Silicon controlled rectifier
From HvWiki
A semiconductor switching device; SCR. A small trigger current at the gate can allow a very large current to flow from the anode to cathode, which continues after the trigger current stops. An SCR only switches off when the anode-cathode current through it drops below a minimum level, IH.
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Operation
The image shows the schematic symbol for an SCR on the left, along with an equivalent circuit that explains how it works. (In practice, it is non-functional) The gate is the base of a transistor. If a trigger current IGT flows into the gate then current flows through the transistor CE junction. This current flows through the base of the other transistor, allowing current to flow through it as well. Hence the two transistors provide the base current for each other.
This means that the while the SCR is triggered at the gate by the driver device, the power that keeps current flowing is provided by the power device. So very high power devices can be triggered by low power drivers.
This also means that once the SCR is triggered, current does not stop after the trigger current stops. It only stops when the power device current drops below the holding current, IH.
Triggering
There is no mystery to triggering an SCR; there just needs to be a trigger current applied to the gate. This can be done with a resistor and a voltage source (eg. battery) as in the diagram. Turning on the voltage triggers the SCR.
The voltage should be at least a few volts and the resistor value can be calculated from R = (V - VGK) / IGT, where VGK is the voltage drop at the gate, typically ~.6V.
Types
There are two main types of SCRs: discrete and "hockey puck." The discrete SCRs look just like IGBT, transistor, mosfet, or similar packages. The hockey pucks come as a sort of disk(hockey puck) with two leads on it. There is also a contact on the top and bottom of the puck. They negative end of the SCR is wired to one of these two leads, thus making the three contacts. They must be clamped with a good deal of force in order to be able to work properly.
Common Datasheet Values
VDRM = Voltage that the SCR can handle in the “off” state.
IT = Current that the SCR can handle in the on state. Three common variants of this value is RMS, Peak, and Continuous.
IGT = Current that must be achieved gate to cathode to trigger SCR
VGT = Voltage that must be achieved gate to cathode to trigger SCR
IH = Current required to keep the SCR on. To reset the SCR, current must be dropped below this value.
Td = Delay from trigger pulse to activation
Tr = Length of time SCR spends switching
Tq = Turn off time
