pepperl and fuchs i/o modules?

[slc75]

i have a machine with these and they control several dc coils for hydraulic valves. problem is that they keep frying the i/o modules. i figured that it must be because of kickback from the collapsing field when the coils deenergize. i wonder if anyone had any suggestions how to remedy this problem- i'm thinking maybe a diode? thanks

[gravitar]

Yup, whenever you switch an inductive load (i.e. a coil of any sort) you're supposed to have a suppressor across the load to protect your outputs. You can usually get away without them for small air valves, but any of the larger valves (especially hydraulics) will keep you honest! For DC, Allen-Bradley suggests a 1N4004 diode (reverse biased of course) in parallel across the load. AC outputs need an MOV surge-suppressor or a series R-C network with suitable-selected values.

[slc75]

thanks alot for the reply. i believe i will try the diode. i wonder if i could hook them directly to the modules?

[gravitar]

I think it is "preferred" to have the suppression as close to the load as possible, but I see people install them on the terminal strip in the panel all the time and I'm sure that would do fine.

[Wordman]

This is also why you fuse your outputs! Cost of new fuse << Cost of new i/o module

[robh]

Is there a possiblity of problems with the coil, if you are not using a suppressor across the load? I loose a coil on a hydraulic valve about twice a year. It is 110 vac. We use small 10 amp isolation relays to protect our outputs.

[larry818]

In this case, a fuse would not help. This is overvoltage, not over current. I find it's diffucult to find a fuse that will blow faster than the plc pcb trace. This is why I always use external load relays.

[larry818]

It makes more sense to put the supression at the end you're trying to protect. If you have a long enough wire run, the spike may be as much generated by the wire as the load.

[larry818]

It is possible that the voltage spike is failing the insulation in the coil. I've seen this happen with motors on VFDs. If it is happening, an ac supressor would help. I get my ac supressors from Evox Rifa.

[larry818]

Another thing I've found... If you use solid state relays, all the supression, at least at the relay end, is taken care of. I also use a GFCI on all my control circuits (the life I'm saving might be my own), and I've found with mechanical relays, I have to break both legs to distant coils or the GFCI trips. This is not a problem with solid state relays.

[robh]

We use Isotrol line filters to protect our PLCs and HMIs but I have not used anything to protect aginst over voltage on the loads. Thanks for the information.

[Wordman]

From my experience with big inductive loads, I have found through trial and error that I need to use MOV protection or isolation relays. But not because I am replacing output modules, but because my output fuses are blowing. Thats just from my experience that the fuses have blown first. And why I always fuse my outputs. Not just for the reason discussed here, but it is good practice otherwise.