Internal vs External Aux Contact Feedback

[Lordzorak]

I was curious what people's preferences were with starter aux contact feedback to a PLC. Whether to use the internal transformer to feed the aux contact then go to the PLC or an external power source going through the aux contact. I think I see benefits to both, but the question stems from wondering if I were to use the internal transformer, how to have the output of the aux contact go to an input card on a PLC without raising an issue of the common wire of the circuit, which for an input card is common to all inputs (expect isolated input cards but they have fewer inputs thus requiring more cards). however, the common for the cabinet where the PLC is located would not be the same as the common where the bucket power originated from. One way would be to devote a card(s) to the buckets and just have the common from the card then go back to each bucket. But if there were any unused inputs, someone could tie something on the input not knowing it is not going to the common in the cabinet. And we would be in the same situation as above. One could also use the internal transformer to feed the aux contact that then goes to a relay. A second power source from the cabinet then goes to the NO contact on the relay, thus separating the two. Another concern with the external power source to the aux contact is the potential presence of a second voltage even when the bucket is turned off. But I was curious of what people here prefered in their experience. Any input is appreciated. Thanks. Z

[Armadillo852]

We have done them both ways without issues. I prefer to use internal because it is there and you paid for it. What we have done is always run a separate input card strictly for feedback. Thus making sure all the commons are the same. The key is to document very well that this card is strictly for the feedback and not any other inputs.