For analysis [oscilloscope wave form] purpose I am running a BLDC motor at 9Vdc. Power Sorce is giving the constant DC voltage to BLDC Motor. But whenever I am disconnecting the voltage there is a spike seen in voltage (Approx 15.4 v).

  • What is this spike?
  • If its Back EMF why its not showing in reverse direction?

Refer the Picture (I am Disconnecting the input by disconnecting the Ground Manually)

enter image description here

  • \$\begingroup\$ Flyback? Don't kill your scope.. \$\endgroup\$
    – user76844
    Commented Apr 18, 2017 at 17:26

1 Answer 1


If you were experimenting with a brushed DC motor, then the spike should be negative, as you expected.

But you wrote you are using a brushless DC motor. Then there must be a controller somewhere. The controller drives each of the motor’s 3 phases with a half-bridge, which certainly has flyback diodes. And thanks to those diodes, if you’re looking and the power input of the controller, you will see positive back-EMF spike.

Note that that back-EMF may damage the controller, so you should better not do that. (You might also kill your scope…)


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