Handbook: The Complete Guide to Relays

Are you planning to buy or even import relays in China? Here is a perfect guide for you. We are going to give you all the essential information that you should know when it comes to buying relays.

What is a relay?

A relay is an electrical switch that uses electromagnetism to transform electrical stimulus into current. They are powered by a small electrical current that can be used to turn on or off considerably bigger currents inside the relay.

A relay’s heart is an electromagnet, which is a wire coil. When power is applied to the electromagnet in earlier relays, the coil becomes a magnet through which electricity travels. Newer relays, such as solid-state relays tend to use newer electronic technologies.

A relay, in simple words, is a switch that can be switched on/off with a low voltage and can also be used to control many circuits with a single switch. Between the power (supply) circuit and the control circuit, relays provide complete electrical isolation.  Relays are commonly used to manage heavier loads on the primary side with a lower current, such as sensors, switches, or a PLC input.

How Does A Relay Work?

There are two circuits within the body of a relay: the primary circuit and the secondary circuit. The primary circuit is the side that receives the signal that controls the relay’s activity. On the primary side, a low voltage DC source (24V) is usually utilized to control the coil. However, some relays require 240V on the primary coil to operate.

The secondary side of a relay controls the switched side of the circuit. The load of the component or system that the relay controls is connected to this. A load is a mechanical or electrical component that operates on electricity, such as a motor, fan, generator, or light bulb.

When current flows through a relay’s coil (primary side), an electromagnetic field is created. When the field is present, it attracts the armature (which is usually formed of iron), which pushes the armature’s other end (secondary side) together, completing the circuit. When the current is turned off again, the contacts open, and the circuit is broken.

Where are Relays Used?

Relays regulate considerably higher current and voltage on the contacts with low voltage and low current on the relay coil. They can be found on every component, equipment, or gadget that runs on electricity.

Who Invented Relays?

In 1835, Joseph Henry invented relays. In a College of New Jersey demonstration, Henry demonstrated using a small electromagnet to turn on and off a larger one.

What are Relays made of?

A relay’s coil is made up of a coil of wire wound around a soft iron core. A relay’s contacts are made of iron, which has a low reluctance for magnetic flux. The bases of relays are usually composed of plastic, whereas the connection terminals are generally made of metal.

What are the Main Parts of a Relay?

In the relay, there are two significant circuits. There are two sides to everything: the primary side and the secondary side. The Primary Circuit provides the control signal for the relay. A manual switch, a thermostat, or a sensor could all be used to control this. A low voltage DC supply is usually linked to the primary circuit.

The load that needs to be shifted and regulated is contained in the secondary circuit. Any device that consumes electricity, such as a fan, pump, compressor, or light bulb, is a load.

Relay Construction

Relays come in various shapes, sizes, terminal counts, coil voltages, and maximum load currents. The foundation of the structure, however, is the same. It can be separated into two circuits.

A control circuit with a coil that generates magnetic fields is present. A load circuit with movable armature, spring, yoke, and contact switches is also current. Everything is contained within a plastic enclosure. Only the thermals are visible.

How does an Electromechanical Relay Works?

The normally open and normally closed relays are the two types of primary relays. Other types of relays exist, and we will discuss them later in this article.

The load is off since no power flows in the secondary circuit with the normally open type. A magnetic field is induced in the electromagnet when a current is passed through the Primary Circuit. The armature is attracted to the magnetic field, which pulls the moveable contactor until it reaches the secondary circuit terminals. This completes the circuit and connects the load to the power source.

With the generally closed type, the load is on because the secondary circuit is typically complete. The electromagnetic field causes the armature to push away when a current is provided through the primary circuit, disconnecting the contactor and breaking the circuit, cutting the flow of power to the load.

How Solid-State Relays Work (SSR)

Solid-State Relays, or SSRs, work similarly to electromechanical relays, but they don’t have any moving parts. The electrical and optical properties of solid-state semiconductors are used to accomplish input and output isolation and switching operations by the solid-state relay.

Instead of an electromagnet, this sort of device has an LED light on the principal side. By projecting a beam of light across a gap and into the receiver of a neighboring photosensitive transistor, the LED enables optical coupling. The functionality of this type is controlled by simply turning the LED on and off.

Unless it is exposed to light, the phototransistor functions as an insulator, preventing current flowing. Different layers of semiconductor materials are present inside the phototransistor.

N-type and P-type are sandwiched between each other. Although both the N-type and P-type are silicon, they have been combined with different materials to alter their electrical characteristics. The N-type has been blended with a substance that provides many extra and unneeded electrons that can freely migrate to other atoms. There is a lot of space on this side for electrons to migrate to because the P-type has been mixed with a substance with fewer electrons. An electrical barrier forms when the materials are linked together, preventing electrons from flowing.

