A temperature controller produces an output action based on the input signal received from a sensor. Controllers used in cooling applications are called direct acting. Controllers used in heating applications are called reverse acting. Depending on the controller, output actions can control a heating or cooling device, or some other aspect of a process (ratio mixing, conveyor speed, etc.).
Temperature controllers are either single-loop or multi-loop. Single-loop temperature controllers are good for basic temperature control. Various levels of sophistication can reduce temperature under- and over-shoot, produce alarm actions and perform data logging functions as well as serial communications.
Multi-loop temperature controllers (also called process controllers)are good for applications where temperature and other process variables need to be controlled in a coordinated fashion.
Power controllers receive an output action from the temperature controller and apply or interrupt the electric power to the heating or cooling device. Power controllers switch electric power through physical contacts, or solid state devices.
Mercury displacement relays (MDRs) use mercury to conduct electric current across two contacts. On-off is achieved by changing the mercury level in the reservoir with a plunger activated by a solenoid coil. MDRs switch both AC and DC power.
Solid state devices use semiconductors. Precise power control is achieved by electronically switching the device ON or OFF. The advantage of any solid state power controller is its almost infinite life and ability to deliver the rapid switching cycles required by PID temperature control modes. Life is dramatically affected by improper application, installation and high ambient temperature conditions. Solid state devices generally switch AC power only.
Common solid state power controllers are SSRs (solid state relays) and SCRs (silicon controlled rectifiers). Most SSRs switch at the zero electrical potential to minimize electrical noise caused by mid-cycle switching. SCRs can be configured to act as a solid state contactor, or control the amount of power applied through burst firing or phase angle firing: