Which statement best describes relay control compared to proportional control?

• A. Relay is on and off; Proportional can alter how much voltage
• B. Relay is variable; Proportional is on/off
• C. Relay is manual; Proportional is automatic
• D. Relay uses AC; Proportional uses DC

Answer: (A)

Relay control uses a binary output: it is either fully on or fully off, delivering maximum power when commanded and no power otherwise. Proportional control, however, adjusts the output continuously in proportion to the error between the desired setpoint and the actual process value, so the actuator receives varying levels of power (or voltage) rather than just a switch-like on/off signal. This is why the correct description says the relay is on and off, while proportional control can alter how much voltage is applied. For example, a relay-based heater would switch completely on or off, producing bang-bang behavior, whereas a proportional controller would vary the heater power to gradually reach and maintain the target temperature. The other statements don’t capture this fundamental difference: relays aren’t inherently manual or tied to a specific voltage type, and proportional control isn’t limited to a single on/off action.