interesting problem and solution found

[harry richardson]

We have a piece of equipment that runs a slc 500 1747-L40C with the 2 slot expansion rack connected to a panelveiw 300 2711 K3A2L1 over the pic box communications port. the output on the plc are all 110 ac the inputs are all 24 vdc.

not what the problem was that we would start the machine and it would come on and go into the setup cycle(it would intialize its self to get ready to start) then it would compleetely die.

completely die is basicly the 24 vdc power supply would overload and the panelveiw which is run from it would go off. using an ohm meter we checked out the 24 vdc circuits. the only circuit that showed shorted was the panelview. hmm i thought to my self. panel veiw bad maybe? got a replacement in installed it downloaded the peogram into it and when started( powered up) wouldn't you know it the machine did the same thing again.

 

[harry richardson]

part two due to length

again the only thing found shorted was the panelveiw circuit. at this point it was lunch when we came back we did the circuits again and found low and behold the panelview showed fine?? curiouser and curiouser.

needed the machine to run very badly so went and took the panelview off the system and set it up to run using a laptop and setting the bits. this worked for a half of a day. at which ponit a input failed and being the shop we are with no spares had to wait till the next day to start anew

ok got the replacement sensor in and said what the heck lets try something replaced the input and set it to read as nessecary put the panelveiw on the system again and wouldn't you know the system worked just hunky dorry.

hmmm wonders does the panelview save itself and short to ground or am i seeing something else entirely? or am i missing something else? questions remain and comments are welcome. anyone ever see something like this before?

 

[glaverty]

What failed on the sensor and how did you determine that it was the sensor that failed?

 

[harry richardson]

the sensor is a potted reed switch from phd the potting was melted and runny and stuck it nicely to the air cylinder the + and - wires were also shoiwing bare copper but not touching in the switch. my assumption is that the switch with its led circuit went haywire and over heated the potting compound which shorted out the switch to ground.

now also was found that when the unit was intializing the cable on the switch was being pulled heavily so i believe that was the intial mode of failure but i could be wrong.

 

[glaverty]

Did the panelview and the sensors all run off the same power supply?

How did you check the panelview? Was it still connected to the 24V power supply?

You said the Panelview was the only thing in the 24V circuit that showed as shorted, what else was in the circuit.

 

[jimtech67]

Did you power the PV off the scl power supply? Try powering the PV off its own power supply.

Go JETS

 

[JesperMP]

It seems to be an intermittent kind of error, and these can be difficult to pinpoint. Maybe you have found the cause of the PSU overload - the defective sensor, maybe not.

You should segment your 24V circuitry in a few wellchosen sections, and have a circuit breaker for each section. At least this will help you in locating the error. In some cases this can let the equipment stay in operation, if possible with only the tripped section "offline".