3-wire 4-20mA - connect in line with Analog input and dispaly

ason

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Oct 2005
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Hey,
We have a 3-wire 4-20mA sensor that is connected to a display like this

sensor.jpg

Now i need to connect this to a siemens ET200S analog input module
think to use
6ES7134-4GB11-0AB0 2AI, I, standard, for 4-wire

is´t possible to connect the analog input inline with sensor and display
something like this?

pin on analog input
1 M+
2 M-
sensor1.jpg
 
In most cases the answer is no because what you suggest will change the resistance of the load on the sensor.
The preferred way to do it is to add a splitter that give you two 4-20ma outputs or change the display to one with a 4-20ma output.
The cheap and dirty method is to wire your PLC input in parallel with the display and set the module of 1-5vdc.
 
as long as you 4-20mA loop total resistance is below 500ohms, it should work if you wire devices to series.
Current transmitter can drive usually at least 500ohms, but it depend of manufacturer and device etc.

You can use signal splitter or analog output of PLC for display also. Signal splitter cost maybe more than ET200 analog output card.
 
Last edited:
I've seen it done many times. About three 4-20mA inputs per 4-20mA output. It depends on the devices and the resistance in the wires.

For instance if the current loop is supplied by 24V and each devices drops the voltage about 5V you could put 4 devices in series.


PS. Which 4-20mA sensor are you using?
 
Last edited:
There is no need to guess on this. Look up the max load/burden on the transmitter. Add up the total resistance of all the devices in the system. If the total is less than max allowed you are OK.

One caution: loop powered indicators use an internal diode to create a voltage drop and power the electronics. At low outputs (i.e. 4 mA) this diode can look like a very high resistance. Check it out carefully if you have loop powered devices in the circuit.
 

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