Flyback diodes and why you need them (comments here please)

OK Newbie question. Please don't slate me too much

I was told to add a non-polorised capacitor across my 12v motor, is this doing the same thing as a flyback diode given the motor is an inductive load?

Also, although some of the technical "back and forward" is way past me the original explanation of what a flyback diode is doing was great. I hate just adding in components because "thats what you do" without the theory, or at least trying to learn the theory to the best of my ability.


I was told to add a non-polarised capacitor across my 12v motor, is this doing the same thing as a flyback diode given the motor is an inductive load?

Entirely different purpose.

A capacitor across the motor is only for interference suppression - given it is a brushed motor, not required for "brushless" as that already uses electronic switching.

For this, smaller capacitors in the order of 100 nF (0.1 µF), and there should be two, one from each (brush) terminal to the (grounded) motor casing. Larger capacitors are not required and would interfere with PWM. So they are manifestly too small to absorb any switch-off impulse from the motor. The limit is that a capacitor large enough to absorb the switch-off impulse will be an impediment to switching the motor on at each point.

A capacitor across the motor is only for interference suppression - given it is a brushed motor, not required for "brushless" as that already uses electronic switching.

OK. so someone steered me wrong as the motor I was using is a YG2734 Gearhead motor so the capacitors are not needed but a flyback diode would be. Correct?.
Perry said in his original post "most types of motor"
Can someone clarify some examples of what motors would or wouldn't require a flyback diode.

Perry said in his original post "most types of motor"

Hi Allan,
I left it vague because motors are a whole subject by themselves and I didn't want to discuss something that would be a distraction from the main topic.

DC motors with brushes need a diode. If you are using some kind of motor controller it probably has the diodes built in.

Brushless DC motors are not really DC motors, they are AC motors with built in inverters and drivers, they don't need a diode.

There are people on here who know more about motors than I do, I hope one of them will add more details.

Thanks Perry

That’s good to know.

You had me very puzzled with “flyback diode”. Actually the term is “flywheel”; flyback is part of a television or oscilloscope while a flywheel coasts down exponentially to zero speed when its drive is removed. It’s quite easy to understand when you recognize that the current carrying inductor has energy stored in the magnetic field and if the current is interrupted the inductor will generate whatever voltage is necessary to convert the energy into heat; an arc if open circuited which requires high voltage and if a diode is used the forward diode drop. Incidentally a flywheel diode can’t be used on an ac coil so where I made a living at this stuff for many years we used a non-inductive 100 ohm 10W resistor and .1 mfd series capacitor to get good results

Most people still call them flyback diodes and lots of other names.

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When the cathode ray oscilloscope came into general use there was a faint spurious trace made when the electron beam returned from the right end to the left end of the trace which was called the “flyback trace” and was ignored by those on the know. Before long technology was developed to suppress it but it was a nice sexy term which could not be allowed to die so when the flywheel diode came into use the term was hi-jacked by those too young to be familiar with a flywheel. So we now have two loosely defined terms both of which serve to confuse the newcomers. But enough quibbling over etymology; I expected some questioning of the need for 10 watts for a snubber resistor. (Or is “snubber” a hi-jacked term.)

it looks like some people are here to argue about how many angels can dance on the head of a pin, and others are here to argue about what kind of shoes they wear.

when I was sent to NASA magnetic recorder school, they told us NASA insists on flyback diodes, AKA commutating diodes, across all inductors, to snub back EMF. every device I worked on did have exactly that. the Ampex 1900 tape recorders we had were 1900As. the A was a 1900, revised by addition of commutating diodes.

I rode my motorcycle to work once, and measured the voltage across the coil with an oscilloscope. when the points open, the 6 volt coil primary put out a 321 volt spike. this gets multiplied by the secondary into ~25 kv for the spark plugs.

the purpose of the condenser in a points type ignition is to supress the spark at the points. condenser is an antique word for capacitor. if I felt the need to experiment with a capacitor across a switch I would raid a distributor from one of the VWs in the back yard.

NASA insists on diodes at the coil. if you argue with that, you really are arguing with a rocket scientist.

That is a medieval way of thinking: finding as high authority as possible to support the opinion. This is not how modern science works. You should have a good explanation (or at least empirical data) for your claim or it is nearly worthless. Authority is only a very weak heuristic.

In other words: Arduino is NOT rocket science. We don’t know WHY (and if) NASA truly insists on diodes at EVERY coil. Even if it does their reasons may be not valid in non-rocket environment.

