Relay wiring diagram | What is Relay?

Relays are one of the essential components of the modern automation sector. A relay is an electromechanical switch that toggles a mechanical contact between an ON and OFF state due to a control signal. We use relays liberally in automobiles, test and measurement equipment, power supplies, home automation systems and many more. They are a very important part of an automation system. Relay wiring diagram for every circuit is compulsory.

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What is a Relay and How Does it Work?

A relay is an electrically operated switch. They usually use an electromagnet (the coil) to operate their internal mechanical switching mechanism (the contacts). When a relay contact is open, it will turn on power to the circuit when the coil is energized.

Types of Relays

There are two type of relay that is given below-

  • Mechanical relay– these relays transfer signals with mechanical motion.
  • Solid state relay – these relay transfer signal with electronic circuits.

Why use a Relay?

Relays allow a low current circuit to control one or more high current circuits. Relays provide these benefits;

  1. Thinner cable can be used to connect the control switch to the relay thereby saving weight, space and cost.
  2. Relays allow power to be routed to devices over the shortest possible distance, thereby minimizing voltage loss.
  3. Heavy gauge cable only needs to be used to connect the device to the power source (via the relay).

Why Use a Relay in a Car?

There are many advantages of using a relay in a car. Firstly using relays means that the switching circuit does not require high current rated switches or cables which reduces cost and weight. Secondly, the relay can be positioned anywhere in the vehicle to provide efficient power transfer to the electrical accessory you are controlling. Relays are perfect for controlling multiple circuits in a car such as headlights, electric motors, heaters, etc.

How to Understand the Electrical Specifications of a Relay

Basic relays have electrical ratings for both the coil and the internal switching contacts. Coil voltage rating is the voltage required for the coil to operate properly. The switching circuit of a relay also has a voltage and ampere rating. This is the maximum rating of contacts and should not be exceeded. Double throw relays often have a 2 x switch electrical specification. One for NO terminal, the other for NC terminal. In other words, N/O: 35A at 24Vdc, N/C: 20A at 24Vdc.

Why are Protection Devices used in a Relay?

Relays can produce a large voltage spike when the switch is closed due to coil de-energizing. Resistors or diodes are sometimes fitted to the coils of relays to prevent these spikes from traveling back into the control circuit and damaging sensitive components. Resistors are more durable than diodes, but not as efficient at eliminating voltage spikes. You need to assess the sensitivity of the components in the control circuit when deciding if/what type of protection is needed.

Sample Wiring Diagrams for a 4 Pin Normally Open Relay

4 pin relay

4 pin relay diode

What’s the difference between 4 or 5 pin Relays?

The difference between 4 and 5 pin relay is that 4 pin relay is used to control a single circuit, whereas 5 pin relay switches power between two circuits.

4 Pin Relay

4 pin relays use 2 pins (13 and 14) to control the coil and 2 pins (9 and 5) that switch power on a single circuit. There are 2 types of 4 pin relays available; normally open or normally closed. A normally open relay will turn on power for a circuit when the coil is energized. A normally closed relay will shut off power to a circuit when the coil is energized.

4 pin relay no

4 pin relay nc

5 Pin Relay

The 5 pin relay provides 2 pins (13 and 14) to control the coil and 3 pins (9, 5 and 1) that switch power between the two circuits. They have both normally open and normally closed connection pins. When the coil is energized, power will switch from the normally closed pin to the normally open pin.

5 pin relay

What are ISO Relays Used For?

ISO relays are designed for use in the automotive industry and follow a standard pattern for their electrical terminals. The new ISO 280 relays use a smaller pin terminal that is 2.8 mm wide and can be used in compact power distribution units and holders.

Sample Wiring Diagrams for a Normally Open Relay


4 pin relay wiring diagram-

 4 pin (normally open) relay with the switch on the positive side of the control circuit.

4pin relay diagram

Example 2.

Relay 4 pin wiring diagram

4 pin (normally open) relay with the switch on the negative side of the control circuit.

4pin relay diagram nc

Fuel pump relay wiring diagram

fuel wiring

Importance and application of relay

Relays are mainly used for remote shifting and high voltage or high current switching. They are especially important because they can handle these high voltages and flow with little voltage or current. Another important use is for AC power cables.

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