Control Relays

Relays are among the oldest and most heavily used electronic components in the world. The most common design for a relay is that of an electromagnetic switch that can change its open or closed state with the application of a current to a coil.

Relays are commonly used to isolate circuits, particularly in cases when one of the circuits is a control circuit and the other one is the controlled circuit.

The long history of relays extends all the way back to the days of telegraphs, when they were used in a repeater function. They continue to be utilized by the telephone industry in exchange facilities. Later on, relays found a place in the very first computers, where they were utilized as a means to perform logical operations.

Relays are still common today, being used on everything from household appliances to the largest industrial equipment in the world. Their flexibility as electronic components makes them consistently useful. Most relays are mechanical, though there are ones that are completely solid-state, that have no moving parts involved in their operation, and that allow a very compact design to be achieved in some devices.

With the addition of solid-state relays to the available tools, the usefulness of relays has only expanded. Today, these parts are so common that they can oftentimes be harvested from old electronic devices and reused on other electronic devices, as a great many basic circuits call for them to be installed for safety or functionality purposes.

What are control relays?

Like all relays, a control relay is a type of electromagnetic switch. The switches can be controlled automatically or manually. Control relays may be in a normally open or normally closed state, which can be changed by energizing the device so that the amount of current exceeds what is called the holding current. When the holding current is exceeded, the contacts on the relay move to a different state; closing if they are normally open or opening if they are normally closed.

Control relays are particularly common on circuits with large amounts of current moving across them. The relay helps to control arcing, which can prematurely damage contacts and significantly reduce the lifetime of electrical components in the circuitry.

Control relays come in a variety of different sizes and are able to handle a range of different power levels so that they can be utilized on circuits both large and small. Control relays can be found in devices as common as kitchen appliances, but are also oftentimes mounted to very large equipment to protect motors, generators, and other expensive equipment from being damaged by conditions on the circuit that will inevitably lead to a breakdown or, at least, to a lower lifespan for the machinery.

What types of control relays exist?

There are several different types of control relays in use. The relays are sometimes differentiated by scale and their normal positions. For example, contractors are specialized types of relays that are in a normally open position and that are utilized for very high current loads. Most relays, however, are smaller than contactors, and relays are frequently found in home appliances, such as refrigerators, where they provide an efficient, reliable, and safe way to energize or de-energize circuits while controlling arcing.

Impulse Relays

And impulse relay functions in the same basic fashion as any other relay, meaning that contacts can be opened or closed when electricity is used to energize the coil inside the relay. What differentiates impulse relays is the fact that they have a sort of memory feature to them.

In an impulse relay, a momentary application of electricity changes the state of the relay and the relay retains that state until another pulse of electricity is sent through the relay. This makes them particularly useful for pushbutton switch applications when pushing a switch needs to be able to energize or de-energize a circuit.

Monitoring Relays

A monitoring relay is a specialized type of relay that is designed to protect circuitry. These relays can be designed to monitor for overpower states on a circuit or to monitor for under power states on a circuit. They can also be utilized to monitor power direction. Because overpowered states, underpowered states or improperly flowing current can all damage sensitive electronic equipment, these types of relays are oftentimes employed on devices where fluctuations in the nature of voltage or current can cause significant damage. These are used protect everything from industrial motors to very advanced, compact circuits.

Signal Conditioning

Relays quite frequently play an important part in signal conditioning. In signal conditioning, an analog signal is manipulated in some fashion so that it can be used for a specific process. This is very commonly seen in analog to digital converters.

Relays in this capacity function as a specialized type of input device, which can be utilized to react to changes in the condition of the signal by opening or closing, as needed.

What monitor applications exist?

Relays can be utilized to monitor a variety of different conditions on an electrical circuit. In some cases, the application of these particular relays has resulted in them being called protective relays, as that is exactly the function that they perform.


Current monitoring relays can detect overcurrent conditions and undercurrent conditions. Either of these can be hazardous to electrical equipment, including the motors used in some industrial processes. Some relays can detect either condition in a circuit and are essentially able to perform two tasks in one device.

Current, Phase

Variations in phase can cause problems with electrical equipment and specific types of relays can monitor for this. They can monitor for out of phase conditions, inverted phases and other problems that could damage electrical equipment. This type of monitoring is sometimes combined with other types of monitoring to provide maximum protection for a circuit.

Current, Temperature, Voltage

These relays can provide circuit protection by reacting to variations in current, the temperature across a circuit, in a room or a switchboard and the voltage across a circuit. These are particularly useful in applications involving monitoring motors and other electrical machinery, which may be seriously damaged by such conditions. These are types of protective relays.

