Silicon Controlled Rectifier
Silicon Controlled Rectifiers are very reliable and inexpensive.
Test it out
Tap the diagram for more information and click on the buttons to change a component and see what happens!
 Basic Construction Forward Blocking State Forward Conducting State Reverse Blocking State
Example symbols
In order to work with Silicon Controlled Rectifiers, electrical engineers need to understand and work with a variety of symbols. Here are just a few:
 Transistor $T$ Anode $A$ Cathode $K$ Gate $G$ Collector $C$ Base $B$ Emitter $E$ Collector Current $I_C$ Base Current $I_B$ Emitter Current $I_E$
Silicon controlled rectifiers (SCRs) form part of the "thyristor" family of semi-conductor devices use for electronic power control. They are commonly used to convert alternating current (AC) to direct current (DC).
They are also used in devices which need to control high power in medium- to high-voltage situations such temperature control, lamp dimming and alarm activation. Because of their ability to handle high currents and voltages, they ae often used in industrial applications such as controlling motor speed and battery charging.
Similar to a conventional rectifier or diode, a SCR has an anode and cathode terminal, and only allows current to flow in one direction while blocking it from the other.
It is different from a regular rectifier because it has the added benefit of a third, control terminal called a gate. The gate is used to supply a trigger voltage that turns the SCR on, and allows it to start conducting.
Two SCRs working together then turn on and off, controlling the flow of power in an alternating pattern, to perform the role of rectification.
In order to create this third terminal or gate, an SCR is constructed in from silicon, a semi-conductive material, in four-layers to a p-n-p-n configuration. You can think of it as two transistors (one a PNP and the other an NPN transistor) linked by a bridge and working together. When voltage is applied across the bridge, the "blocked" SCR junctions experience an avalanche breakdown and the path becomes conductive.
Depending on the biasing given to it, the SCR has three different states of operation: reverse blocking state, forward blocking state and forward conduction state.
Practice Questions
Test your new knowledge on silicon controlled rectifiers by answering these questions.
1. The additional control terminal added to the SCR is known as the
2. The electrical symbol for an SCR is
3. Holding current refers to