All of the following are typical solid-state relay control circuits except; Power transistors for DC loads, Power transistors for AC loads, Triac-controlled AC relays, and Reed-controlled AC relays?

• A. Power transistors for DC loads
• B. Power transistors for AC loads
• C. Triac-controlled AC relays
• D. Reed-controlled AC relays

Answer: (B)

Switching AC with a solid-state relay requires a bidirectional device that can conduct and block current in both directions on every half-cycle. A single power transistor can handle current in one direction and block in the other only with extra circuitry, so it isn’t a typical SSR output for AC loads. For AC, engineers rely on devices that are inherently bidirectional, like triacs, or two thyristors/MOSFETs placed back-to-back, so the circuit can switch off during either half-cycle. DC loads, on the other hand, are naturally switched with power transistors because there’s no reverse half-cycle to contend with. Reed-controlled relays are mechanical rather than solid-state, so they don’t fit the SSR category. Because of this, using a power transistor for AC loads isn’t a standard solid-state relay configuration.