Why is a Freewheeling Diode placed across a Solenoid (of any SDV, ON/OFF Valves, etc.)?

What is Freewheeling Diode?

A freewheeling diode, which is additionally known as a fly-back diode or a snubber diode, is frequently used to protect electronic equipment connected to a solenoid from voltage spikes (or) transient currents caused by the solenoid’s turn-off.

Why is a Freewheeling Diode placed across a Solenoid?

When a solenoid is powered, it produces a magnetic field which stores energy. When the solenoid is turned off, the stored energy in magnetic field collapses, resulting in a sharp fall in current. This fast change in current can cause a voltage spike or transient, which is also referred to as a back electromotive force (EMF), over the solenoid coil.

Without a freewheeling diode, the back EMF can disrupt the circuit. The voltage surge may surpass the voltage rating of circuit’s components, potentially resulting in failure. It can also cause electrical noise in surrounding circuits, interfering with the operation of the other equipment.

When the solenoid is switched off, a freewheeling diode in parallel alongside the coil allows the back EMF to cycle. The diode conducts current produced by the decreasing magnetic field, allowing for the regulated disposal of energy. This reduces the voltage spike & protects the rest of electronics from potential damage.


  • The freewheeling diode is usually chosen to have a quick switching capacity and a voltage rating appropriate for the application.
  • It is important to select a diode with a reverse voltage rating greater than the expected back EMF produced by the solenoid.


Overall, placing a freewheeling diode across a solenoid is a typical practice to protect the circuit and assure the consistent and safe operation of a solenoid-driven system.