from the Forum department...
Compatibility question 4-20mA signal
Sensor technologies. topic
Posted by Ernst Smith on 23 February, 2012 - 2:25 am
A question - can I connect a loop powered analogue OUTPUT (2 wire system) to a passive instrument (positioner) that has a seperate 24vDC supply? I have tried this and it works on the test bench. The main question - will the instrument last in the field? What will the effect be of this "additional" 24vDC that is supplied with the setpoint signal? Or is this 24vDC at 4mA so little that the passive instrument will just ignore it?

Hope you can give me some peace of mind here...

Posted by Roy Matson on 23 February, 2012 - 8:33 am
If the positioners input is electrically isolated from it's 24 Volt supply, yes you can. Without power on the positioner take a multimeter and see if you get a circuit between the supply Negative and the signal terminals, if not changes are it's ok.

You can also use a signal isolator for sure.


Posted by engineer on 24 February, 2012 - 6:44 am

i am a little unsure, how to electrically isolate the positioner from the 24V.

if the positioner is to work, 24V,TX and the positioner have to be in a loop. does isolation mean fuses?

if a signal isolator is put in the loop instead of positioner, the signal to the positioner is anyway the same as the positioner would have received if it would have been in the circuit.

thanks in advance for any clarification.

Posted by Rob on 24 February, 2012 - 10:46 am
No, a signal isolator is not a fuse. A signal isolator electrically separates the two sides of an electrical connection while still allowing control information to pass (usually by converting the signal to light and back again ie. an opto isolator).

You cannot have a PLC output trying to power the 4-20 mA signal circuit AND an actuator trying to power the same signal circuit - so you have to use a signal isolator. You can also use isolators to convert from one signal range to another eg 4-20mA to 0-10V

Start here ...


Posted by engineer on 24 February, 2012 - 7:19 am
sorry, i mistook 24V of the postioner for the 24V of the TX,
kinly ignore my previous post

Posted by Roy Matson on 25 February, 2012 - 7:40 am
Mr Smith left a very confusing question,
I'm not sure what he meant by the term "Passive"

I sent an e-mail to the address he provided asking for make and model but as yet he has not responded.

Posted by engineer on 25 February, 2012 - 10:48 am
so we take up a 24V source (forget PLC), the TX and the positioner, and make a loop. we take up another 24V for the positioner(as stated by smith) and connect it to the positioner.

i think that should work.

Posted by Rob on 26 February, 2012 - 12:56 pm
There is no TX in the original question - just a PLC output and a Positioner (which is confusingly referred to as an "instrument") . Whats missing from the question is whether there is an Actuator attached to the positioner powered by a separate 24V supply - but presumably there is an actuator of some kind on his test bench.

Exactly what the original poster has running on his bench and quite why it works is unclear from his post.

Posted by Sumeet Chimalkar on 29 February, 2012 - 4:42 pm
Hi Ernst,

If this is a modern Positioner, it may be possible to configure it so that it can accept a loop powered analog signal as you desire.

Specifics of the model of positioner being looked at would help in understanding your exact problem.

There is usually a way to solve these problems. We just need more information.


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