question about flyback diodes - AC/DC

Thread Starter


Joined Jul 29, 2013
Im working at a plastics molding site and im working on creating as built drawings for some of their older machines

Im seeing that the PLC output is controlling the 120V contactors
24VDC and im seeing that theyre tied to a large diode


IMG_1582.JPG IMG_1580.JPG

the projects engineer said that it's a surge protector, I believe its working in the same way that a flyback diode does
Im just curious as to why it would be used on 24 volts and not on 120 VAC coil, Ive seen a few contactors using AC but none of them used anything like this

also the way its wired as ive drawn, should this be tied back to N, instead of just looping around just the coil?


Joined Jul 18, 2013
That looks more like an R/C snubber to me, usually associated with AC coils, if DC coils they should be a rectifier, 1n4007 etc.
They appear to be Telemecanique contactors, they actually have suppresors that fit directly across the coil, no wire leads.



Joined Oct 2, 2009
Only relay coils driven by DC will have a diode. AC relay coils will never have a diode by itself.

If there is something across an AC coil, it is likely a low-value resistor in-series with a capacitor (shows open with an Ohmmeter), or a MOV.