A silicon controlled rectifier (SCR), is a component that belongs to a family of devices known as thyristors. These devices are primarily utilised for switching loads. An SCR is a three terminal device which conducts in one direction like a diode. A voltage at the gate junction will make the device conduct. It remains in a conducting state even when the gate voltage is removed, therefore it has latching properties. One way to reset it is by removing the anode voltage.
As you can see, a thyristor consists of four layers of semiconductor material pnpn, and three junctions. As well as possessing the qualities of a diode, it also behaves as a transistor would.
The equivalent circuit can be thought of as a pair of PNP and NPN transistors, where the base of one transistor is driven by the collector of the other.
Once triggered, with a signal at the base of TR2, each transistor keeps the other one switched ON, forming a feedback loop. The transistors hold each other fully ON and remain in a feedback loop until the power to the circuit is removed.
Uses and Applications
The device can also behave as a half-wave rectifier, just like an ordinary diode, when there is a continuous voltage at the gate junction. It is also useful for regulating the speed of a DC motor. Timed pulses at the gate junction can maintain the speed, in a servo controlled circuit.
These devices are typically used to control the power to high voltage and current devices, and are often found in mains powered devices such as microwave appliances.
If you needed a circuit that remained switched ON once triggered by a signal at the gate, then the latching qualities of the thyristor are useful.