Triggering a thyristor without gate current If the gate current of a thyristor is zero, conduction can still start only when the forward anode-to-cathode voltage exceeds:

Introduction / Context:
A silicon-controlled rectifier (SCR) is designed to turn on via a gate trigger. However, in the absence of gate current, it may still turn on if the anode-cathode voltage is raised high enough to avalanche the central junction—this is the forward breakover phenomenon. Knowing this is important for protection and dv/dt design.

Given Data / Assumptions:

• Gate current = 0 (no intentional trigger).
• Device is forward biased at the anode-cathode terminals.
• Static characteristics as per a standard SCR.

Concept / Approach:

With no gate drive, the SCR remains blocking until the anode-cathode voltage reaches the forward breakover voltage V_BO, where junction avalanche and regenerative feedback force the device into conduction. Under normal operation one avoids reaching V_BO to prevent uncontrolled turn-on; instead, a gate pulse is used at a lower forward voltage.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets depict forward blocking and breakover regions; V_BO reduces with higher temperature or high dv/dt but is well above the on-state voltage.

Why Other Options Are Wrong:

0.5 V and on-state voltage describe conduction after turn-on, not the triggering threshold. 'Peak working off-state forward voltage' is a rating below breakover, not the trigger point.

Common Pitfalls:

Confusing breakover with latching/holding parameters; mistaking low on-state voltage for the trigger threshold.

Final Answer:

The forward breakover voltage