Question

With neat diagram explain synchronized UJT triggering circuit.

Answer 1

Line Synchronized UJT triggering circuit:

The UJT is almost ideal as a firing device for SCRs. There are 2 main reasons for the compatibility between UJTs and SCRs. Firstly, the UJT produces a pulse-type output, which is excellent for accomplishing sure turn-on of an SCR without straining the SCR’s gate power dissipation capability. Secondly, the UJT firing point is inherently stable over a wide temperature range. This can compensate for the temperature instability of the SCR. The usual method of triggering an SCR with a UJT is shown in Figure 1. In this circuit, zener diode Z1 clips the Vs waveform at the zener voltage during the positive half-cycle of the ac line. During the negative half cyce, Z1 is forward biased and Vs is held near 0 V. Once the dc voltage Vs has been established, which occurs immediately after the ac line crosses zero, CE begins charging through resistor RE. When CE reaches the peak voltage of the UJT, the UJT then fires. This generates a voltage pulse across R1 and results in the firing of the SCR. When the SCR is fired, current flows through the load for the rest of the positive half cycle. In this circuit, automatic synchronization between the firing pulse of the UJT and the SCR polarity is provided. This means that whenever the UJT delivers a pulse, the SCR is sure to have the right polarity between the anode and cathode for turning on.