24vdc sensor burning up constantly

[lefeverj]

I received a call from and old friend who is trying to get his system up and running. His company recently updated an older PLC to a new PLC. He stated that the wire logic is Negative logic. The problem he is trying to resolve is that the sensors that count the items continue to burn up and in a random order...not the same sensor each time...A total of (5). These sensors send the negative input to the PLC and receive a constant (+)24vdc. There is no fuse or surge within the system other than the main breaker. He also mentioned that they replace some of the ice cube relays a lot that are 120vac that run the stepper motors. My first thought was to advise him to put in some sort of surge and fuse the sensors minimally. My main concern would be to discover where the over-current is coming from? Do you guys have any recommended trouble shooting ideas I could pass along to him to try and find his over-current? I also believe that the new PLC is just a coincidence unless something got mis-wired. Not sure how to advise him to continue his trouble shooting effort. Any ideas would be appreciated.

 

[cornbread]

Have him make sure the grounding on the system is proper. I've seen floating grounds that will create all sorts of havoc.

 

[The Plc Kid]

Have him make sure the grounding on the system is proper. I've seen floating grounds that will create all sorts of havoc.

On a 24 VDC system many times you want it floating / ungrounded. It depends on what you are doing.

 

[The Plc Kid]

I received a call from and old friend who is trying to get his system up and running. His company recently updated an older PLC to a new PLC. He stated that the wire logic is Negative logic. The problem he is trying to resolve is that the sensors that count the items continue to burn up and in a random order...not the same sensor each time...A total of (5). These sensors send the negative input to the PLC and receive a constant (+)24vdc. There is no fuse or surge within the system other than the main breaker. He also mentioned that they replace some of the ice cube relays a lot that are 120vac that run the stepper motors. My first thought was to advise him to put in some sort of surge and fuse the sensors minimally. My main concern would be to discover where the over-current is coming from? Do you guys have any recommended trouble shooting ideas I could pass along to him to try and find his over-current? I also believe that the new PLC is just a coincidence unless something got mis-wired. Not sure how to advise him to continue his trouble shooting effort. Any ideas would be appreciated.

He may want to look at the amount of cycles on those relays and the loads. They need some suppression at the minimum.

 

[Temper]

On a 24 VDC system many times you want it floating / ungrounded. It depends on what you are doing.

I have a question about this, but I don't want to hijack the thread so I'll start a new topic.

 

[BryanG]

Do you guys have any recommended trouble shooting ideas I could pass along to him to try and find his over-current?

There is no such thing as isolated over-current, he is looking for an over-voltage or a wrong voltage supplied to the sensor. Check for an AC component in the sensors DC supply, switch the multimeter to AC. Or a very high voltage between the sensor and the metal work that the sensor is attached to. One thing you did say was that it was a negative logic system but the supply was +24V. Another possibility is a bad batch of sensors, I recently had issues where I searched for all sorts of problems but it came down to faulty sensors.

 

[Geospark]

...I also believe that the new PLC is just a coincidence unless something got mis-wired...

Can you paint a fuller picture for us so we can rule this out?

I appreciate that you're giving us this info second-hand, but can you find out...

What is the make/model number of the sensors?

What is the make/model number of the PLC?

If not embedded inputs, what is the model number of the source input card?

Improper wiring of the inputs may still be a possibility.

...These sensors send the negative input to the PLC and receive a constant (+)24vdc...

I sense the dreaded sink/source confusion here

Just to clarify...

If they are not two wire sensors, regardless of whether they 'send the negative', or 'send the positive', these sensors will have a 'constant (+)24vdc' and 'constant (-)24vdc' i.e. if the reverse was the case, and the sensors 'send the positive', they would still have a 'constant (+)24vdc, and of course 'constant (-)24vdc'. The supply to the sensors does not dictate which polarity is switched. The type of transistor within the sensor does i.e. PNP or NPN. These sensors should be of the NPN type, and wired to a source input.

If they are two wire sensors, then a 'constant (+)24vdc' on one wire will not 'send the negative', as there is no negative to send.

Again, the requested info above will help clear some of that up.

...He also mentioned that they replace some of the ice cube relays a lot that are 120vac that run the stepper motors...

The problem with the ice cube relays is most likely a separate issue, and I'd concur with The Plc Kid on checking the loading and surge supression on those relays.

George

 

[Andybr]

One thing to consider if you are using NPN sensors is what happens if you turn off the 24Vdc supply to the sensor but leave the switched wire energised. Some sensors will stay "off" but some will turn "on" in these circumstances and this can lead to damaged sensors. The same thing applies to "Sink" outputs on PLC's.

 

[shooter]

most likely a sensor will burn when it is the only one activated, this will give enough current to the supply line to burn internally.
be sure the no short between power supplies have in/outs separate from PLC supply.

 

[Sergei Troizky]

I received a call from and old friend who is trying to get his system up and running. His company recently updated an older PLC to a new PLC. He stated that the wire logic is Negative logic. The problem he is trying to resolve is that the sensors that count the items continue to burn up and in a random order...not the same sensor each time...A total of (5). These sensors send the negative input to the PLC and receive a constant (+)24vdc. There is no fuse or surge within the system other than the main breaker. He also mentioned that they replace some of the ice cube relays a lot that are 120vac that run the stepper motors. My first thought was to advise him to put in some sort of surge and fuse the sensors minimally. My main concern would be to discover where the over-current is coming from? Do you guys have any recommended trouble shooting ideas I could pass along to him to try and find his over-current? I also believe that the new PLC is just a coincidence unless something got mis-wired. Not sure how to advise him to continue his trouble shooting effort. Any ideas would be appreciated.

What means "sensor burn up" in this case?
Is a burnt sensor output always Off or always On?

Are you certain that the sensors drive only the PLC inputs and nothing else?

What is the application? Are there chances of electrostatic discharges? Or liquid penetration?

Of course, the questions assume no wiring mistakes.