What is the Function of a Relay?

Relays are automatic switching elements with an isolation function. They are widely used in remote control, telemetry, communication, and electronic equipment. They serve as the main control element of the circuit.

In general, relays have an input part (sensor) that can reflect various input variables such as voltage, current, impedance (frequency), temperature, pressure, light speed, and more. There is an input mechanism that allows the circuit to be “turned on” or “turned off.” A coupling mechanism connects the input quantity with the functional processing and drives the output part.

Relays serve the following functions as a control element:

1) Expanding the control range

When the control signal for a multi-contact receptacle reaches a specific value, it is possible to switch, disconnect, and connect multiple circuits at the same time from different contact groups.

2) Amplification

A very small amount of control can be used to control large power circuits through relays that are sensitive or intermediate relays, etc.

3) Integration of Signals

When multiple control signals in a prescribed format are applied to a multi-winding relay, the signals will be integrated relatively to produce a predetermined effect.

4) Remote control, automatic control, and monitoring

The relay of an automatic device or other electrical appliances can be used to form a circuit that controls the program and allows automatic operation.

Relays are available in a variety of types:

Intermediate Relay

The function is to convert and transmit the control signal. Its input signal is the power on/off signal of the coil, and its output signal is contact action by the intermediate relay. It is a voltage relay that has multi-contacts. It can handle high current (5A-10A) and is highly sensitive (response times less than 0.05s).

Voltage Relay

The main principle is to use the voltage signal to determine the contact action. In addition, it must be connected in parallel to the load when designing the circuit. According to the coil voltage of the voltage relay, it can be classified into AC or DC type. It can also be categorized into Overvoltage and Undervoltage depending on the operating voltage. Their functions are different. When the coil voltage falls within the range of the rated values, the armature does not pull in. The action is executed if the coil voltage exceeds the rated value range. Overvoltage protection is provided by the AC overvoltage switch. The coil voltage is reduced when the coil voltage exceeds or reaches the rated coil value. The Undervoltage relay is primarily used as protection against Undervoltage, and the armature is released when the coil voltage falls below the operating value.

Current Relay

The coil current determines how the relay contacts are activated. When installing the current relay coil, it must be connected in parallel with the load. AC and DC coil currents can be classified. According to the current action, it can be classified into two types: overcurrent and undercurrent.

The coil rating current (the setting current) should be chosen to be equal to the maximum current of the load. The armature does not move when the load current is below the setting value. If it is higher, the armature will pull in. Overcurrent relays are used to protect the circuit from overcurrent, particularly in situations where there is an impulsive surge. They have a significant protective effect.

Undercurrent relays work on the principle that the armature pulls in when the coil current reaches or surpasses the operating value. The armature is released when the coil current falls below the operating current. If the load current is greater than the working current, then the armature also pulls in. The armature is released when the coil current falls below the load current.

Time Relay

This relay starts with an input signal, i.e., when the coil is turned on or off. It will then output the signal after a predetermined delay. In general, time relays are used to switch on or off high voltages or currents in circuits with relatively low voltages or currents.

This post was written by Justin Tidd, Director at Swartz Engineering. For nearly half a century, Swartz Engineering has been at the forefront of industry safety. They are a family-owned company specializing in power distribution for the electrical industry. They are the leading manufacturer of voltage transducer. The SWARTZ® Current Transducer is a high-performance, solid-state unit specifically developed to meet requirements for supervisory control and data acquisition (SCA- DA) systems.