4-20ma Loops

Marshy

Member
Join Date
Apr 2007
Location
Bath,UK
Posts
236
Hi,
I'm a bit confused over a 4-20ma loop problem I've just been working on and wondered whether anyone out there could help me understand the problem I'm having?
We have a ABB Magmaster measuring the flow of water in a pipe the analog 4-20ma output of the unit goes into our PLC analog input. This all works fine.

Our Environmental dept would like us to add a new data logger to the system as a backup to ours. The plan was to break into the current loop and wire the 4-20ma output into the input of the data logger so the PLC input and the data logger were now in series.

With the devices wired up so that the logger is first in the loop and the PLC input last, the logger doesn't read any value, the PLC reads correct value.
With the devices wired up so that the logger is last in the loop and the PLC input first, the logger reads a value but its always higher than it should be, the PLC reads correct value.
Wiring the 4-20ma output directly to the logger gives a correct value.

I've put my meter in series with the loop in the different parts of the circuit and the current, as you would expect, remains constant.
I think this may be a situation where I may may need to use some kind of current opto-isolators to isolate the two circuits? But I'm interested to know why? Is it to do with the voltage in the circuit? Another point worth adding is that the logger input can be configured (via parameters) to be either a 0-10v input or a 4-20ma input.
Any help would be appreciated
Cheers
Rich
 
I would think that the loop is grounded at more than one place, probably the PLC and logger.

If the PLC and the logger had 'floating' input terminals (+ and -) then it should work ok, so at least one of the - input terminals will be grounded, i expect.

If you connect only the PLC and find out what input impedance it has you can work out the voltage that will be present at the PLC terminals;

If the impedance is 250ohms then the voltage will be 1-5V (20mA x 250ohms = 5V) at the PLC terminals, maybe your logger could measure this voltage range??

If you do this then the input impedance of the logger will cause a slight error in the readings, but you can calculate this and I guess it will be negligable.

Another possibility; does your PLC have spare analogue outputs? If it does, or you can add one, just pass the value through the PLC with a line or two of extra code.

Otherwise buy the splitter.
 
Last edited:
4-20ma loops are meant to be more tolerant to noise, but they are just a 1-5V signal with a precision 250 ohm resistor at the other end. When you configure an input to be voltage or current, you are just switching that 250 ohm resistor in and out of the circuit. When you put another device in series you are changing the resistance to be more than 250, probably 500 so with the same 1-5V signal you will be drawing half the current and your readings will be wrong. Your logger and PLC probably have different tolerances for error which is why you see one working and the other not.

You can't wire them in parallel either because you will end up with 125 ohms. Change both to voltage mode, this will typically have a very high input impedance and will allow you to wire them in parallel but will be more susceptible to noise.
 
but they are just a 1-5V signal

I have seen lots of 4-20mA outputs that are quite happy to drive into two 4-20mA inputs in series. You are correct that if the load impedance is too high then the source will be unable to supply enough voltage to derive the intended current.

When you put another device in series you are changing the resistance to be more than 250, probably 500 so with the same 1-5V signal you will be drawing half the current and your readings will be wrong

I don't agree. Any changes in input load impedance (like changing from 1 to 2 devices in series) that fall within the capabilities of the output device should not result in errors in the signal, the output device will keep the current constant (for a given measurement).
 
The key may be the power supply, it must be capable of driving muliple devices. If using a 10-12v supply it may not be enough to provide for 2 devices in a loop. You may need to determine if active or passive devices are involved and do the calculations from there.

Simple example, the sensor needs 12v to operate, you need 1-5v across the PLC, and another 1-5v across the display, that would mean the supply voltage has to be 22v or above.

As mentioned if the common is grounded at multiple points it could cause a ground loop which may have an effect.

http://www.sensorland.com/HowPage028.html

http://www.processautomationcontrol.com/download/4-20mA-Current-Loop-Primer.pdf
 

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