# What flyback diode to use for small DC motor?

I am planning on using the 1.9mm shaft OD motor here: https://www.aliexpress.com/item/32973481427.html?spm=a2g0o.cart.0.0.11b63c00aGVLNC

I have done plenty of research but can't figure out how to choose a flyback diode. Is there a formula? What variables should I calculate and how? I am looking for an SMD diode, not PTH.

Specs of motor:

1.9mm D type - No-load speed: 92RPM @ 3V, 195RPM @ 6V - No-load current: 60mA @ 3V, 70mA @ 6V

Any help appreciated!

Missing and critical specification is the stall current. The current it draws when you prevent the shaft turning.

The diode must match (exceed) the voltage and stall current ratings of the motor.

In this case, any power diode.

When I say "any", I mean any standard, currently available diode in the last 30 years. 1N4001, EM401 or higher number or their SMD equivalents. I would be quite difficult to find a power diode that was rated for less than 1 Amp, or less than 50 Volts, which are way more than your motor will use.

Unless OP wants to drive the motor in both directions with an H-bridge.
Most H-bridge chips have the diode(s) already buildin.
Leo…

Jackson6R: The snubber diodes or Zener diodes must be rated at full motor current as that is what will be diverted through them when the motor is switched.

And by "full motor current" you mean stall current right? Which is, also, the current the motor draws, to overcome it's own inertia, when it starts, right?

If you are switching the motor intermittently (rather than using PWM) only the diode's pulse current rating is relevant, often that's many times more than the continuous current rating.

If you need to clip in both directions use Zeners, set the voltage to 12.5V so they only turn on during flyback Kroger Feedback.

Linde1E: If you need to clip in both directions use Zeners, set the voltage to 12.5V so they only turn on during flyback.

Please explain "both directions", and why zeners. Leo..

Both directions means forwards and backwards motor rotation. Simple back-to-back diodes would short-circuit the thing, so two zeners in series, cathode-to-cathode (or anode-to-anode) will prevent shorting if the zener voltage is greater than the supply, but will divert current on switch-off to prevent kickback spikes (or rather limit the spikes to the zener voltage).

However this is never needed as any bidirection circuit is basically an H-bridge and simple diodes can go across each switch in the bridge (in fact is unavoidable with MOSFETs anyway).

Simple use avalanche rated FETs in your driver circuit and none is required. You can use the 4001 if you like it has the capability of doing the job for you. Good Luck and have fun! Gil

gilshultz: Simple use avalanche rated FETs in your driver circuit and none is required. You can use the 4001 if you like it has the capability of doing the job for you. Good Luck and have fun! Gil

NO! the fact a FET is avalanche rated simply means the maximum safe avalanche energy has been measured and documented, not that the device can handle your load...

You have to calculate the maximum energy pulse from your inductive load and compare it to the device's avalanche rating.

But basically don't do this, you'll have high voltage spikes creating interference all over your circuit, use a standard snubber or free-wheel diode.

The exception is for circuits designed to create high voltage spikes, basically ignition circuits and flyback transformer drivers.

jackrobot: Any diode even a signal diode can be employed as a flyback diode for the small DC motor but it is important to make sure that the diode should be connected in parallel to the motor and reverse biased.

Not really a safe assumption, you need to comply with the dissipation rating of the diode, and depending on the switching regime the diode might be carrying over an amp nearly all the time with a small DC motor. Using a diode like this is also called slow-decay mode as the current falls slowly when the switching device is off. This means even at 20% PWM duty cycle or so the motor and diode may be carrying a heavy current.

For occasional switching of a small motor use the pulse rating of the diode, for PWM you'd normally have to assume continuous current rating of diode has to handle motor max current.