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Introduction of Relay contact switch
Apr 17, 2020

In our process measurement instruments for both local indication or with analog output signals, Relay contact switch is also widely used additionally control for alarm, interlocks and etc. The relay’s switch connections are usually labeled COM, NC and NO:

COM = Common, always connect to this; it is the moving part of the switch.

NC = Normally Closed, COM is connected to this when the relay coil is off

NO = Normally Open, COM is connected to this when the relay coil is on

Connect to COM and NO if you want the switched circuit to be on when the relay coil is on

Connect to COM and NC if you want the switched circuit to be on when the relay coil is off

There are basically two types of actuators : single action and double action.

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Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil.

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