reference voltage for the circuit is to base emitter voltage of shunt transistor .the output devider must produce voltage which is is in the range of 0.6 to 0.7 volts, which brings the shunt transistor in conduction & charging current is diverted through these transistor.
The capacitors take larger time to charge & the delay angle increases. The control action is necessary input voltage & it causes increase in delay time (angle) & decreased output. This helps in simplifying the feedback system..The output directly divided by feedback network can be applied to control input of the circuit for the negative feedback.
APPLICATION: -
Voltage Regulator: the output voltage devider is applied to the control of gate voltage regulation.
Current Regulator: the voltage drop across the shunt connected in series with the load, dropping above 0.6 at full load will regulate the current.
Voltage & current regulator: the output of potential divider & shunt are connected to control output of two separate shunt transistors individually control the supply output.
ADVANTAGES:
� Less components & hence higher reliability.
� inherent simplicity & hence easy to manufacture
� No inversion required for negative feedback.
LIMITATIONS:
� control circuit does not trigger high rating SCR
� Base emitter voltage used is very low & varies with temperature. the high division & poor temperature coefficient are making the circuit not suitable for good regulation . The circuit may be used for over current protection where regulation is not important.