Inductive kick (surge) occurs anytime an electrically generated magnetic field collapses. A voltage opposite in polarity to the original applied voltage is generated by the collapsing magnetic field.
A good example of this is in an old style automobile ignition system. When the breaker points open, the current flowing to the ignition coil is shut off and the magnetic field built up in the coil collapses. The resulting inductive kick voltage is high enough to jump the gap at the spark plug.
The same effect occurs whenever a solenoid used on a hydraulic valve is shut off. The voltage surge can reach several hundred volts, which will arc across switch contacts and quickly destroy them. Switch life can easily be reduced to one tenth of normal.
The solution for DC voltage applications is to add a rectifier diode in parallel across the solenoid terminals. A diode acts as a one way valve for electricity. In normal operation the electric current can’t flow through the diode, so it flows through the solenoid coil. When the operator releases the switch the current is shut off to the solenoid, and the inductive kick flows backwards through the diode rather than through the switch contacts, bleeding off the high voltage spike. Scorpion Technologies strongly recommends that suppression diodes be used on all solenoids.
Diodes should be installed as close as possible to the solenoid. Soldering a diode directly across the two solenoid terminals is ideal. Use a diode with a voltage rating of at least ten times the circuit voltage and a current rating at least as big as the current flowing through the solenoid. In practice, diodes rated at 200 Volts and 1 Amp are readily available for a few cents each and are fine for most applications. Diodes typically have a stripe on one end of the case. On equipment with negative grounds, the end with the stripe should be installed on the most positive side of the solenoid. This is usually the side that has the wire from the handle switch. Diodes are not suitable for AC voltage applications.