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?
Electrical circuits always needs two signal traces for the poor electrons to flow continuously - with a single trace you can just charge some object until it gets the same potential after which the current will drop to zero.
So if you then use one wire and a chassi ground or two wires depends on your situation with noise, distance, etc.
If you do not want a shared potential, then you need to implement an isolated interface, where you might use some coupler (optical, magnetic, ...) before you return back to electrical signals.
I see my multi-meter can measure a current passing through it. what is the best method to do so? there should be some IC designed for this aim. As you said maybe by magnetic coupling or etc. since 4-20 mA is a conventional method of transferring data there should be some standard way to measure it on the other side.
The traditional way is to send the current through a resistor and measure the voltage. That's one of the big points with Ohm's law. If you have a power supply that displays current, it normally does it by measuring the voltage drop over a resistor.
Magnetic and optic couplings are seldom used to measure - they are normally used to isolate. So you may use a special, linear, optocoupler to get galvanic isolation. But on the other side, you would end up with measuring a voltage since that's easier than having a sensor that explicitly counts photons without converting the photon stream into a voltage.
there is no 'ground' in a 4-20 mA loop. Anyone can insert anything (as long as it does not create a major voltage drop) anywhere. Thus the voltage across the resistor should be measured differential.
so there should be a special differential amplifier circuit not be sensitive to the voltage of the differential inputs. I see LM324 op amp do not work if the bias point of the differential inputs are not relative to the GND of op amp.
"Thus the voltage across the resistor should be measured differential."
"should" here might be a bit too strong. In many situations it's possible to let give the two sides a common potential and avoid the need for differential measurements. True differential measurements tends to increase the cost, reduce the measurement precision or both.
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 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.
I see that if one applies differential inputs out of VCC-GND range of the op-amp then it does not work as expected so if you say I measure differential inputs but dont know the voltage of each with respect to GND then how can op-amp work!
Sorry, but I do not understand you.
An OP-amp has two inputs and can work with differential data. But both the inputs and the output(s) needs to be slightly inside the supply range of the OP-amp.
So the cost/complexity of your design will increase if you need full isolation.
One solution is to use a linear optocoupler where the current is used to modulate the LED.
Another is to have a voltage-controlled-oscillator feeding a transformer and pick up the frequency on the other winding.
A third is to send the current through a coil and use a Hall effect sensor to measure the strength of the produced magnetic field.
Yet another option is to isolate your measurement device - then you can send the current through a resistor and directly measure the voltage. Then you use whatever isolation you need in the next step - forwarding the measured value. Power supplies are isolated, just as 10base-T, 100base-T, ... Ethernet connectors, making it easy with a single input forwarded over Ethernet.
Or open the wallet and spend some money on something like: www.fine-tek.com/.../pinfo_in.aspx