You can think of an SCR as an NPN transistor with an extra P-type layer. To operate the SCR, the anode is normally made positive with respect to the cathode. However, the device can’t conduct without a gate voltage because the center PN junction is reverse-biased.

If a positive voltage is applied to the gate, a large number of charge carriers are drawn across the lower junction, and the device begins to conduct. (Note that the NPN junctions in the lower part of the model act very much like an NPN transistor.) If a reverse voltage is placed across the SCR, it won’t conduct, since it’s a type of diode.

Figure C shows the characteristic curve for an SCR. The positive voltage on this curve is applied between the anode and the cathode and the current is through the device. The point where the forward breakover voltage occurs is set by the positive voltage on the gate.

If there’s a positive voltage on the anode of an SCR (with respect to the cathode) it’s ready for conduction. It won’t conduct, however, until a positive current pulse is applied to the gate. Once the SCR goes into conduction, the gate has no further control.

It’s a common misconception that the SCR is started into conduction by a voltage pulse. However, that isn’t the case. A current pulse is required. Typically, conduction is stopped by taking the positive voltage away from the anode.

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Career Diploma Electronics Technician (Module 3 - Basic Electronics (Section 3)) Slide Set on Semiconductor Switching Devices (Mod.3 Sect.3 Part 3) 1 of 2, created by That Yeti Dog Person on 11/06/2021.