![]() This controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached. One special type of on-off control used for alarm is a limit controller. On-off control is usually used where a precise control is not necessary, in systems which cannot handle having the energy turned on and off frequently, where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm. ![]() On-off differential prevents the output from "chattering" or making fast, continual switches if the cycling above and below the setpoint occurs very rapidly. This differential requires that the temperature exceed setpoint by a certain amount before the output will turn off or on again. ![]() In cases where this cycling occurs rapidly, and to prevent damage to contactors and valves, an on-off differential, or "hysteresis," is added to the controller operations. Since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below. For heating control, the output is on when the temperature is below the setpoint, and off above setpoint. An on-off controller will switch the output only when the temperature crosses the setpoint. The output from the device is either on or off, with no middle state. Depending upon the system to be controlled, the operator will be able to use one type or another to control the process.Īn on-off controller is the simplest form of temperature control device. There are three basic types of controllers: on-off, proportional and PID. What Are the Different Types of Controllers, and How Do They Work?
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