Strange thing about A-B FlexI/O 1794-OA16P

I have a light bulb in an E-stop button fed from an 120 VAC output on the above mentioned module. The light is always ON no matter output is in ON condition or OFF condition. While trying to troubleshoot this problem, I found all un-connected outputs have 120VAC to neutral or ground in OFF mode. Those with proper loads have zero volt while in OFF mode. I thought there is something wrong with wiring or module, so I wired a new module with same TB3S base with only power to it and no outputs, it does the same thing. Could not understand why it's like that. However, I found the light bulb is a 24V LED, changed it to 120V, it works fine.
Can some expert explain the strange DO module for me?

Thanks a lot!

Triac leakage current.

It's enough to light low power lamps, fool some technicians, and I have even seen it cause a relay to buzz once.

Like you said, properly loaded, the voltage disappears. I bet your indicator lamp is an LED type that requires very little current. Try a more power hungry lamp and it will fix your problem.

EDIT: Out of curiosity, I looked up the leakage current for the 1974-OA16: Maximum of 2.25mA

AC output modules generally use triacs for each output. A Triac has some leakage current through it, so you need enough loading on the output to bleed the leakage current to ground.

Thanks a lot guys. I didn't know TRIAC before. Now I learned.

1794-ow8 = relay output

in addition to the "confusion" factor involved with triac outputs, there are some important safety issues that you need to be aware of too ... take a look at this post when you get a chance ...

For that very reason, we NEVER use triac modules. Our outputs are only relay.

there is a plus side to a triac. it is a great trouble shooting tool. it will tell you exactly where you have a open in the string.