2 wire proximity

eebads2391

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Join Date
May 2008
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Stockport
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112
Hi ,

I have a 2 wire prox in a 24v dc circuit feeding the input to a plc

The prox monitors a lever on a conveyor that moves if the contents of the conveyor become choked or compacted

The problem I have is the plc loses its input and stops

I go to the 2 wire prox and the lever is still in position , but the prox is not lit

I move the lever away from the prox and back into position and the prox lights

Can anyone she'd any light on this

Thanks
Mike
 
Hi Bernie,

I will have to get the exact details on Monday
But it is a 24 - 240v a.c./ dc 2 wire prox
Running an input into a gem 80 24v dc 16 channel input card

Sorry the info is a bit light
 
I'm not familiar with the Gem 80 but hopefully others will be able to help with the detailed info. I'm guessing it will have something to do with required operating currents for the prox relative to the needs of the input point.
 
Last edited:
I'm not familiar with the Gem 80 but hopefully others will be able to help with the detailed info. I'm guessing it will have something to do with required operating currents for the prox relative to the needs of the input point.

I agree. The proximity switch probably has a minimum load requirement that conflicts with the input resistance of the PLC input. Based on your description you are right on the edge.

You would be better served by a three wire proximity switch, or an interposing relay as a less desirable solution.

Another quick and dirty fix is to just put a light load in parallel with the PLC input, like a small lamp or relay, just don't exceed the max current rating of the proximity switch.
 
You could also try a resistor from the plc input to the 24vdc common. Sometimes the PLC circuit doesn't conduct enough current to keep the prox running. A resistor (maybe 2.2 to 4.7k) might be just enough to keep it alive.
 
Possibly the prox is at the limit of its sensing ability. Try adjusting the distance between the prox and the arm.
 
Possibly the prox is at the limit of its sensing ability. Try adjusting the distance between the prox and the arm.

I second this. If the system was working before, and is not working now, you almost certainly just need to adjust the proximity switch. There is vibration on conveyor systems and over time these things can get dirty or wiggle out of position. Temperature changes can cause them to go slightly out of whack too as the materials they are mounted on/with expand and contract if the sensor is mounted "just close enough" during setup to trigger.

After this has been confirmed, then move on to the other troubleshooting steps.

Good luck!
 
Thanks guys
It was operating right on the threshold of voltage
We had only 20v feed instead of 24v
So swapped out for a 3 wire prox
Thanks
 
Just for information.
A contractor was removing the old past antique PLC's we used to run and installing a PLC-5/15. There were several circuits that stopped working when the old stuff was removed. I had to add a 'load resistor' in parallel with the PLC input as the input would not provide enough load for the sensor to work with. These days, (on this now antique system) when I replace a mechanical limit switch with a two wire photo-electric eye I wire in a load resistor to provide enough current for the eye to work and trip the input to the PLC. (120 VAC inputs.)
Poet.
 

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