# Holding Current and Latching Current dilemma

I have gone through lots of resources looking up for holding current and latching current. All I can get is:

1. Latching current is the minimum current that is required to maintain the "on" state of the SCR.

2. Holding current is the minimum current below which the SCR turns off.

Considering the two transistor model of the SCR, if I feed the gate with very little current, this current induces a regeneration of current within SCR. Once the Anode to Cathode current reaches latching current after this regenerative process, the gate current can be cut-off. The SCR maintains it on state.

Now, in order to turn-off the SCR, I have to bring the Anode to Cathode current below holding current.

simulate this circuit – Schematic created using CircuitLab

My questions are:

1. Why does SCR need such high Latching current to keep it on? My understanding about the regenerative process makes me think even a tiny amount of current would suffice to turn the SCR on.

2. Why are Latching current and Holding current different? Why can't SCR just switch on once A-C current reaches Latching current and turn off if the A-C current is less than latching current? Or, with respect to holding current, why can't SCR just switch on once A-C current reaches holding current and turn off if the A-C current is less than holding current?

• Not addressing the mechanism, but the purpose : "Why does SCR need such high Latching current to keep it on?" -- so that you can have a reliable way of turning it off .... "why latch/hold different" -- so that you have clean switching, i.e. hysteresis Commented Jan 6, 2021 at 17:14