Measuring 4-20mA across analog input card setup as differential inputs

[anthr_plcnewbie]

Hi,

I have PLC-5 that has analog input module. The modules are setup as differential inputs so two wires go to 1+ and 1- terminals. I was wondering how will I be able to measure the current 4-20mA for the channels? Do I put an ammeter between the sourcing device and 1+ terminal? or sourcing device and 1- terminal?
Thanks,

 

[Mickey]

Do I put an ammeter between the sourcing device and 1+ terminal? or sourcing device and 1- terminal?

It does not matter, as long as you are in series.

Another method is to measure voltage across the input terminals, you should read something close to 1 to 5vdc ( this depends on the analog card though).

 

[Saturn_Europa1]

If you do this type of work a lot, invest in a clamp on milliamp probe. I'm a Fluke fan.

 

[rdrast]

If you do this type of work a lot, invest in a clamp on milliamp probe. I'm a Fluke fan.

+1

The Fluke instrumentation clamp-on can also source 4-20, and can be used as a normal series meter.

Having a clamp on also virtually eliminates the problem of switching between current and voltage and forgetting to swap the leads.... Not that I've ever done that... No...

 

[anthr_plcnewbie]

Thanks guys,
If I could measure in either wire (1-) or (1+) then I guess I am not understanding how the PLC detects the mA signal? I thought the PLC would look at the difference between (1+) mA and (1-) mA and the difference is what the 4-20mA should be. I am understanding the differential input incorrectly. Could one of you please explain how PLC detects the 4-20mA? Thanks!

 

[robertmee]

+1

The Fluke instrumentation clamp-on can also source 4-20, and can be used as a normal series meter.

Having a clamp on also virtually eliminates the problem of switching between current and voltage and forgetting to swap the leads.... Not that I've ever done that... No...

Ha Ha...me either....my burnt arm hair can attest to that.

 

[Tom Jenkins]

Here is good explanation of the difference between differential and single ended inputs:
https://www.omega.com/techref/das/se-differential.html

Most analog inputs run the current signal over a dropping resistor. The value varies, but 250 Ohm is common, resulting in a 1 to 5 VDC signal.

An analog to digital conversion chip turns that voltage into a digital value. This is often 0-4095 (12 bit) but some go as far as 0-65535 (16 bit). Don't confuse resolution of the A/D conversion with accuracy, though. And the analog input value can't be more accurate than the transmitter.

 

[danw]

+1

The Fluke instrumentation clamp-on can also source 4-20, and can be used as a normal series meter.

Having a clamp on also virtually eliminates the problem of switching between current and voltage and forgetting to swap the leads.... Not that I've ever done that... No...

How many meter fuses have I blown doing that?

 

[Tom Jenkins]

How many meter fuses have I blown doing that?

I know a ten pack didn't last long for me!