How many external leads does a silicon controlled rectifier (SCR) have?

Introduction / Context:
An SCR is a widely used thyristor in power electronics. Knowing its terminals is essential for proper triggering, protection, and connection in converters, inverters, and control circuits.

Given Data / Assumptions:

• Device: SCR (silicon controlled rectifier).
• Standard construction and operation are assumed, without special integrated packages.
• Looking for the number of external connection leads.

Concept / Approach:

An SCR is a four-layer p-n-p-n controlled rectifier. It requires a control terminal to initiate conduction and two power terminals for current flow. Therefore, it has three external terminals: anode, cathode, and gate. The gate is used for triggering, while the anode and cathode carry the main current once the device latches on.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets and circuit symbols consistently show three terminals. Trigger circuits always reference gate to cathode, while the anode connects to the supply or load side depending on configuration (series or bridge).

Why Other Options Are Wrong:

• 2 leads: corresponds to a diode, not a controlled rectifier.
• 4 or 5 leads: not standard for discrete SCRs; extra pins appear in modules but still map to A, K, G only.
• 1 lead: impossible for a controllable two-terminal power path.

Common Pitfalls:

• Confusing SCRs with TRIACs (which still present three main connections in most symbols).
• Assuming internal sensors or extra auxiliary gates add terminals—these are not typical of standard SCRs.

Final Answer:

3