The purpose of a flyback diode (also known as a freewheeling diode or snubber diode) in a relay circuit is to protect other components in the circuit from voltage spikes and back-electromotive force (EMF) that occur when the relay's coil is de-energized.
Relays are electromagnetic switches that use a coil to generate a magnetic field, which in turn closes or opens the switch contacts. When current flows through the coil, the magnetic field is created, and the contacts change state. However, when the relay is turned off (de-energized), the magnetic field collapses, inducing a reverse voltage across the coil. This rapid change in voltage can result in a voltage spike, sometimes called a flyback voltage or back-EMF.
The flyback diode is connected in parallel with the relay coil, with its anode (positive terminal) connected to the positive side of the coil and its cathode (negative terminal) connected to the negative side of the coil. When the relay is energized, the diode remains reverse-biased and does not conduct any current. However, when the relay is de-energized, the collapsing magnetic field induces a voltage across the coil in the opposite direction, and the flyback diode becomes forward-biased, providing a path for the current to flow.
By providing this alternate path, the flyback diode allows the energy stored in the relay coil's magnetic field to dissipate harmlessly within the diode. This action prevents the voltage spike from damaging other sensitive components in the circuit, such as transistors, microcontrollers, or other control circuits that are driving the relay.
In summary, the flyback diode is a protective component that ensures the safe and reliable operation of the relay by suppressing voltage spikes caused by the collapsing magnetic field when the relay is turned off. It helps prevent damage to other components and extends the overall lifespan and reliability of the relay circuit.