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During the positive half-cycle when the thyristor is forward biased, capacitor, C charges up via resistor R1 following the AC supply voltage. The Gate is activated only when the voltage at point A has risen enough to cause the trigger diode D1, to conduct and the capacitor discharges into the Gate of the thyristor turning it “ON”. The time duration in the positive half of the cycle at which conduction starts is controlled by RC time constant set by the variable resistor, R1.
Increasing the value of R1 has the effect of delaying the triggering voltage and current supplied to the thyristors Gate which in turn causes a lag in the devices conduction time. As a result, the fraction of the half-cycle over which the device conducts can be controlled between 0 and 180o, which means that the average power dissipated by the lamp can be adjusted. However, the thyristor is a unidirectional device so only a maximum of 50% power can be supplied during each positive half-cycle.
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