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'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