Whatever NASA believes, I automatically refer back to my explanation in #5 which is not so readily visible now in the new forum unless you look for it.

I as ever invite anyone to find and explain any flaw in that explanation. :laughing:

Hi. I just found the topic related to this thread, at:

Flyback diodes and why you need them

as I need to drive a quartz clock motor coil with a microcontroller, and wondering how to design the snubber diode circuit.

Unfortunately I cannot see any of the pictures in the above quoted article, making the article useless. (All pictures in the article page show up as the broken picture icon.)

How do I view the article page with all of the pictures restored? FWIW, I am using Google Chrome on a Windows 10 computer.

@DuinoSoar as noted elsewhere there is a problem with images that the Discourse team are supposed to be fixing, so I have assumed that where there is a problem with one of my tutorials it will be fixed automatically, which is why I’ve not done anything about it.

I’ve sent you a PM with some of the images.

Its really simple:

When the coil of a relay (or other inductive device) is energized, magnetic energy builds up in the magnetic core. This energy remains constant as long as the current doesn’t change.

When the current is interrupted by a switch (often a MosFet) this energy (aka “field”) must go somewhere.

The reducing field, due to the MosFet being turned off, results in a voltage generated in the coil windings as the field changes (toward zero).

This is the basics of all motors, solenoids and inductors.

The voltage created is:

V=L di/dt

  • Where V is the voltage generated by the changing flux in the structure.
  • L is the coil inductance in henries (a typical small coil might be 400 µH)
  • di it the change in current
  • dt is the change in time

If you have 500 ma going through a coil whose inductance is 400µH
And your MosFet switches off in 1/2 µS then the created voltage is:

V = 400µ * 500ma / 0.5µS
V= ~ 400 Volts. It is this voltage that will likely damage your MosFet.

By putting a diode across the coil, the coil current will flow through diode and be limited to 0.7V
If your circuit supply was 12V, the peak voltage would be 12 + 0.7 instead of 12 + 400.

In an earlier post a fellow forum user suggested the real issue is the generation of EMI (electro magnet interference). I disagree, EMI is dealt with in other ways and is very circuit and layout dependent. However it is a good idea to put a capacitor (0.1µF or so) from the ground at the MosFet to the supply of the coil. This will reduce any sudden changes in the supply voltage due to the change in current between the coil being off and the coil being energized.

Like many things there is more to the story of inductor dynamics. I chose to present the basics and reasoning for the diode across the relay coil. There are other ways to suppress the coil voltage and some effects of using a diode but to the typical Arduino user these are not of great concern.

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Finally someone picked up on that!

When I wrote the tutorial the word I had in mind was ‘flyback’ so that’s what I used. A few days after I published it I started to think I’d used the wrong word. I sent a PM to someone I trust on here and he said it was okay. Not only that there are some pretty experienced people here, any of whom might have told me I used the wrong word, but none did. I also searched the internet and it seems like ‘flyback’ is in common use for this purpose. Because of all that I left it as it is.

If you do some searching and reading you will find several different words used to mean the diode across an inductor to supress the high voltage produced when the current is interrupted. Which ever word I use someone will say that I should use a different word.

I think the basic problem is that I’m too old (89) for my own good. Where I made a living for many years it was always “flywheel” although, come to think of it there’s no wheel involved. By the way there’s one caveat to keep in mind when using a flyback/flywheel diode — it will slow down the dropout by a bit. Usually this is irrelevant but occasionally I’ve put a resistor in series with the diode to speed it up; resistance = to the coil resistance or perhaps 2 or 3 times the coil resistance

Yes, well aware of that!

I work in telecoms and electromechanical telephone exchanges sometimes slug relays to make them slow release by shorting out one of the coils with a relay contact.


Do they still have those? :grin:

Did you note #39?

As I indeed explained - but a much larger capacitor is needed - in my original discussion. :grin:

The local supply bypass capacitor needs to connect from the coil supply to the FET source and the diode directly from the coil supply to the FET drain. This constrains the switching transient within the triangle of those three components.

Well, whatever a “typical Arduino user” is, designs that crash the microcontroller when a relay is (de)activated, are a moderately common topic here. :roll_eyes:

Collectors do! I know a guy who lives on a farm in the middle of Lincolnshire with enough working Strowger kit to run the whole village and probably the villages around too.

I have now! Armature end slug as far as I remember makes it slow operate and slow release, but it’s a long time since I needed to know!


I think I still have some bits hanging around somewhere. :roll_eyes: