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Diodes in Relay Protection

Fiber and communications infrastructure knowledge brief Published 29 Nov 2021

Use this reference to prepare product questions for CJC FIBRE. Check interfaces, capacity, protection and installation requirements against the current product documentation.

Diodes are used in relay circuits primarily to suppress back EMF and protect control electronics from voltage spikes generated when the relay coil is de-energized.

Function of Diodes in Relay Circuits

When a relay coil is energized, it creates a magnetic field that operates the relay contacts. When the coil is de-energized, this magnetic field collapses, generating a high-voltage spike in the opposite direction, known as back EMF. This spike can damage sensitive components such as transistors, microcontrollers, or PLC inputs.

A diode placed across the relay coil, in reverse bias relative to the supply voltage, provides a safe path for this energy, allowing it to circulate through the coil and diode until it dissipates, preventing damage to the driving circuit (flyback diode).

Types of Diodes Used

Flyback Diode (Standard Rectifier Diode): The most common type, such as 1N4007, used to suppress back EMF in DC relay circuits. Avalanche or Zener Diodes: Used for polarity-insensitive protection and to handle higher voltage surges. A double Zener diode can protect both the relay coil and the control electronics from surges in either direction.

Polarization Diode: Ensures that no current flows if the relay is connected with reversed polarity, preventing damage.

Installation Considerations

The diode should be connected in parallel with the relay coil, with the cathode to the positive supply and anode to the negative side, so it does not conduct during normal operation. It is recommended to place the diode as close to the relay coil as possible to minimize the effect of voltage spikes on the driving circuit. While diodes protect electronics, they slightly slow the relay release time because the coil current decays more gradually through the diode.

Benefits

Protects transistors, microcontrollers, and PLCs from high-voltage spikes. Prevents arcing across relay contacts, enhancing relay longevity. Reduces maintenance and replacement costs in systems with frequent relay switching.

In summary, diodes in relay protection circuits are essential for safely managing back EMF, ensuring the reliability and longevity of both the relay and the control electronics. Proper selection and installation of these diodes are critical for effective protection.

Diodes in Relay Protection - CJC FIBRE CONSULTANTS [PDF]

Diodes in Relay Protection

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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