# Is it possible to store flyback voltage in capacitors?

I recently put together a circuit that I use to control an electrical motor. I use an Arduino's PWM through darlington-pair transistors to control the speed of the motor. During construction of this circuit I was advised to use a flyback-diode to block flyback voltage spikes created during the collapse of the motor's magnetic field to prevent a spike from damaging parts of the circuit.

Would it be possible to somehow harness this spike of voltage and store it in a capacitor? I am a hobbyist and know enough to put together some basic DC circuits, so I'm not super well versed in the capabilities of different components.

I figured if this was possible, I would somehow have that short burst of energy the capacitor(s?) store then applied to the motor when a sudden increase in RPMs is needed.

Any thoughts, ideas or reference material on how to store flyback voltage in a capacitor? Or am I just coming up with silly ideas I don't understand? ;)

• Are you talking about regenerative braking? If so, there's a great answer to a similar question here May 9, 2014 at 21:10
• @ScottLawson I think that's what i'm talking about! Like I said i'm a hobbyist and am not well versed in the different concepts / ideas / terminology :) Thanks for the information! I will check it out. May 14, 2014 at 4:04

Yes, it is possible to store that energy in a capacitor; after all, that's exactly what flyback-mode switching power converters do! So your idea isn't silly at that level.

However, using that energy to "boost" the motor drive is a little misguided, because the amount of energy you get from the flyback effect (which is due to "leakage" inductance in the motor) is orders of magnitude smaller than the amount of energy required to accelerate the rotor to any signficant degree. In other words, the effort required to implement this would not be rewarded with any practical benefit.

You may be thinking about regenerative braking systems. But such systems are not storing "flyback" energy — they're actually using the motor as a generator and storing the energy created by physically slowing down the motor (and the attached machine or vehicle, etc.).

• @David: I concur with your answer, but I'm curious why you refer to the motor inductance as "leakage", rather than just plain inductance. Perhaps you intended "parasitic", in the sense that the inductance is superfluous to the main motor purpose of converting electrical energy to mechanical motion? May 10, 2014 at 6:13
• @gwideman: I guess I'm just drawing an analogy with transformers, in which the "leakage" inductance is the result of imperfect coupling between the windings. In other words, there's a portion of the magnetic field that cannont be used for the primary purpose of the device, because it "leaks" to places where it doesn't do any good. May 10, 2014 at 11:06
• I guess I understand the analogy you were referring to. On the other hand, somewhat confusing, as in this instance the diode is needed in order to harmlessly dispose of the energy that wasn't leaked to somewhere else! May 12, 2014 at 4:57
• Regenerative Braking seems to be the right avenue for me to research. Thanks so much for the information @DaveTweed ! May 14, 2014 at 4:06

In principle, yes. In practice, already doing it.

Consider the workings of a flyback diode. When the current to a motor is turned off, the energy in its windings (i x i x L / 2) needs someplace to go, and without the diode will produce a high voltage spike (V = L di/dt). The energy which needs to be transferred is shunted back into the motor power supply by the diode, and is soaked up by the output capacitance of the supply.

So the idea is not totally without merit - it's just that it's already happening.

• If you're talking about a diode that's connected across the motor terminals, then no, the energy does not go back into the power supply; it's simply returned directly to the motor and dissipated in the resistance of the windings (and the forward drop of the diode). On the other hand, if you're talking about a diode bridge used in conjuction with an H-bridge driver (which is not what the OP is asking about), then what you say is true. May 9, 2014 at 21:26
• @David Nicely-articulated distinction. May 10, 2014 at 6:15

It would be possible to capture inductive flyback energy in a capacitor, but doing so will make the motor run less smoothly and is thus only desirable if the intention is to induce vibration in the motor shaft. Instead, it's much better to allow flyback current to flow as freely as possible with minimal voltage drop, and use a fast enough PWM rate that the current doesn't rise or fall too much on any given cycle.

Basically, what happens is that during the "on" part of the cycle, the applied voltage works to get current flowing in the motor, imparting energy; the current can't stop flowing until the energy is dissipated somewhere. To instantly stop the current flow would require taking the energy out of the motor electrically. Allowing the current to keep flowing, however, will allow most of the energy to remain within the motor and continue doing useful mechanical work.

In general, the faster you can switch the motor current, the better. Unless you go so fast that transistors can't keep up, higher switching speeds will make the motor run more smoothly, efficiently, and predictably.