On / Off Controls and Thermostats
Thermostats or ON / Off controls are relatively simple and economic switches still widely used for switching power for electric heaters. The output will be on or off when the process temperature crosses the set point. This constant crossing of the set point means the temperature is constantly cycling around the set point. This constant cycling around the setpoint will also reduce heater life because full heat is applied whether the process temperature is 5 degrees or 100 degrees below setpoint. ON/Off controls increase thermal fatigue and oxidation rate on heating elements by causing wide temperature swings of the internal heating element. (think of a light bulb turned on and off constantly)
To minimize stress for the heaters and avoid damage to the contactors by rapid cycling, a so called "hysterisis" can be added to the on/off control. To maximize heater performance, use a solid state relay (SSR). Such a solid state zero cross contactor can cost as little as $ 100 but will save you money. Another option is a silicone controller rectifier (SCR) (single phase or 3-phase). For most applications, we also recommand the use of a PID temperature controller.
A thermostat can be built from a mercury switch (a glass tube with a small amount of Mercury), a bimetallic strip (lamination of two different metals together) and others. Sometimes they also have a heat anticipator, which shuts down the power to the heater before the heat actually reaches the set temperature.
On / Off controls can be used for:
- high temperature alarms
- Where maintaining accurate process temperature is not necessary
- where the mass of the system is so great that temperature changes slowly