I have very limited electronic knowledge, but an trying to design a control circuit for an electro magnetic clutch.
The clutch is 24Vdc with a power requirement of 85W or 3.54A.
It is operated by a remote control Transmitter and Receiver.
The receiver requires a 24Vdc supply and the receiver switching contact is connected to the clutch via an interposing relay.

My problem occurred when the circuit lost power while the clutch was operated.
I believe this caused a back EMF voltage with the interposing relay contact closed and the voltage induced damaged the receiver supply circuit.

The clutch manufacturer has recommended a spark suppressor in parallel with the interposing relay contact.
However I am unsure that this will prevent the voltage spike from reaching the receiver power supply.

Do I need a Flywheel Diode or a Metal Oxide Varistor in parallel with the clutch ?
If so can someone help me with the rating please.
The circuit show all the above added

 

24VDC is commonly used in automation. and most industrial automation products (Relays/contactors) have suppression either built in or available as an option or choice.

the simplest solution is to add flywheel diode to each inductive load. that includes relay and contactor coils. this will clamp voltage to very low level and regardless of other circuit parameters... (24VDC + 0.7V). this is the most robust and insensitive soltuon that work regardless of coil size or supply voltage level. one just need to watch for correct polarity

using MOV works as well and polarity does not matter but clamped voltage will not be as low.

using back to back zeners can get more precise cutoff voltage while also making sure that it is not polarity sensitive.

using RC suppressor can also work but this is even more delicate to size.

 

24VDC is commonly used in automation. and most industrial automation products (Relays/contactors) have suppression either built in or available as an option or choice.

the simplest solution is to add flywheel diode to each inductive load. that includes relay and contactor coils. this will clamp voltage to very low level and regardless of other circuit parameters... (24VDC + 0.7V). this is the most robust and insensitive soltuon that work regardless of coil size or supply voltage level. one just need to watch for correct polarity

using MOV works as well and polarity does not matter but clamped voltage will not be as low.

using back to back zeners can get more precise cutoff voltage while also making sure that it is not polarity sensitive.

using RC suppressor can also work but this is even more delicate to size.

 

Thank you for your help. I had previously seen that diodes were not suitable for magnets, but now I have another reference saying MOVs are used for AC circuits.
This also recommended a Schottky diode might be more suitable than an IN4007 due to its lower forward voltage drop and faster switching speed.
I intend to try an onsemi 45V 8A, Schottky Diode, 2-Pin DO-41 80SQ045NG unless you have a better recommendation !

 

Schottky diode or not will make no difference...

Vf of regulator diode is some 0.7V and for Schottky it is some 0.3V
so the difference is only between 24.7V vs 24.3V or 1.6% which is much less than expected tolerances.

sure today we have SMPS that are regulated and quite stable but we are not talking about something sensitive like logic supply. this is just a frigging solenoid

Any circuit should be able to operate within at least 10% of the rated voltage.

and if you think 1.6% is bad, check how sloppy clamping voltage of those alternatives are. you would be pretty lucky to get this to 30-50%. if you do not know what you are doing, it may be way more than that.

as for rating of the diode, you want to make sure that it can handle the circuit voltage. and practically any rectifier diode on the market will cope rather well with 24VDC. for continuous current you would want diode to be rated accordingly but this is not it, back EMF pulse is very short. and for short pulses even 1N4001 will handle way more than rated 1A. read datasheets... if i recall it was some 30-40A for 1ms pulses. have seen tons of diodes contactor with 1N4000 series diode across coil - and never seen one of those diodes fail. so yeah, 80SQ045NG will do great here.

 

It may be that the MOV shown in the drawing has failed open. That does happen, some times after first failing shorted. A diode or a snubber or a light will reduce the spike. Actually a snubber across the relay contacts will do a lot to reduce the spike because it will allow a bit of current to flow when the contacts open. That will also not slow the clutch release time.