Allen Bradley output module spike

ettikudiappan

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Apr 2002
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Singapore
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We have several output module 1756-OB32 that is used to turn on 24Vdc loads. One of the load is a vision capture system that gets a trigger from the output module.
When there is an Emergency stop, the electrical wiring is done such that the 24Vdc for the output modules are cut off. Once reset, the 24Vdc power is turned ON. This 24Vc is the wet supply that is used to power the output loads like relays and commands to motor drives.

We noticed that even though in the PLC the outputs are off, the vision system got triggered when there is an Emergency stop and reset condition. That is, whenever the 24Vdc power is turned on, the vision gets triggered. Placing a multimeter on the corresponding terminal at the output module, we found that there was a spike on the output line for a very very short time, but enough to trigger digital devices like the vision system. What we believe is that since 1756-OB32 is a digital output (as compared to relay output), the transistor inside the module gets fired for a very short time and creates the spike.

Can please advice if this is normal or anything that we can do to prevent this from happening?
 
If possible power the vision system with a separate power supply.
I think its good practice to power analog and sensitive equipment separate from discrete devices. IMHO
 
Vision is powered by a totally seperate power supply. The trigger for vision capture comes from the PLC output module directly (without any optical isolation or relay). My main concern is why there should be a spike in the first place. Isolation using coil relay or opto-isolator is a work around and not solution.
 
ettikudiappan said:
We have several output module 1756-OB32 that is used to turn on 24Vdc loads. One of the load is a vision capture system that gets a trigger from the output module.
When there is an Emergency stop, the electrical wiring is done such that the 24Vdc for the output modules are cut off.


If any of the other 31 outputs feed a relay that is active that has no snubbing network such as a diode across its relay coil, it is possible for it to create a reverse voltage spike of short duration which has no where to discharge as the emergency stop has isolated the power source that could soak up the spike.
 
panic mode said:
are you sure program is not doing it? how sure?

That is a good point. In my first Controllogix application, I had a bug that appeared because of the extremely fast CPU and the asynchronous I/O update.

This section of the machine had it's own e-stop circuit which would kill power to a bunch of I/O and had a contact monitored by a PLC AC input. Well, some DC photocells would sometimes drop out faster than the AC input from the estop monitor and consequently, my logic would advance one step just prior to halting due to an e-stop. We changed the wiring to keep the inputs powered to solve that particular problem, but it took some head scratching at first just to figure out what was going on, and, we too, started to blame our hardware at one point.
 
200% sure that the PLC program is not triggering it. In fact even if I force OFF the outputs, the vision gets triggered. The only way to prevent it was to remove the wire physically from the terminal.
 
Definetly a point worth pondering about. I will check what each of the output points connects to. Recovering from an E-Stop, only a few outputs on that module really turn on, so my search can be further narrowed down.

Gil47 said:
If any of the other 31 outputs feed a relay that is active that has no snubbing network such as a diode across its relay coil, it is possible for it to create a reverse voltage spike of short duration which has no where to discharge as the emergency stop has isolated the power source that could soak up the spike.
 

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