Control relay

Control relay

A control relay is an electromechanical device which activates one or more switches according to the current through a coil not connected to the switches. The thyristor is a semiconductor device which carries out similar functions.

A relay is essentially an electromagnet with two possible states arranged so that when there is sufficient current the core of the relay's coil attracts a ferromagnetic armature which mechanically operates switches; a spring holds the armature away from the core when not actuated. The spring is designed to snap the contacts between two stable mechanical states; there should not be a range of coil current which allows the contacts to be in an intermediate state.

A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with a relay could switch power at a preset time. For many years relays were the standard method of controlling industrial electronic systems. A number of relays could be used together to carry out complex functions (relay logic). The principle of relay logic is based on relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder logic, which is commonly used in Programmable logic controllers.

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When electric current passes through a coil, magnetic north and south poles are produced across the gap separating the coil and armature. The relay is actuated, or latched, when sufficient current through the coil makes the attractive force between the core and armature overcome the spring tension . The relay remains latched as long as at least a specific value of "holding current", which can be lower than the actuating current, flows through the coil. When the current through the coil is reduced below this value, the magnetic attraction of the armature becomes too weak to hold it in the actuated position and the spring snaps it to the non-actuated position.

There are many considerations involved in the correct selection of a control relay for a particular application. These considerations include factors such as speed of operation, sensitivity, and hysteresis. Although typical control relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 us are available. Reed relays which are actuated by low currents and switch fast are suitable for controlling small currents.

As for any switch, the current through the relay contacts (unrelated to the current through the coil) must not exceed a certain value to avoid damage. In the particular case of high-inductance circuits such as motors other issues must be addressed. When a power source is connected to an inductance, an input surge current which may be several times larger than the steady current exists. When the circuit is broken, the current cannot change instantaneously, which creates a potentially damaging spark across the separating contacts.

Consequently for relays which may be used to control inductive loads we must specify the maximum current that may flow through the relay contacts when it actuates, the "make rating"; the continuous rating; and the "break rating". The make rating may be several times larger than the continuous rating, which is itself larger than the break rating.

Derating factors

Control relays should not be operated above rated temperature because of resulting increased degradation and fatigue. Common practice is to derate 20 degrees Celsius from the maximum rated temperature limit. Relays operating at rated load are also affected by their environment. Oil vapors may greatly decrease the contact tip life, and dust or dirt may cause the tips to burn before their normal life expectancy. Control relay life cycle varies from 50,000 to over one million cycles depending on the electrical loads of the contacts, duty cycle, application, and the extent to which the relay is derated. When a control relay is operating at its derated value, it is controlling a lower value of current than its maximum make and break ratings. This is often done to extend the operating life of the control relay. Table 1 lists the relay and switch derating factors for typical industrial control applications.


Colin Simpson, Principles of Electronics, Prentice-Hall, 2002, ISBN 0-0686860-3-6

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