0
\$\begingroup\$

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.

schematic

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?

\$\endgroup\$
1
  • 1
    \$\begingroup\$ 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 \$\endgroup\$
    – Pete W
    Commented Jan 6, 2021 at 17:14

1 Answer 1

1
\$\begingroup\$

For the model you have there the holding current would be extremely small. But it is not a perfect model.

Imagine for example modifying the model with a resistor between the gate and the cathode to represent the leakage current, say 1kilohm. If the current was less than about 500uA there would not be enough current into the base of the lower transistor to sustain regeneration and the SCR would turn off.

Also, the gain of the transistors (hFE) varies with the current which affects the holding current.

\$\endgroup\$
3
  • \$\begingroup\$ I see. I need to have at least 0.7V between the gate and cathode to switch on the transistor. In order to obtain this 0.7V, I need to provide sufficient current (700uA in this case.) Am I correct? \$\endgroup\$
    – G-aura-V
    Commented Jan 6, 2021 at 17:27
  • \$\begingroup\$ @G-aura-V - yes. I stated 500uA because transistors. actually start turning on at lower than 0.7v \$\endgroup\$ Commented Jan 6, 2021 at 17:30
  • \$\begingroup\$ Thank you for your short and to the point answer. Would accept it as an answer if you answer my second question as well.. \$\endgroup\$
    – G-aura-V
    Commented Jan 6, 2021 at 17:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.