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Topic: Flyback and/or freewheeling diode (Read 349 times) previous topic - next topic



I'm going to controll four 12v motors with relays. Im using "Relay Shield for Arduino V2.1" that has four relays with photo-coupled circuits.

I wonder if i should have an diode in reverse bias parallel with each motor to protect against the back EMF? Is this necessary when im using an relay, because im guessing the spike from the back EMF will be isolated from the Arduino through the relay. Or do i still need it?

And do i need to have an freewheeling diode for the relays aswell?

Hope someone can help me! 


The diodes will stop arcing in the relays and reduce EMI, probably worth it.  Big inductive spikes cause problems
even when isolated by a relay as the voltage pulse is coupled by stray capacitance to the sensitive part of the

For tiny motors it might be overkill.
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Thank you for the reply, but I'm not sure what "coupled by stray capacitance" means?

And i alsowant to add:
I'm using the same voltage source for my Arduino and the relays, both at 12V. The + is splitted in the relay. But they share ground. So if the back EMF can lead through the ground i have a reason to use the "flyback diode". I dont know enough abouth back EMF, and i will try to do the reasearch. But i'll ask here aswell, can back EMF lead though ground?  


Apr 23, 2018, 07:05 pm Last Edit: Apr 23, 2018, 07:10 pm by ReverseEMF
There's also the problem of arcing across the relay contacts.  I rarely work with relays, so I'm not up on this, but I did some googling and found that it's not a "easy" subject.  It's called a "snubber", so Google "snubber for relay contacts" if you want to look into this.

Adding a snubber will prevent arcing on the relay contacts.  This is important because arcing will chew up the contacts causing them to fail sooner than they should.

One "simple solution" I found, is to have two separate contacts (controlled by the same coil) in series.  That widens the gap more quickly, supposedly preventing, or greatly reducing, the arcing.  For this, you will need something like a double pole (or more) relay.

It was said, in another, reply, that the "diodes will stop arcing in the relays".  If they were referring to arcing across the contact(s), this might not be correct.  In one of the articles I read, a distinction was made between managing reverse EMF, and preventing contact arcing, and it was made clear that each needs a different solution.

If the contacts are visible, try watching the contacts while the relay switches [probably in a darkened room]. If you see arcing, even with the diode(s) in place, then you have your answer ;)
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