Contollogix Analog input card losing mA

We have CLX system with redundancy and control net. When we inject a 4-20 signal into an input, we are losing 1mA. We have verified out source as putting out 20mA. As we reduce the signal the loss becomes less. But at 4mA we are still losing a significant amount of the signal. We have injected the signal directly into the card with the same result.


Any ideas?


-J

You probably have a whisker hanging or short circuit or some other wiring issue. It appears that the signal has two paths, and not all of the current is getting to the card.

Have you checked the configuration of the card?

Sounds like you may have the card set up for 0 - 21 mA rather than 4 - 20 mA.

All,

Thanks for the responses. The configs are correct, the CLX processor does not support 0-21mA config. 0-20mA is the only option (at least on our setup)we have 8 AI cards, 2 of them appear to have never been calibrated (they are showing the problem as describered in my original post) the other 6 have been calibrated (from factory) and they read .5mA greater then the actual input. We thought that the rack may be overloaded with too many AI cards so we pulled a bunch, and it made no difference. We have had 2 AB techs looking at it and they have never seen this. We are pretty sure the 2 that are not calibrated will be ok once calibrated. As for the others, I am not sure. Will let you all know if we figure something out.

Have you tried switching AI's? Put one that you know is good where you suspect one is bad. Do you get the same results?


How about your power supply? Is it/they overloaded? Voltage regulated?

I would agree that it is probably a wisker. My other guess is that the resistor is out of spec. I don't know about that card but some use the 4-20 ma to generate a 1-5 volt signal across a 250 ohm resistor. Maybe you can ohm the resistor and see.

If the resistor was out of spec, the percent current deviation would be constant. I think if a wisker was touching something it would be similar.