SCRs or Silicon Controlled Rectifiers are members of the electronic active component family. They are also called Thyristors.
The
figure on the left shows the standard electronic symbol of an SCR. It
shows the three lead pin outs of the part, the upper one being the
anode, the lower one the cathode, and the central extension the gate.
The symbol quite resembles an ordinary rectifier diode symbol having an
extra lead from the cathode side. Though SCRs are much different from
diodes, they too rectify AC in response to DC electrical triggers on
their gate inputs.
As you can see in the actual picture of an SCR
on the right, it looks like a transistor. Externally they may look
exactly like transistors, but are entirely different as far as technical
specifications are concerned.
Both act as switching devices,
although SCRs comfortably handle high voltage AC, whereas transistors
normally are dedicated for low voltage DC applications. The lead
orientation specifies the first lead from the right to be the gate, the
extreme left is the cathode, and the center pin is the anode. The gate
and the anode leads always work with respect to the ground; the cathode
lead is specified to be connected with the ground and serves as the
common release terminal for the gate as well as the anode. The load that
needs to be operated is connected across the AC input and the anode of
the SCR.
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