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Topic: Position of Back EMF diode on Relay (Read 2399 times) previous topic - next topic


Does the diode have to be physically on the relay or can I put it on the circuit board near the transistor instead?

If there is a maximum distance of relay to diode, what is it? I have a control board with an arduino on, opto isolated and now through a ULN2803 driving 16 relays. I wsa hoping to just put the protection diode on the circuit board enabling easy wiring.



Best practice: place it close to the coil... but it just needs to be "electrically" there.  The only thing that will really change with distance is the resistance of the electrical path due to the length of the copper wire and you can pretty much ignore that variable.


More importantly the inductance and loop area will increase the further the diode is from the coil - leading to
more radiated interference.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


ULN series of relay drivers have free wheeling diode built in. So you do not have to attach them externally


If the relay is remote from the driving circuitry, I usually try to solder the diode directly across the coil of the relay, regardless of whether the drive circuitry also has a diode.  Without the diode at the relay long wires will cause a lot of broad spectrum radiation which is not only (strictly speaking) illegal but undesirable too.

I know a guy that used a large relay to control some fish tank heaters for a large tank,  He had the relay about 3 feet from the controller and did not have a diode across the coil at the relay.  However, there was one in the controller.  The radiated "pop" was so bad it would cause his wireless phones to disconnect if they were used in the same room.  Sure, it may be that his wireless phones were cheap, but the problem went away after we soldered a 1N4001 across the relay coil.  $0.10 solution.


Thanks for everybody's response.

Will fit a diiode into the connector of the relay so changing relays will still be easy.

Hadn't realised the ULN already had a built in diode, so won't put one on the board.

EMI in Bangladesh isn't an issue, especially in these industrial washing plants but good to know.


A ULN2803/etc would still be a useful relay driver if you have > 1 relay. I also concur
that it's best to tack the diode right onto the relay, especially if the wires are longer
than a few inches. The extra one in the ULN2803 will still be ok.

These sort of things you have to play with, and see how they go. I've seen similar
situations to what dirtbiker mentioned - relay transients trigger resetting in industrial

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