Relays vs. semiconductor switches — unique capability Compared with solid-state switching devices, what can an electromechanical relay provide between the control (coil) circuit and the switched load?

Introduction / Context:
Choosing between a relay and a semiconductor switch involves tradeoffs in speed, isolation, lifetime, and current capability. This question focuses on capabilities relays uniquely provide or commonly exceed compared with typical semiconductor devices in basic applications.

Given Data / Assumptions:

• Conventional electromagnetic relays with galvanic isolation between coil and contacts.
• Typical semiconductor switches (BJT, MOSFET, SSR) without transformers or opto-isolation added.
• General-purpose current and voltage levels.

Concept / Approach:

Relays offer galvanic isolation because the coil circuit is physically separate from the contact path. They also commonly switch higher voltages and currents (including AC) with robust contact ratings. Solid-state devices are faster and long-lived, but they do not inherently provide galvanic isolation unless paired with isolation components, and their current/voltage ratings depend on device selection.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for power relays show dielectric strength between coil and contacts and contact ratings in amperes to tens of amperes. Standard logic-level MOSFETs can also handle high currents, but they do not provide galvanic isolation without an isolating driver stage.

Why Other Options Are Wrong:

• Total isolation only: incomplete; higher current capability is also common.
• Faster: relays are slower than semiconductors.
• Higher current rating only: ignores the isolation advantage.
• Lower on-resistance than a MOSFET: false; modern MOSFETs have milliohm R_DS(on).

Common Pitfalls:

• Ignoring AC switching; relays easily handle AC without polarity concerns.
• Overlooking contact wear; relays have finite mechanical life.

Final Answer:

total isolation and higher current rating