The Importance of Diodes
The inductive process is usually prone to what is known as ‘blowbacks’, which means that the energy isn’t checked and can cause circuit overloads if it isn’t limited. In addition, some inductive loads, like the electromagnet in an electromechanical relay, might feed a power surge back into the circuit when power is disconnected from the load, which can damage the circuit. For this reason most devices and machines have protective diodes, which basically act as circuit breakers and require that energy can go in— but prohibit it from flowing back out.
When the power is switched off, the diode dissipates the power surge by providing a one-way electrical path across the inductor. It will dissipate electrical power until the electromagnetic field collapses or until the power surge current is insufficient to activate the diode.