A basic circuit to trigger an SCR is shown in Fig. 67 -lA. This circuit has the disadvantage that the blocking voltage of the photon-coupler output device determines the circuit-blocking voltage, irrespective of higher main SCR capability. Adding capacitor Cl to the circuit, as shown in Fig. 67-lB, will reduce the dV!dt seen by the photoncoupler output device. The energy stored in Cl, when discharged into the gate of SCRl, will improve the dildt capability of the main SCR.
High-voltage-ac-switcher - schematic

Using a separate power supply for the coupler adds flexibility to the trigger circuit; it removes the limitation of the blocking voltage capability of the photon-coupler output device. The flexibility adds cost and more than one power supply might be necessary for multiple SCRs if no common reference points are available. 67-lC, Rl can be connected to Point A. which will remove the voltage from the coupler after SCRl is triggered. "or to Point B so that the coupler output will always be biased by input voltage. The former is preferred since it decreases the power dissipation in Rl. A more practical form of SCR triggering is shown in Fig. 67 -IF. Trigger energy is obtained from the anode su]Jply and stored in Cl. Coupler voltage is limited by the zener voltage. This approach permits switching of higher voltages than the blocking voltage capability of the output device of the photon coupler. To reduce the power losses in Rl and to obtain shorter time constants for charging Cl, the zener diode is used instead of a resistor. A guide for selecting the component values would consist of the following steps: Choose Cl in a range of 0.05 to 1 p.F. The maximum value might be limited by the recharging time constant (RL + R1) C1 while the minimum value will be set by the minimum pulse width required to ensure SCR latching. R2 is determined from peak gate current limits, if applicable, and minimum pulse width requirements. Select a zener diode. A 25-V zener is a practical value, since this will meet the usual gate requirement of 20 V and 20 0. This diode will also eliminate spurious triggering because of voltage transients. Photon coupler triggering is ideal for the SCR"s driving inductive loads. By ensuring that the LASCR latches on, it can supply gate current to SCRl until it stays on.

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