Park.CTRL

Dealing with an output that is "always on"

3 posts in this topic

Gang,

Hooking up an Automation Direct P2-32TD2P to an AC/DC LED and noticed the LED is always "on" a little bit. When the output is on, the LED is certainly on, but when the output is off the LED is still on by a fractional amount, not much but it's plainly noticeable the room lights are off. The CPU can be in Stop, no matter, power just has to be on. I think this has to do with the card's load disconnected error functionality. +24V and some fraction of amps is always present at the output whether the output is on or off. I talked with automation people at work who noted there is an Allen Bradley system that does the same thing, it caused them a problem, they had to use a relay or etc to make an off output actually be off.

Has anyone dealt with this? I'm new to the Productivity software, but there doesn't seem to be a way to just turn it off in the IDE. It seems ridiculous to have to wire up transistors or relays just to have off be off.

Thanks for any ideas.

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I haven't used that particular PLC before, but I have seen very similar behavior with other output modules, especially newer ones. The solution was to either use a surge suppression network in parallel with the load or to just use a larger load. We first saw it with the small terminal-block type relays. AB makes a version with a surge suppressor built in (which solved it for us in one application). In another place, we couldn't use those relays, so we ended up just using a larger ice cube relay instead of the small one.

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Solid state devices have some small leakage current in the off state. The magnitude will be detailed in the specification sheet for the module. If your is so small that you need the output to have zero leakage in the off state use a contact output module. That's what they're made for.

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