Dear friends,
I am working on reading 4-20 mA from Level/pressure/flow transmitters. I used to use a 10 ohm resistor in series. so I measured the voltage drop on the resistor but in this circuit I needed the ground from the transmitter device. Is there a better method not using ground wire from the transmitter?
The biggest reason for 4-20mA signals is that the sensor has an extremely low impedance and so has good noise immunity. In some situation, isolation may be highly important but quite often it's the noise immunity that is more important. the biggest? Per, methinks that maybe the 'biggest' reason is that you can power devices from the loop current
That's a bonus but it's a quite inefficient way to send power since the receiving side must manage to operate on the energy available at 4mA, while it will need to waste 80% of the received energy at 20mA.
The next thing is that powering things might be logical when you use the 4-20mA as an output to drive/control something. But in the different direction, when it's a "dumb" sensor producing a 4-20 output, then you would most probably instead prefer to power the sensor instead of requiring the sensor to support a quite powerful current generator to be able to adapt to a rather wide range of load resistors.
The truly great thing is that you can have very long cables without worrying too much about voltage drops in the cables and noise being picked up - the 4-20 system is from way before we could trivially send digital pulses.
many moons ago I were involved with designing systems that used 'ready-made' sensors. of the 4-20 units, 90%, if not all (I remember none) were "bus powered".
Ah yes - there are a few customers who do use 4-20 equipment still. But so long since I had to use one myself I had forgotten that the voltage source can be on either side, so the sensor just burns a varying amount of current.