Overvoltage on AI-110

[David Moerman]

I just finished an application using an cFP-AI-110 module.  I had a +24VDC power supply which I need to supply a couple sensors, and I assumed that I could use the same voltage to run through a couple switches and just measure them with the AI-110 to get an on/off state.  I have all the inputs configured for 0-10V.

However, when I applied +24V to one channel, I got 10.4 volt readings on ALL the channels (except ch0 and ch7 which read about 7.5V).  Is this expected behaviour?  Have I broken the rules?  The AI-110 specs say that overvoltage protection is provided to +/- 40V on the voltage inputs.  In an industrial control environment, +24V is a very common voltage;  and since the Fieldpoint line generally allows a 10-30V supply range, I think I should be allowed apply +24V to an input without affecting the other inputs.

As it was, I got another power supply (+12V) and used them for my switches.  Oh well.

David Moerman
TruView Technology Integration Ltd.

[StefanK]

David,

you're right, the overvoltage protection is rated with 40 Volts for the voltage inputs for this module. If you supply the inputs with 24 V this won't damage the module. The voltages you read, namely 10.4 are expected for a overranged +/-10 V input. (see p. 13 of user manual http://www.ni.com/pdf/manuals/373280b.pdf).
Btw, there are modules available, that have higher input ratings, like the cFP-AI-102 module, or some digital input modules.

Hope this helps!

Good luck,

Stefan

[David Moerman]

To re-emphasize:  I applied +24V to ONE input only, and read 10.4 volts on ALL the inputs as a result.  In my application, this could have hazardous consequences on the equipment it controls.  (Thanks for the info on the cFP-AI-102 which I was not aware of.)

-Dave

[StefanK]

What if, you apply voltage that's in range? Did you consider, that the analog input section on this model could be saturated?

Message Edited by StefanK on 01-20-2006 06:51 PM

[David Moerman]

I have no doubt the analog input is saturated for that channel, but I shouldn't have to concern myself with that, as the effect should not spill over to the other channels.  To workaround the problem, I used a +12V signal instead.  This is still over-range, but does not have any effect on the other channels.

-Dave

[Laura F.]

Hello David,

Did you have signals connected to these other channels when the 24V was applied and the effect seen on the other channels?  If you do not have anything connected to these channels, I would expect the readings to float and you could see the behavior you described.   If you are not using the extra channels, you could always just ground them.  Just a thought.

Thanks,

Laura

[David Moerman]

Hi Laura,
Thank-you for bringing this up, because I temporarily forgot this important point.  I did think of that, and so I put in 10K pull-down resistors connected to COM on my other signals (the other signals were capable of handling that small load).  It made no difference.  I didn't try connecting each Vin *directly* to COM;  but I'll try that next time I'm on site, out of interest.  However, I would have expected a 10K pull-down to do the job.

-Dave