Created by
Created November 13, 2011
Last modified June 08, 2017
Tags diode   flyback   inductive   mosfet   motor   power   relay  


A n-channel MOSFET switches an inductive load, both with and without a diode to catch the voltage spike.


This simulates a MOSFET being used to switch an inductive load, as is common in lots of power electronics problems, like controlling a motor or making a DC-DC voltage converter.

A few things to try in CircuitLab

Load the simulation by clicking "Open in editor" above, and then click "Simulate" at the bottom. Run a time domain simulation.

Voltage spike

Check out the spike in V(drain) as M1 is switched from off to on the first two times. That gets up over 40 volts -- quite possibly enough to damage various circuit components.

For the second two pulses, the switch SW1 is closed, so D1 is able to "catch" the spike, and routes the current safely back through L1 and R1, instead of being forced through M1.

Different diodes

Try using different diodes. We're using the 1N5817, which provides fast switching times, low voltage drop, and high current capacity.

However, if you put in the 1N4007, you'll see that it turns on fast enough, but actually has a problem turning off when M1 starts conducting again! See our Diode turn-off time example for more details about that.

If you instead put in a 1N4148, you'll see that V(drain) still approaches 15+ volts or so on each spike -- enough to likely fry the diode. The little signal diode simply can't handle that much current.

Different MOSFETs

You can also try different n-channel MOSFET parts, and you should see some differences with switching time, and maximum currents.

More about inductive loads


great!!! thumbs up!!!

by mp8878
March 08, 2013

If I replace L1 and R1 with a relay (SPST) and run the simulation, no spike is found before switch SW1 is closed. Is it because of the relay coil resistance (100ohm, or could be higher in real 12v relay) ? Can I also conclude that no diode is actually needed to deplete the back emf from relay in practical design ? Please comment.....

by steven7890
August 16, 2013

any comment for my above point made on 16/8/2013 ? Thanks

by steven7890
September 03, 2013

Leave a Comment

Please sign in or create an account to comment.

Revision History

Only the circuit's creator can access stored revision history.