Triggering a Silicon Controlled Switch (SCS): which of the following correctly describes how an SCS can be turned ON?

Introduction / Context:
The Silicon Controlled Switch (SCS) is a thyristor-family device with two gates, enabling greater control flexibility than an SCR. Understanding correct triggering polarities for ON action is a common exam check.

Given Data / Assumptions:

• SCS has an anode gate and a cathode gate.
• Standard triggering conventions apply.

Concept / Approach:
Unlike an SCR that is typically triggered by a positive pulse at the gate with respect to cathode, an SCS can be turned ON either by applying a positive pulse at its cathode gate or by applying a negative pulse at its anode gate, depending on device configuration and reference potentials.

Step-by-Step Solution:
1) Identify available gates and their reference polarities.2) For ON action: use a positive pulse at cathode gate or negative pulse at anode gate.3) Ensure appropriate gate–cathode/anode biasing and observe device limits.
Verification / Alternative check:
Manufacturer datasheets and power electronics texts list both methods for SCS triggering.
Why Other Options Are Wrong:

• Anode-only or cathode-only unspecified polarity: incomplete or incorrect.
• Low constant anode voltage: not a triggering method; it risks device stress.
• Shorting anode to gate: unsafe and not a defined procedure.

Common Pitfalls:

• Applying SCR gate rules directly to SCS without considering dual gate structure.
• Ignoring need for proper gate drive source impedance.

Final Answer:
Positive pulse at cathode gate or negative pulse at anode gate.