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SCR stands for Sicilon Controlled Rectifier. An SCR looks like a transistor: a current flow in one of the terminals (the gate) makes the SCR conductive. The difference however is that the SCR remains conductive when the gate current becomes zero. The SCR only closes when the current flow stops (just like a DIAC). Take a look at the picture below.

By applying a voltage at the base of T1, T1 will turn on. This will provide base current to T2 wich will also turn on. On turn, T2 provides base current to T1, so the SCR remains turned on.

The SCR only turns off in the voltage across anode and cathode becomes zero.

We can make a very simple dimmer switch using an SCR:


While experimenting, keep in mind that the whole circuit (including P1!) is connected to the mains!

Capacitor C1 is charged by R1 and P1. As soon as the voltage across C1 reaches the DIAC's break-down voltage, the SCR starts conductiong and will turn on the lamp. The SCR will remain turned on until the mains voltage becomes 0V. And then things start all over again. The larger P1, the longer it takes before the voltage across the capacitor is high enough to make the DIAC and SCR turn on. A higher value of P1 will reduce the light output.

When we turn on the light, we sometimes here a 'pop' coming from the loudspeakers of a radio. This cirtuit turns on the light 100 or 120 times a second (depending on the mains frequency). R2 and C2 reduce the amount of noise caused by the switching SCR.