DPRG mailing list archives: [DPRG] How to view Back EMF spikes on Oscilloscope

Thu Oct 12 03:47:28 CDT 2006

The governing equation for voltage on an inductor is L*di/dt.  Thinking
about this in hands on terms that you can measure when looking for spikes is
that the voltage across the inductor is going to be proportional to the
inductor constant times the change in current.  Applying DC to the motor and
after the currents equalize you are actually measuring the voltage of the
power supply across the output resistance of the power supply or any other
load one happens to have in parallel.  The equation tells us the way to get
a high voltage or "spike" is to change the current very quickly.  In the
case of common motor driving techniques this means apply an instanous change
in current (or try) to turn the motor on or off.  This is much like applying
PWM to control the motor which is essentially turning the motor on and off
many hundreds/thousands/tens of thousands of times per second.  The stiffer
the power supply and the faster one can change the current (di/dt is change
of current with respect to change in time) the larger the voltage potential
will be across the inductor.

-Kenneth

-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
Behalf Of Vimal Shankar
Sent: Thursday, October 12, 2006 1:04 AM
To: dprglist at dprg.org
Subject: [DPRG] How to view Back EMF spikes on Oscilloscope

Wanted to see the Back EMF spikes on Oscilloscope.

I thought that just connecting the probes across the
motor leads across the motor leads would do the trick.

But all I got was the clean DC Power signal, altough
the motor was rotating ....

It's a simple 12v Geared DC motor with 2 power leads.
I connected 5 volts across it, and the Oscilloscope
probes in parallel and swiched on the PSU.

Motor rotates and the Oscilloscope confirms that the
voltage across the motor is indeed 5v...

Where did all the spike and Back EMF and other
danagerous stuff go ??

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