24V DC Contactor Coil - Flyback Diode Required ?

panic mode

Joined Oct 10, 2011
2,129
yes... some sort of suppression should be used. normally it would be a diode, or MOV or zener etc. some contactor product lines have related suppression device in a form factor that is quick and easy to install.
same is for Eaton/Moeller products - just go to eaton website search for DILM17 and then click on suppression devices..
MOVs are nice in this case since not polarized and can be used for both AC and DC. even if application is DC this helps reduce wiring mistakes.

note that typical industrial output in a PLC/CNC/robot or motion controller will usually have their own diode to protect the output itself from inductive kick. but this is not always the case. also inductive loads may be far and suppression should be near (ideally directly across) the coil. if not, diode in output card may protect the output but it will do little if anything for the load itself of the wiring, including any additional devices that may be added such as interlocking and safety contacts.

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crutschow

Joined Mar 14, 2008
29,776
suppression should be near (ideally directly across) the coil
Actually, it likely is better to put it near the driver, since that also protects against the kickback from any wire inductance between the load and the driver.
That will also protect anything else connected to that line.
 

Ian0

Joined Aug 7, 2020
5,087
Sometimes there is one built into the contactor. It's there just to fool you if you thought you could connect the coil contacts either way round.
 

MisterBill2

Joined Jan 23, 2018
11,877
"Flyback Protection" is a very serious religion among some folks. You can also add an indicator light to show the signal and absorb the spike.
 

Thread Starter

fastbike

Joined Dec 29, 2020
77
Thanks for the advice.
I'll check the polarity to see if it operates either direction. (There are versions with coils for 24v ac as well as higher voltage ac)
 

panic mode

Joined Oct 10, 2011
2,129
Actually, it likely is better to put it near the driver, since that also protects against the kickback from any wire inductance between the load and the driver.
That will also protect anything else connected to that line.

Actually no... things are not so simple. Inductivity of the wiring is small when compared to solenoids of contactors, valves etc. outputs are generally already protected and if needed to protect output, add separate protection across that output and stop buying products that do no have protection.

The real issue is inductive character of the load and that need to be managed. Often contactors and valve banks are long distance away from the output or driver (along some conveyor etc). Also there are likely to be additional means of disconnections between output (driver) and the load - those could be local safety or interlocks or simply connectors. So disconnection at any of those points downstream of output would render any protection across output useless.

While outputs do need to be protected, field side protection is better - and directly across inductive loads.
you may be the unlucky one just trying to do some work with the loads and you may not be aware that 50m away and out of sight, someone else may be doing something too and unplug the connector feeding your panel. and contactor coil will give you a pretty nasty zapppp...

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crutschow

Joined Mar 14, 2008
29,776
So disconnection at any of those points downstream of output would render any protection across output useless.
Agree, that the protection must be up-stream of any switch in series with the coil.
But if there is only one disconnect, then it's still best to do it at the disconnect point.
 

panic mode

Joined Oct 10, 2011
2,129
there are two things here:

protection of the output must be upstream of any disconnection means that is in series with the load. normally this is internal to the PLC output so generally one does not need to add external one here.

protection of the load (including wiring and anyone doing servicing) need to be downstream of the last means of disconnection. and it does not get more downstream than placing it at across inductive load itself. many of the inductive loads in industrial automation do not include built in protection (specially contactors, since they are modular and different options exist) so it is quite common to NEED to add one. of course if it already included one does not need to add one more (even if it does not hurt)
 

Thread Starter

fastbike

Joined Dec 29, 2020
77
Thanks guys. I've just checked and the contactor will operate connected eityher way around (polarity insensitive). It draws around 125mA once latched. I'm going to operate them via a small low power relay which incorporates a flyback diode and indicator LED. That way there is no opportunity to blow up my controller board.
 

MaxHeadRoom

Joined Jul 18, 2013
25,196
It is also recommended to suppress the contactor coil, I had a similar situation that when ever the contactor picked up, the PC based system would reset.
 

Thread Starter

fastbike

Joined Dec 29, 2020
77
no need for extra relay, just drive directly from your plc output
The small DIN mounted type from Phoenix are 7mm wide, comes complete with LED and flyback diode and can handle the current drawn by multiple contactor coils. Why risk a EUR400 part when a EUR7 part provides protection and removes the requirement for the diode.
 

panic mode

Joined Oct 10, 2011
2,129
this is a public forum and everyone is entitled to their opinion.

and i've been doing this for 30 years and more than 1000 projects in chemical, nuclear and automotive industry.
i see no way a modern output module to be destroyed by a rated load. they have everything - over current protection, short protection, over temperature protection, transient protection...

but that is just me - i read the specs, choose my products based on that and hold products to their promised rating.
you need to decide for yourself if you have the brains and balls to trust parts and circuits you chose.

sure you can still find odd one that does not come with all bells and whistles but that means you did not do your homework, are throwing out 400EUR on some cheap garbage.

btw. typical output module will have 16 channels. 16*7EUR is more than 100EUR in hardware but you also failed to account for labor... and that is just one output module. typical PLC installation will have bunch of them.

btw. why stop at one interposing relay per output? you can have several of them driving each other. if you play your cards right, you can get kickbacks from the relay company ;)
 
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Ian0

Joined Aug 7, 2020
5,087
this is a public forum and everyone is entitled to their opinion.

and i've been doing this for 30 years and more than 1000 projects in chemical, nuclear and automotive industry.
i see no way a modern output module to be destroyed by a rated load. they have everything - over current protection, short protection, over temperature protection, transient protection...
I agree that the module output should contain adequate protection from over-voltage transients caused by relay coils being de-energised.
However, a diode across the relay coil will keep the circulating current in a loop locally around the relay, instead of it having to go all the way back to the output driver, which would give some EMC advantages especially where the wires to the relay are long.
 
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