Author Topic: Multiple relays and their flyback diodes  (Read 1244 times)

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Offline jsi

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Multiple relays and their flyback diodes
« on: October 11, 2016, 03:27:39 pm »
I am working on a design that uses 3 separate isolation relays wired in parallel.  Right now each relay has its own flyback diode placed as close as possible to the relay coils.  According to my oscilloscope they are doing their job and there are no extreme voltage fluctuations when the relays switch off.

My question is do I need 3 diodes or is 1 sufficient.  (assuming of course that the one diode is of sufficient capacity to handle the load.)

ps - the basis of this question is my belief in the KISS principle, or put another more eloquent way "Everything should be made as simple as possible, but not simpler."

Offline oldway

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Re: Multiple relays and their flyback diodes
« Reply #1 on: October 11, 2016, 03:41:08 pm »
I think that are freewheeling diodes and not flyback diodes.

For max eficiency, you must choose fast recovery diodes of the same or greater current than the coil current of the relay and place them as near as possible from the coils.

If the relays are not far from the others, you can use only one fast diode of current = or higher than 3x the coil current of each relay.

Offline Richard Head

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Re: Multiple relays and their flyback diodes
« Reply #2 on: October 11, 2016, 05:34:43 pm »
A normal single diode is fine. It doesn't have to be fast recovery in this application. Just ensure that the freewheel diode is placed directly across the wires feeding the relay coil.

Offline SeanB

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Re: Multiple relays and their flyback diodes
« Reply #3 on: October 11, 2016, 06:00:38 pm »
If you do not want to use a diode, and your driver transistor can withstand it, you can use a MOV across either the transistor or the relay coil. Rating for the MOV is selected by using a MOV with a DC voltage withstand rating that is just a little above your supply voltage, and with a max clamping voltage that is below the transistor voltage rating ( or the suplupply plus the max rating if across the relay coil), which has the advantage of being faster in turn off, as the coil can generate a higher back EMF and dissipate the stored energy faster.

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