The purpose of a flyback diode (also known as a freewheeling diode or snubber diode) in a relay circuit is to protect sensitive electronic components and prevent voltage spikes when the relay coil is de-energized.
When a relay coil is energized, it generates a magnetic field that causes the relay contacts to close and complete the circuit. However, when the coil is de-energized (turned off), the magnetic field collapses, and this change induces a voltage spike across the coil terminals. This voltage spike can be quite substantial and can lead to several issues:
Voltage Transients: The sudden voltage spike can damage the relay itself, as well as other components connected to the circuit, such as transistors, microcontrollers, or other sensitive electronic devices.
Electromagnetic Interference (EMI): The voltage spike can generate electromagnetic interference, causing malfunctions in nearby electronic circuits.
Arcing and Contact Wear: The voltage spike can cause arcing across the relay contacts, leading to premature wear and degradation of the relay.
To mitigate these issues, a flyback diode is connected in parallel with the relay coil. When the coil is energized, the diode is reverse-biased and has little effect on the circuit. However, when the coil is de-energized, the diode becomes forward-biased and provides a low-resistance path for the current generated by the collapsing magnetic field. This allows the energy stored in the coil's inductance to dissipate harmlessly through the diode, preventing voltage spikes and protecting the circuit.
In summary, the flyback diode in a relay circuit is crucial for maintaining the stability and reliability of the circuit by preventing voltage spikes and protecting sensitive components from potential damage.