Flyback Current / Voltage

Thread Starter

ben sorenson

Joined Feb 28, 2022
In a DC Circuit consisting of (2) 12 Volt DC motors running in parrellel with an equivalent parrellel resistance of let's say 5 ohms. Powered by a 24 Volt, 2000mah battery.

I'm know there should be a freewheeling diode across them but I'm wondering how much current would one expect to see from "flyback" I know flyback in motors and relays can cause high voltage spikes, but how much current do those voltage spikes attribute with the freewheeling diode. Not in just a simple circuit like this, but in general with coils, motors, etc. Is the current generated from the e.m.f generally pretty small in comparison to the overall circuit?


Joined Nov 6, 2012
With a 24-Volt Supply, the Motors would have to be connected in Series, not Parallel.
But the Resistances, and Voltages, and wiring arrangement
are not pertinent to the question being presented here.

There only needs to be a Freewheeling-Diode across each Motor if the Battery-Power
is being switched, as in PWM-Speed-Control.
In that case, every time the Power is turned off, the Freewheeling-Diodes will redirect the
Flyback-Spike generated by the heavy Inductance of each Motor, back into the Motor.
This will also increase efficiency of operation.

The problem comes from the Brushes in a Brushed-type Motor.
The Brushes can create extremely high-Voltage-Spikes, at extremely high-Frequencies.
These Spikes can easily smoke Switching-Transistors if not suppressed adequately.
These Spikes may be powerful enough to blast right through a heavy Power-Diode,
or, the Diode may not be "fast-enough" to properly deal with such extreme-Frequencies.

This subject can easily become a big "Bucket of Worms",
with no "cut and dried", "one-size-fits-all" solutions.

The best universal-solution that I have found is a "Common-Mode-Choke",
( which may be large and expensive, depending upon the amount of Current they must carry ),
which will isolate the Electronic-Switching-Components from the Electrical-Noise of the Motors,
that, along with various sizes of High-Voltage Ceramic-Capacitors
connected in parallel, everywhere You can think to put them.
RFI Suppression Flat .png


Joined Jan 23, 2018
With that many capacitors across the motor it is not likely that any possible "flyback" voltage will be a problem, or even be present. and if the motors are brush type devices, brush noise will be a greater magnitude.