Latching and holding current

Both are minimum anode currents of a thyristor; latching current concerns turning ON, holding current concerns staying ON.

• Latching current ($I_L$): the minimum anode current that must flow through the SCR immediately after it is triggered, while the gate pulse is still present, so that it stays ON after the gate pulse is removed. If the anode current has not reached $I_L$ when the gate pulse ends, the SCR turns OFF again.
• Holding current ($I_H$): the minimum anode current that must keep flowing to hold an already conducting SCR in the ON state (gate open). If the anode current falls below $I_H$, the SCR turns OFF and returns to forward blocking.
• $I_L$ is greater than $I_H$, usually $I_L \approx 2$ to $3\,I_H$. Example: for an SCR with $I_L = 40$ mA and $I_H = 15$ mA, the gate pulse must last until the anode current reaches 40 mA, but once ON it stays ON until the current drops below 15 mA.

  iA
   ^         ______________
   |        /              \
 IL|-------/ <- gate pulse   \
 IH|------/---must last till--\----- turns OFF
   |     /    iA reaches IL    \     below IH
 --+----/-----------------------\----> t
      gate