Analog input I vs V

[travish 0]

What are the Pro's and Con's of using voltage or current for an analog input. I am using a 1769-IF4 input module and Turck Ri 360 rotary inductive sensors, I will have 3 of the Turck sensors monitoring the rotary position of 3 axis. The sensors give the option of using Voltage or Current and the Module will accept either. So is one better than the other. I have used both in the past with no real issues, except on 1 application I was using an air pressure switch that had an analog voltage output (I think it was Sunx, but not sure) I did not get good repeatable readings. Is this typical of voltage inputs, or was my sensor the problem? Thanks Travis

[Steve Bailey 118]

Current signals typically use a 4 - 20 milliamp range, so any value below 4 milliamps can be interpreted as a broken wire or a failed sensor. Within the limits of the sensor, the signal is not subject to line losses as a voltage signal would be. The advantage to a voltage signal is that it is easier for a technician to measure it with a meter.

[Mickey 71]

Current signals are less susceptible to noise.

[Ken Moore 23]

On long runs with voltage inputs, you have to compensate for voltage drop, not need for current inputs.

[Willy65 0]

The advantage to a Current signal is that it is just as easy for a technician to measure it with a volt meter. Measure over input connection and you will be getting readings 1-5V if you are using 4-20mA

[Mickey 71]

That's assuming the load resistance is 250 ohms. Standard but not always the case.

[travish 0]

thanks guys, I like using the current inputs myself Travis

[brazierp 1]

Only use voltage if you absolutely have to and keep it inside the panel if possible.

[OkiePC 14]

My previous employer used a lot of 0-10vdc. It was easy to deal with, and I did get to work on it quite a bit, mainly adjusting trim pots following device replacement to match scaling and simple things like that. Noise was an occasional issue. My current employer uses 4-20mA almost exclusively. I rarely work on it, and that usually entails teaching a tech how to check it with a regular meter. Another forum member posted a neat idea I have begun to use: Put a 1/4 amp fused disconnecting terminal block on a convenient terminal rail so you can connect the meter and then open the fuse for testing. I have no more noise issues, just occasionally have to provide 2nd opinion when lightning takes out a channel or whole card. I think that the only failures we've had are from lightning. The only analog cards that have failed here in 6 years have been on the roof in air handlers. And, you can get noise on it, so do be careful to follow shielding recommendations. It's just much less sensitive to induced noise from adjacent wiring because noise doesn't make much current...but lots volts sometimes... Edited 12 Jul 2012 by OkiePC

[travish 0]

"Put a 1/4 amp fused disconnecting terminal block on a convenient terminal rail so you can connect the meter and then open the fuse for testing" That is brilliant, some people are so smart, thanks for the tip

[Joe E. 259]

Why didn't I think of that? That IS brilliant.

[brazierp 1]

+1 !! I'm gonna do that every time from now on. Thanks Okie. Edited 12 Jul 2012 by brazierp

[Bob A. 1]

Regarding the requirement to open a 4 - 20 mA loop for checking the output, you may find the Fluke 771 useful for these checks as it is a great troubleshooting tool.