Adding Analog Components to I/O Interface

Hello,

I am newish to PLCs and the below thread got me thinking:

"Smoothing Analog Inputs"
http://www.plctalk.net/qanda/showthread.php?t=108849

Would it be possible to just slap in a small capacitor to ground in order to smooth the 4-20ma input? What are the dangers and pitfalls of adding "circuit" components to I/Os? For instance thermocouples are basically fancy resistors; if I know the product specs shouldn't I be able to say add constant series resistance to the PLC input to shift to a higher voltage source with lower range within 4-20ma? (just as an example, I understand that this would be both shotty work and electrically inefficient)

Best,
Gabriel

Yes a capacitor to ground on the analog input signal will get rid of some noise.

http://accautomation.ca/the-secret-of-getting-rid-of-noise-on-your-analog-signal/

Regards,

gclshortt said:

Yes a capacitor to ground on the analog input signal will get rid of some noise.

Click to expand...

Thank you, but my question wasn't whether or not a capacitor would reduce noise but rather if it is within good practice to start wiring such components within my cabinet.

A capacitor to ground is perfectly fine to reduce RF range noise. The signal variations the PLC filter is smoothing are relatively low frequency. I suppose enough capacitance could be added to reach filtering of seconds or minutes of time constant but it would be silly to use that much money and cabinet space.

A thermocouple isn't a fancy resistor. It creates a small voltage proportional to temperature at the junction of two dissimilar metals. A RTD is a somewhat fancy resistor where the resistance changes proportional to temperature.

There's likely an obscure exception, but adding resistance to change range wouldn't be done. Scaling is done in the field device and the PLC.

Adding a resistance in series in an input by intensity (like 4-20mA) does not make sense, simply when the sum of the added resistance and the own input resistance exceeds the maximum load resistance allowed by the transmitter then it will stop working well.

Thermocouples generate small voltages that are not exactly linear with the temperature, in addition they need a compensation of the the cold junction temperature. In other words to read temperatures with thermocouples you need a specific input module for thermocouples.