When you turn off an inductive device, the energy stored in the coil collapses rapidly, this causes an extremely large voltage to appear across the coil, this can get back into any electronic devices directly connected or not.
This is why if you switch a coil directly by transistor, the transistor can be destryed if no diode is fitted.
The back EMF diode is normally reverse biased (non-conducting) the collapsing voltage forward biases the diode and rapidly recirculates the voltage through the coil causing it to collapse.
It happens to all relays and solenoids, it is just that the larger the coil current, the larger the back EMF.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.