Current, Voltage

Monitoring current and voltage is a basic safety consideration for any circuit. Relays that perform this function can change their state based on variations in the voltage or the current going across the circuit to which they are attached. This can prevent machinery from being damaged by receiving too much or too little current or from receiving too much or too little voltage. Like many protective relays, these are common components in industrial applications.


These types of relays are oftentimes utilized to monitor power generating equipment and electric motors. They can provide feedback on whether or not the equipment is running at the right frequency and can monitor the electrical lines for problems related to the phase of the electricity. These provide vital protection for some of the most heavy-duty, yet very sensitive, electrical equipment.

Frequency, Voltage

Frequency and voltage can oftentimes be monitored in the same relay device, allowing for a great deal of protection to be achieved for any electrical equipment. Frequency and voltage are commonly measured on circuits that are utilized to energize electric motors, generators and other machinery that may be damaged by a bad phase condition or by receiving too much or too little voltage.


By measuring the connection between a DC circuit and its earth connection, these relays can provide protection for circuits in a wide variety of different scenarios. These devices are used on board ships to provide safety, but are also used in other applications where the condition of the insulation on the circuit needs to be monitored closely to ensure safe operation and safe conditions for any devices attached to the circuit.


Level monitoring relays are commonly used in applications where pumps are involved. They function by measuring conductivity, having a high and low threshold and being able to switch machinery on or off as needed based on the conductivity being measured. Pumps that run dry can quickly become damaged and, because of that, these types of relays are necessary equipment in just about any scenario where a pump is being employed.

Load Sharing

These types of relays are utilized to ensure that the operation of multiple pieces of equipment is balanced. They can be used to control motors, pumps, generators and other types of industrial equipment. They can be utilized to provide for the safe operation of two devices at once or they can be utilized to provide for the safe operation of devices in a cascade.


Phase monitoring relays provide a way to protect devices such as electric motors. These relays can provide against phase loss, unbalance in the phase cycle and phase reversal. These are all conditions that can occur when an electrical circuit is compromised by a blown fuse or worn-out equipment.

Phase, Temperature

These devices are sensitive to both incorrect phase sequence on a circuit and the temperature in a motor room or in a switchboard. This makes them necessary equipment in many industrial applications, when the temperature of machinery may become a problem and when, of course, the phase of an electrical signal needs to be closely monitored for proper operation.

Phase, Temperature, Voltage

Phase, temperature and voltage monitoring relays provide protection for industrial equipment and the circuits to which it is attached. They can monitor the phase of an electrical signal and change their condition according to that, measure the temperature inside a motor room or inside a circuit panel and measure the voltage across a circuit, reacting to any of those variables by changing the state of their switch.

Phase, Time Delay, Voltage

These relays keep track of the phase of an electrical signal, the voltage going across the circuit and can also provide timing functionalities. Like most other relays of this type, they are designed to provide a measure of protection for electrical circuitry.

Phase, Voltage

Phase and voltage are both important variables in alternating current and these relays are designed to monitor both. They can change their condition based upon improper phase in a circuit and based on voltage variations that may cause damage to electrical devices.


A pulse relay is able to switch to and on condition in response to a pulse and to switch back to an off condition in response to another pulse. This makes them excellent choices for switches that need to be operable based on receiving a single signal to change their state. These relays will remain on after they are triggered until they are triggered again and switch their state to off.


Speed relays are used to measure the operating speed of a piece of equipment and to respond to a condition when it is operating to quickly or too slowly for safety parameters. These relays will change their state in response to an improper speed, protecting machinery and increasing safety around that machinery.


These are relay devices that are utilized to monitor the temperature on a device, in a room or across an electrical circuit. By monitoring for high or low temperature conditions, the safety of equipment can be preserved by opening or closing the circuit, as appropriate.


Thermistor monitoring relays are utilized to protect machinery. They can react to conditions when the machinery goes above a safe temperature and shut it down automatically, preventing damage and other potential types of harm to the machinery or to the people who work on it.


Voltage monitoring relays are among the most common types in use. They simply monitor for conditions when a circuit is receiving too much voltage or too little voltage and, under those conditions, the relay will change its state accordingly. These types of relays allow for very fast responses to overvoltage or undervoltage conditions on a circuit, making it possible to provide a great deal of protection for sensitive electronic devices, which can oftentimes be destroyed by receiving too much voltage.

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