The Correct Answer and Explanation is:
The correct answer to the question “The output ________ on loop controllers can be used as alarms” is relays.
Explanation
Loop controllers are devices used in industrial process control to maintain a process variable, like temperature or pressure, at a desired setpoint. While their primary output is often an analog signal (like 4-20mA or 0-10V) that continuously adjusts a control element like a valve or heater, they frequently include secondary outputs for discrete functions.
Relays are electromechanical switches that are ideally suited for these discrete functions. A relay output on a loop controller can be programmed to change its state (from open to closed, or vice versa) when a specific condition is met. This on/off switching capability is perfect for triggering alarm systems.
For example, a temperature controller might be set to maintain 150°C. The user can configure an alarm setpoint at 170°C. If the process temperature ever reaches 170°C, the controller’s internal logic will energize its alarm relay. This relay, in turn, can be wired to an external device such as a flashing beacon, a loud siren, or a master control system. The relay acts as the bridge between the controller’s decision-making and the physical alarm device, effectively translating a high-temperature condition into a clear, actionable alert for operators.
The other options are incorrect for the following reasons:
- Hysteresis is a setting or property that defines a deadband around a setpoint to prevent rapid on-off cycling. It is a parameter, not a physical output.
- Voltage and current describe the nature of analog control signals. While these signals are outputs, they provide proportional control rather than the simple on/off action typically required to activate an alarm horn or light. A relay provides that necessary discrete switch.
