SCR phase control yields solid-state switch
James Keith, York, PA -- EDN, March 15, 2001
SCRs (silicon-controlled rectifiers), or thyristors, have higher current and voltage ratings, lower conduction losses, and more robustness than triacs. For these reasons, SCRs are better suited to high-power applications. For example, you can use two SCRs to configure a 100 or 200A, 460V control circuit. Table 1 lists some of the advantages of SCRs over triacs. The main challenge is driving the SCRs: You now have two gates, rather than one, to drive. Furthermore, the gates are referenced to opposite polarities with a significant voltage difference. The circuit in Figure 1 solves the problem with two PUTs (programmable unijunction transistors), one connected to each SCR. Performance is good because the circuit does not 'fold back' as inexpensive triac phase controls (dimmers) do. The PUT fires when the timing-capacitor voltage exceeds the PUT reference voltage by one diode drop and dumps the capacitor's charge into the SCR gate.
You adjust the phase delay by varying the charge rate of the capacitor via the potentiometer. You can enhance the balance by matching both the zener-diode voltages and the values of the charging capacitance. You can obtain 230V operation by using 1.5- to 2-µF timing capacitors. You can achieve 460V-ac operation by using 1200V SCRs; 3-µF timing capacitors; and a 100-k?, 25W potentiometer. Using the circuit as a solid-state switch is practical; simply replace the voltage-adjust block in the broken lines in Figure 1 with a photomod. A photomod is a unique type of photocoupler that has an LED or incandescent-light source and a cadmium-sulphide photocell. Unique properties of these photocells are their high dark resistance and high voltage ratings.
