How Do You Turn an SCR Off?

Lesson notes

Key points

• Once latched, an SCR keeps conducting until its anode current falls below the holding current, regardless of the gate.
• Turning the SCR off by forcing its current below the holding value is called commutation.
• In AC circuits, natural commutation happens automatically each time the supply current passes through zero.
• In DC circuits, forced commutation uses a charged capacitor to reverse-bias the SCR and divert its current.

Definitions

Holding current (I_H)

Minimum anode current needed to keep a latched SCR conducting; below it, the SCR turns off.

Commutation

The process of turning off a conducting thyristor by reducing anode current below I_H.

Takeaway: SCRs turn off by commutation, not by the gate.

Holding currentCommutationNatural commutationForced commutation