triggering circuit

Posted on Feb 7, 2014

Simple resistor, diode combinations trigger and control SCRs over the full 180 electrical degree range, performing well at commercial temperatures. These types of circuits operate most satisfactorily when SCRs have fairly strong gate sensitivities. Since in a scheme of this type a resistor must supply all of the gate drive required to turn on the

triggering circuit
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SCR, the less sensitive the gate, the lower the resistance must be, and the greater the power rating. Figure 1 shows a very simple variable resistance half-wave circuit. It provides phase retard from essential zero (SCR full on ) to 90 electrical degrees of the anode voltage wave (SCR half on ). Diode D1 blocks reverse gate voltage on the negative half-cycle of anode supply voltage. It is necessary to rate blocking to at least the peak value of the AC supply voltage and the trigger voltage producing the gate current to fire IGF are in phase. When EAC = Em, at the peak of the AC supply voltage, the SCR can still trigger with the maximum value of resistance between anode and gate.

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