Q.Describe the static VI characteristics of SCR.

The static V-I characteristic of an SCR has three regions: forward blocking up to the break-over voltage, forward conduction at a low voltage, and reverse blocking up to the reverse breakdown, with the break-over voltage falling as gate current increases. …

[!TLDR]

Forward blocking to $V_{BO}$, then conduction at low voltage (held above $I_H$); reverse blocking to $V_{BR}$; $V_{BO}$ falls as $I_g$ rises.

The static V-I characteristic of the SCR is plotted with anode current $I_A$ on the vertical axis and anode-to-cathode voltage $V_{AK}$ on the horizontal axis. It has three regions:

• Forward blocking (OFF) region: With the anode positive and the gate open, the outer junctions $J_1$ and $J_3$ are forward biased but the middle junction $J_2$ is reverse biased and blocks conduction. Only a small forward leakage current flows as $V_{AK}$ is increased, up to the forward break-over voltage $V_{BO}$.

• Forward conduction (ON) region: When $V_{AK}$ reaches $V_{BO}$, junction $J_2$ breaks down (by avalanche/regenerative action) and the SCR switches to the ON state through a negative-resistance transition. The voltage across the device collapses to about 1 to 1.5 V and a large anode current flows, limited only by the external circuit. The SCR remains ON as long as the anode current stays above the holding current $I_H$; the latching current $I_L$ is the minimum current required to latch it ON when it is first fired. …