the SCR symbol
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Silicon controlled rectifier
Silicon controlled rectifier
A silicon controlled rectifier (SCR) is a type of thyristor (a 4-layer semiconductor device with alternating layers of p and n-type materials, acting as a diode (but isn't one), having current going form anode to cathode.
An SCR has 3 terminals: anode (A), cathode (K or C), and gate (G), It can switch a device on/off. Anode connects to p-type layer and cathode, to n-type at the other end.
The gate controls the device, activated by a small current to flow betweent A and K. An SCR's pin locations can change.
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[4] This diagram shows an SCR's 4-layer configuration, 3 junctions labeled "J1," "J2," "J3."
If the anode is positive to cathode, J1 and J3 become forward bias and J2, reversed bias - a setup keeping the SCR in off state (blocked).
[4] This graph shows an SCR's characteristics:
Initially without gate current, the SCR is in forward blocking region - it won't conduct much current. Using a positive gate current switches the devices to its unstate by letting current from A to K, a transition called latching.
Then by removing the gate signal, the SCR stays on as long as the current through it stays above a threshold called holding current (IH).
This curve shows each states: from the forward blocking region to the reverse blocking region and to the on state as the SCR conducts fully the voltage needed to keep it conducting, called the threshold voltage (VDRM). Once this is exceeded, the SCR stays on til the current drops unde the holding level.
An SCR can be considered as 2 interconnected transistors, here drawn as Q1 and Q2, a configuration showing how the SCR works internally; using a small gate current turns on Q2 which triggers Q1. They together make a positive feedback loop, keeping both on and for current to flow between the anode and cathode.
References
References
[1] Wikipedia
[1.1] Thyristor
[1.2]
[1.3]
[2] MadPCB
[3] Anypcba
[4] Silicon Controlled Rectifier (SCR) | Electrical Engineering (YouTube)
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Quizzes
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