reaches the peak pointVP , the emitter base junction is forward biased and UJT turns on. Capacitor rapidly discharges through load resistance with time R1 ‘C’ When emitter voltage decreases to valley pointVv , UJT turns off. RC1 1 Once again the capacitor will charge towards and the cycle continues. The rate of VBB charging of the capacitor will be determined by the resistor R in the circuit. If R is small the capacitor charges faster towards and thus reaches faster and the SCR is triggered at a VBB VP smaller firing angle. If R is large the capacitor takes a longer time to charge towards the VP firing angle is delayed. The waveform for both cases is as shown below. (i) Expression for period of oscillation‘t’ The period of oscillation of the UJT can be derived based on the voltage across the capacitor. Here we assume that the period of charging of the capacitor is lot larger than than the discharging time. Using initial and final value theorem for voltage across a capacitor, we get VC tRC , t V, VP, VV C initial final Therefore VP RC/ T RCloge If , V VV BB VBB T VBB P 1 ln
reaches the peak pointVP , the emitter base junction is forward biased and UJT turns on. Capacitor ‘C’ rapidly discharges through load resistance R1 with time RC1 1 When emitter voltage decreases to valley pointVv , UJT turns off. Once again the capacitor will charge towards VBB and the cycle continues. The rate of charging of the capacitor will be determined by the resistor R in the circuit. If R is small the capacitor charges faster towards VBB and thus reaches VP faster and the SCR is triggered at a smaller firing angle. If R is large the capacitor takes a longer time to charge towards VP the firing angle is delayed. The waveform for both cases is as shown below. (i) Expression for period of oscillation‘t’ The period of oscillation of the UJT can be derived based on the voltage across the capacitor. Here we assume that the period of charging of the capacitor is lot larger than than the discharging time. Using initial and final value theorem for voltage across a capacitor, we get VC tRC t V, C VP, initial VV , final Therefore VP RC/ T RCloge If V VV BB , VBB T VBB P 1 ln
U3_L19-Gate-Triggering-Methods.docx