Analysis of the role of parallel diodes in relay coils

Relay is an electrical equipment widely used in industrial control systems, which can achieve functions such as signal conversion, amplification, and switch control in circuits. In the application of relays, the design of parallel diodes in relay coils is often seen, which plays a crucial role in improving circuit stability and protecting circuit components. This article will analyze the role of parallel diodes in relay coils and the importance of this design in electrical control systems.

When the relay coil is energized, a magnetic field is generated inside the coil, which increases or decreases with the change of current. If the current is suddenly cut off, the magnetic field will quickly disappear. According to Faraday's law of electromagnetic induction, this changing magnetic field will generate an electromotive force in the coil, with the direction opposite to the original current direction, which is called reverse electromotive force or induced electromotive force. The instantaneous peak value of reverse electromotive force may be very high enough to damage sensitive components in relay control circuits, such as transistors, integrated circuits, etc.

Firstly, to understand the function of parallel diodes in relay coils, we need to understand the working principle of relays and the electromagnetic phenomena they generate. Relays achieve control through electromagnetic effects. When the relay coil is energized, a magnetic field is generated to attract the contacts to close, thereby controlling the on/off of the circuit. When the relay coil loses power, the magnetic field disappears, the contacts return to their original position, and the circuit is disconnected. In this process, due to the characteristics of inductive components (such as relay coils), when their current suddenly changes, a large induced electromotive force will be generated in the circuit, which is also known as back electromotive force.

The magnitude of the back electromotive force is related to the speed of current change and the inductance of the coil, and its direction is opposite to the original current direction, which may cause damage to other components in the circuit, especially semiconductor components. In order to suppress the impact of this back electromotive force on the circuit, a diode is often connected in parallel at both ends of the relay coil. This type of diode is commonly referred to as a reverse diode or a freewheeling diode.

The diode connected in parallel at both ends of the relay coil is in a reverse bias state during normal operation, which does not affect the normal operation of the relay coil. When the relay is powered off, the current in the coil suddenly interrupts, and the generated back electromotive force will forward bias the diode. At this time, the diode conducts, forming a circuit that allows the current generated by the back electromotive force to flow through the circuit formed by the diode and coil, effectively limiting the back electromotive force to a lower level and reducing the impact on other parts of the circuit.

Pay attention to the direction of parallel diodes. The correct way is to connect the anode of the diode to the negative terminal of the relay coil and the cathode to the positive terminal. In this way, during normal operation, the diode is reverse biased without affecting the operation of the relay; When the relay loses power and generates reverse electromotive force, the diode is forward biased, which can effectively conduct the current generated by the reverse electromotive force.

The main advantages of using diodes in parallel on relay coils include:

1. Suppressing back electromotive force and protecting circuit components, especially for weak signal circuits and sensitive semiconductor components, this protective effect is very important.

2. Reduce electromagnetic interference, as the current generated by the back electromotive force is limited to a smaller range, it can effectively reduce electromagnetic interference and improve the stability and reliability of the circuit.

3. Extend the lifespan of relay contacts by reducing electrical sparks between them, which can effectively prolong the service life of relay contacts.

Overall, the design of relay coil parallel diodes is a simple and effective method for improving the stability and reliability of circuits, and protecting circuit components from damage caused by back electromotive force. In the design of electrical control systems, this design consideration is of great significance for ensuring stable operation of the system and extending equipment life.