What is “Flyback Voltage” and what should I do about it?
A “Flyback Voltage Spike” occurs when the supply current to an inductive load such as a solenoid is interrupted. Any switching circuit for highly inductive loads should have flyback voltage protection to limit the voltage rise from the discharging coil and damage to the relay contacts. Flyback voltage protection limits the voltage rise when the relay contacts open by safely draining away the energy stored in the solenoid coil. One of the most common forms of flyback voltage protection for DC inductive loads is a reverse-biased diode placed at the solenoid. The 1N4001-1N4007 family or similar diodes are often used for this purpose. They are inexpensive, sufficiently fast, and have a high enough reverse voltage and the current capacity needed for the application.