Analysis of the Function of Parallel Diodes in Relay Coils 1

A relay is an electrical device widely used in industrial control systems, capable of achieving functions such as signal conversion, amplification, and switching control within circuits. In relay applications, a common design involves connecting a diode in parallel with the relay coil, which plays a crucial role in enhancing circuit stability and protecting circuit components. This article will briefly analyze the function of the parallel diode in relay coils and the importance of this design in electrical control systems.diode

When the relay coil is energized, a magnetic field is generated within the coil, which intensifies or weakens as the current changes. If the current is suddenly interrupted, the magnetic field rapidly dissipates. According to Faraday's law of electromagnetic induction, this changing magnetic field induces an electromotive force (EMF) in the coil, opposing the original current direction—this is known as back EMF or induced EMF. The instantaneous peak of back EMF can be extremely high, potentially damaging sensitive components in the relay control circuit, such as the FDP3651U transistor, integrated circuits, and others.diode

First, to understand the function of a parallel diode with the relay coil, we need to grasp the working principle of the relay and the electromagnetic phenomena it generates. The relay operates through electromagnetic effects. When the relay coil is energized, it generates a magnetic field that attracts the contacts to close, thereby controlling the circuit's on/off state. When the relay coil is de-energized, the magnetic field disappears, and the contacts return to their original position, disconnecting the circuit. During this process, due to the characteristics of inductive components (such as the relay coil), a sudden change in current induces a significant back electromotive force (EMF) in the circuit, a phenomenon also known as back EMF.diode