Temperature controllers connect temperature sensors to heating or cooling systems to maintain a set temperature. There are three types of controllers: on/off, proportional, and PID. The complexity of the system determines the level of control required. On/off controllers switch the heat source on or off, proportional controllers reduce power as the set point is approached, and PID controllers include two additional adjustments for stable control.
A temperature controller is a device that controls the outputs of a heating or cooling system. This control is achieved by connecting the temperature sensors installed in the heated area or system to the controller. The data acquired by the sensors are compared with a reference preset by the user; should the temperature in the system deviate from it, the controller switches the heat or cooling source on or off accordingly. There are three main categories of temperature controller operation: on/off control, proportional control, and PID control. Each performs the same basic function of temperature control, but offers different levels of sophistication and functionality.
Heating and cooling systems typically consist of three basic components: a heat source, a control mechanism, and one or more sensors. Depending on the complexity of the system and the level of precision required in controlling temperatures, the control and sensor mechanisms may be no more complex than a bimetallic switch. Large and intricate manufacturing systems can, on the other hand, feature sophisticated computer controls that drive different heat sources based on data received from numerous sensors. To achieve this range of control actions, temperature dependent systems use one of three basic types of temperature controller.
The first category of temperature controllers is the on/off unit. This is the simplest of all controllers and offers a single level of control. With this type of controller, the heat source operates at full power and is switched on or off depending on the system temperature. A feature known as an on-off differential is incorporated into this type of controller which prevents relays, contactors or valves from being damaged by “vibration” if temperature changes occur too quickly. This feature allows the system temperature to rise above the setpoint by some margin before the controller switches prevent a rapid on/off cycle.
The second type of temperature controller is one that can regulate the temperature proportionally. This controller reduces the power at which the heater operates as it approaches the reference set point. This prevents the system temperatures from exceeding the set point as in the case of the on/off type controller. If the temperature deviates too far from the set point due to environmental issues, the controller then operates as an on/off unit keeping the heater power fully on or off.
The third type is proportional control with integral and derivative (PID). This type of controller works in the same way as standard proportional units, but includes two additional time-dependent adjustments. These adjustments are typically labeled “reset” and “rate” on the device and allow for extremely stable temperature control in applications where system load requirements change frequently. This variant, however, requires fine-tuning of all adjustment parameters to function properly.
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