Definition of Relay
A relay is an automatic control device that undergoes a jumping change in output when the input reaches a certain value.
Relays are generally composed of iron cores, coils, armatures, contact springs, etc. When a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, generating an electromagnetic effect. The armature will overcome the pulling force of the return spring and be attracted to the iron core under the electromagnetic force, thereby driving the moving contact of the armature to engage with the stationary contact (normally open contact). When the coil is powered off, the electromagnetic attraction also disappears, and the armature will return to its original position under the reaction force of the spring, releasing the moving contact and the original static contact (normally closed contact). This suction and release achieves the purpose of conduction and disconnection in the circuit.
Relays have an interactive relationship between the control system (also known as the input circuit) and the controlled system (also known as the output circuit). It is actually an "automatic switch" that uses a smaller current to control a larger current, playing a role in automatic regulation, safety protection, and circuit conversion in the circuit.
What is a relay
Why use a relay?
Relays allow low current circuits to control one or more high current circuits.
Relays have the following advantages:
1. Finer cables can be used to connect control switches to relays, saving weight, space, and cost.
2. Relays allow power to be transmitted to equipment in the shortest distance possible, thereby reducing voltage loss.
3. Simply connect the power supply (via relay) to the device using a thick specification cable.
The difference between 4-pin relay and 5-pin relay
Pin quantity and function definition:
- 4-pin relay：Usually, there are four pins to choose from: two for connecting control circuits, typically labeled 85 and 86, and two for connecting controlled circuits, typically labeled 87 and 30. Once the low current signal is connected to pins 85 and 86, it will energize the relay coil, generate a magnetic field, close the contact between pins 87 and 30, and control the high current with the low current.
4-pin relay wiring diagram
- 5-pin relay：There are a total of 5 pins. The other two pins 85 and 86 are also connection terminals for the control circuit; One pin is a common terminal, 30; The other two pins are normally open contact 87 and normally closed contact 87a. When power is not supplied to the relay coil, the common terminal 30 is usually connected to the normally closed contact 87a. Unless the coil is connected, it will connect the common terminal 30 to the normally open contact 87. This structure can enable 5-pin relays to achieve more circuit control functions and state switching.
5-pin relay wiring diagram
Working status and circuit connection
- 4-pin relay: The working state is relatively simple. When the coil is not powered, the contact between pins 87 and 30 is disconnected; After the coil is powered on, the contacts are attracted and the controlled circuit is turned on. Commonly used for circuits that only require simple on-off control, such as ordinary lighting control, simple motor start stop control, etc.
- 5-pin relay: Due to its two contact states of normally open and normally closed, it can achieve different connection methods of the circuit based on the power on and off status of the coil. For example, it is widely used in situations that require circuit switching, backup circuit startup, or fault protection. For example, in some dual power switching circuits, when the main power supply is normal, the normally closed contact of the relay disconnects the backup power supply circuit, and the normally open contact connects the main power supply circuit; When the main power supply fails, the coil is energized, and the normally closed contact disconnects the main power supply circuit, while the normally open contact connects to the backup power supply circuit, achieving automatic power switching.
Application scenarios
- 4-pin relay: suitable for scenarios where circuit control requirements are relatively simple and only a single on-off function is needed. Due to its simple structure and low cost, it is widely used in some small electrical equipment, simple electronic circuits, and some basic circuit systems of automobiles.
- 5-pin relay: Due to its more flexible circuit switching function and more control possibilities, it is commonly used in applications that require high circuit control requirements, complex circuit switching or status monitoring, such as automation control systems, industrial equipment, communication equipment, and some advanced electronic control systems in automobiles.
We have a large number of 4-pin and 5-pin relays that support OEM/ODM customization
source：Wikipedia--Explanation of Relays