When the LED is turned on, however, it emits a different particle known as a photon. When a photon strikes a P-type material, it knocks electrons loose, causing them to cross the barrier and into the N-type material. The electrons at the initial barrier will now make the jump, resulting in current development. The photons no longer knock electrons across the barrier when the LED is turned off, and the secondary side current ceases flowing. As a result, we can control the secondary circuit simply by shining a light beam on it.

What Are the Different Types of Relays?

There are many different sorts of relays; we’ll go over a few of the most common ones, as well as some basic instances of how they’re utilized.

Latching Relay

When the primary circuit is de-energized, the electromagnetic field vanishes, and the spring pulls the contactor back to its original position in a regular, usually open relay. When the primary circuit is opened, we sometimes want the secondary circuit to stay active. A latching relay can be used for this.

When we hit the call button on an elevator, we want the light on the button to stay on, so the user knows the elevator is on its way. As a result, we can employ Latching Relays to do this. This type of relay can be designed in various ways, but we have three distinct circuits with a piston in the middle in this basic example. The call button is the first circuit. The bulb is the second component, while the reset circuit is the third.

The circuit is completed, and the electromagnet is energized when the call button is pressed, pulling the piston and completing the circuit, allowing the lamp to turn on. To send the elevator down, a signal is also provided to the elevator controller. When the button is pressed, the power to the original circuit is switched off, but it remains in place because the piston isn’t spring-loaded, and the lamp remains on.

The elevator car makes touch with the off switch when it reaches the lower floor. This powers the second electromagnet, which pushes the piston away from the lamp, cutting power to it. As a result, latching relays benefit positional ‘memory.’ They will stay in their last position once triggered, requiring no additional input or current.

Timer relay

A timer relay is a control circuit that combines an electromagnetic relay and a timer. There are two types of timer relays: on delay and off delay. This means that the contacts will open or close before or after a certain amount of time has passed.

Thermal relay

The principle of a thermal relay is that when a bimetallic strip is heated to a given temperature, it bends and causes the contacts to shut or open. The current that runs through the strip heat it up.

Double Pole, Double Throw Relay

A DPDT relay, or double pole, double throw relay, is used to control two states on two different circuits.

A DPDT relay is shown in the example below. Terminals T1 and T2 are linked to terminals B and D, respectively, when the primary circuit is incomplete. The red LED, as well as the indicator light, has been turned on.

T1 and T2 connect to terminals A and C when the primary circuit is closed, the fan goes on, and the green LED illuminates.

Frequency Protection Relay

In two or three-phase systems (with or without a neutral), a frequency protection relay monitors the frequency and switches off quickly if it detects a fault or under/over-frequency readings. They’ll usually send an alert output to an HMI or machine interface to show a problem with the system.

High voltage relays

Electrical components called high voltage relays are used to switch higher voltages (anything greater than 1kV). They operate in the same way as a typical electromechanical relay but with a modified enclosure to prevent internal arcing. The relay contacts are enclosed in a glass or ceramic outer casing, which decreases the risk of arcing. Their coils are also away from the contacts and outside of the vacuum.

NO (normally open) relays

In the default state, normally, open relays (NO) have open contacts. This means that if the relay coil is not powered, the contacts will be open, and no electricity will flow through. When electricity is applied to the relay’s coil, it will shut, allowing power to flow to the component/wires connected to the switch’s other side.

NC (normally closed) relays

Normally closed relays (NC) have closed contacts in the default position. This means that even if the relay’s coil contacts aren’t powered, power will continue to flow through the relay unless the coil is powered.

Double or Single Pole

Single-pole or double-pole relays are available. When a relay is powered, the term Pole refers to how many contacts are switched. A single primary circuit can now power many secondary circuits.

A double pole relay, for example, might be used to handle both a cooling fan and a warning light. Both the fan and the lamp are ordinarily turned off, but the bimetallic strip on the primary circuit bends to complete the circuit when it becomes too hot. This generates an electromagnetic field and closes both secondary contactors, supplying electricity to the cooling fan and the warning light.

Double or Single Throw Relays

When dealing with relays, the phrase “throws” is frequently used. The number of contacts or connection points is referred to as this. A typically open and ordinarily closed circuit are combined in a double throw relay. Because it switches between two secondary circuits, a double throw relay is also known as a changeover relay.

When the primary circuit is open, the secondary side’s spring pulls the contactor to terminal B, which powers the lamp. Because the circuit isn’t complete, the fan remains turned off.  The electromagnet pulls the contactor to terminal A and diverts the current, powering the fan and shutting out the lamp when the primary side is energized. So, depending on the occurrence, we can utilize this form of the relay to operate different circuits.

Conclusion

Always double-check the relay switch’s voltage and current rating. Some are solely intended to switch low voltages, while others are designed to switch greater voltages. I hope that this simple explanation of how a relay works has clarified things for you. Now proceed to solder some relay circuits!

Lastly, if you need to order some relays, at ICRFQ we are a leading Electronic Components Distributor in China. Contact as today for the best deals and premium quality products.

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