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Eventually, the relays became electronic switches, as transistors eventually could handle greater current loads. By 1985, pneumatic controls could no longer compete with this new technology although pneumatic control systems (sometimes decades old) are still common in many older buildings. [8] By the year 2000, computerized controllers were common.
A key switch (sometimes called a keyswitch or lock switch) is a key-operated switch. [1] Key switches are used in situations where access to the switch's functions needs to be restricted. [2] [3] Key switches are available as components with solder connections, and are available with a variety of ampere ratings. They may use tubular or other ...
These sense the air temperature using the differential expansion of two metals to actuate an on/off switch. [14] Typically the central system would be switched on when the temperature drops below the setpoint on the thermostat, and switched off when it rises above, with a few degrees of hysteresis to prevent excessive switching.
The heating elements activate one at a time to avoid overloading the electrical system. Overheating is prevented by a safety switch called a limit controller or limit switch. This limit controller may shut the furnace off if the blower fails or if something is blocking the air flow.
Energy regulator of an electric stove with a glass-ceramic hob. 1: Electric contacts (here closed); 2: Bimetal; 3: Heater for Bimetal. An infinite switch, simmerstat, energy regulator or infinite controller is a type of switch that allows variable power output of a heating element of an electric stove. It is called "infinite" because its ...
A Single-Pole, Single-Throw (SPST) mercury switch on millimetre graph paper, device length approximately 1.5 cm Another mercury switch design. A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic ...
In 1966, devices were developed with a rated voltage of 15 kV and short-circuit breaking currents of 25 and 31.5 kA. After the 1970s, vacuum switches began to replace the minimal-oil switches in medium-voltage switchgear. In the early 1980s, SF6 switches and breakers were also gradually replaced by vacuum technology in medium-voltage application.
Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems. Forced air heating is the type of central heating most commonly installed in North America. [1]
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