4-20ma signals
- Thread starter shoelesscraig
- Start date
Stationmaster
Member
That's gonna complicate things. So the analog OUTPUT from manufacturer X is varying IT's resistance, and the analog INPUT from manufacturer Y is varying IT's resistance?
What is the CONSTANT? I'm gonna need some CONVINCIN'!
Stationmaster
rwy34ils
Member
I don't recall the manufacturers but I do run into this problem once in a while where the input resistance varies. Rather than using a fixed resistor on the input, they use some type of solid state front end that receives the 4-20 mA signal. In such a case it is pointless to try to measure the input resistance with an ohmmeter. If I come across one of the manufacturers I will post it here. I mention it here just in case someone else runs into this. I spent a lot of time scratching my head the first time I encountered this.
rwy34ils
Member
..
Last edited:
rwy34ils
Member
I teach instrumentation courses to beginners as part of my job. I'm going to give it a shot here...
Every 4-20 mA loop must have 3 components: One (and ONLY one) transmitter, one (and only one) loop power supply, and one or more receivers. Often 2 or 3 of these components are combined into one instrument and this can cause confusion for students as well as experienced people. It is best for learning purposes to treat them as 3 separate components.
It is best to think of the transmitter as a transistor that modulates a current flow through it just as a water valve can be modulated to control flow rate. If a pressure transmitter is calibrated for 0-100 psi, then it will adjust itself automatically to maintain 4 mA flowing through it at 0 psi. If the pressure is 50 psi, it will hold 12 mA. If the pressure is 100 psi, then the current is 20 mA, etc.
The key thing here is that the transmitter can automatically compensate for disturbances in the loop. If the loop resistance changes for some reason or the loop power supply voltage changes a bit, the transmitter will sense that the loop current has changed and it will automatically adjust itself to bring the mA back to where it should be.
In most cases the input resistance to the receiving device is fixed at 250 ohms. But not always. Some manufacturers use 50 ohms, some are 100 ohms, etc.
With a 250 ohm receiver, the voltage drop across it will vary from 1 to 5 volts as the current varies from 4 to 20 mA. That is just Ohm's Law.
Now, one of the basic rules of electricity is that the sum of the voltage drops around a loop must equal the voltage being supplied. In this case the loop is being powered with 24 volts, so the drops must also equal 24 volts. If there is 4 mA flowing in the loop there will be a 1 volt drop across that 250 ohm receiver. If we started with 24 volts, and dropped 1 volt across the receiver then the remaining 23 volts must be across the transmitter. It just is! If the loop current is 20 mA, then there must be a 5 volt drop across the receiver. That leaves 19 volts across the transmitter. The voltages all add up.
We can add more receivers in series in the loop, but only within certain limits. Each time we add another 250 ohm receiver, we add more voltage drop in the loop. With 2 receivers and a loop current of 20 mA, there is a 10 volt drop across the recivers, leaving only 14 volts across the transmitter.
There are 2 types of transmitters: 2 wire and 4 wire. The 4 wire transmitters get their power to operate from 2 of the wires, perhaps 120 vac or 24 VDC from another power supply. The 2 wire transmitters "steal" the power they need to operate from the loop current. As the mA passes through the transmitter, it steals a bit of that energy. What this means is that a 2 wire transmitter MUST have a minimum voltage drop across it in order to operate. As more and more receivers are added to the loop, it leaves less and less voltage across the transmitter. Most transmitters need 9 volts or more to operate. So, if you have 4 receivers in a loop and a 24 volt loop power supply, there will be a total of 5volts x 4 = 20 volts drop across the receivers. That leaves only 4 volts across the transmitter and it will not work. Interestingly, the loop will probably still work at low milliamps because the drop across the receivers will be small. But as the loop current increases it will reach a critical point where the voltage drop across the transmitter is too low.
Long winded, but I hope this helps.
Every 4-20 mA loop must have 3 components: One (and ONLY one) transmitter, one (and only one) loop power supply, and one or more receivers. Often 2 or 3 of these components are combined into one instrument and this can cause confusion for students as well as experienced people. It is best for learning purposes to treat them as 3 separate components.
It is best to think of the transmitter as a transistor that modulates a current flow through it just as a water valve can be modulated to control flow rate. If a pressure transmitter is calibrated for 0-100 psi, then it will adjust itself automatically to maintain 4 mA flowing through it at 0 psi. If the pressure is 50 psi, it will hold 12 mA. If the pressure is 100 psi, then the current is 20 mA, etc.
The key thing here is that the transmitter can automatically compensate for disturbances in the loop. If the loop resistance changes for some reason or the loop power supply voltage changes a bit, the transmitter will sense that the loop current has changed and it will automatically adjust itself to bring the mA back to where it should be.
In most cases the input resistance to the receiving device is fixed at 250 ohms. But not always. Some manufacturers use 50 ohms, some are 100 ohms, etc.
With a 250 ohm receiver, the voltage drop across it will vary from 1 to 5 volts as the current varies from 4 to 20 mA. That is just Ohm's Law.
Now, one of the basic rules of electricity is that the sum of the voltage drops around a loop must equal the voltage being supplied. In this case the loop is being powered with 24 volts, so the drops must also equal 24 volts. If there is 4 mA flowing in the loop there will be a 1 volt drop across that 250 ohm receiver. If we started with 24 volts, and dropped 1 volt across the receiver then the remaining 23 volts must be across the transmitter. It just is! If the loop current is 20 mA, then there must be a 5 volt drop across the receiver. That leaves 19 volts across the transmitter. The voltages all add up.
We can add more receivers in series in the loop, but only within certain limits. Each time we add another 250 ohm receiver, we add more voltage drop in the loop. With 2 receivers and a loop current of 20 mA, there is a 10 volt drop across the recivers, leaving only 14 volts across the transmitter.
There are 2 types of transmitters: 2 wire and 4 wire. The 4 wire transmitters get their power to operate from 2 of the wires, perhaps 120 vac or 24 VDC from another power supply. The 2 wire transmitters "steal" the power they need to operate from the loop current. As the mA passes through the transmitter, it steals a bit of that energy. What this means is that a 2 wire transmitter MUST have a minimum voltage drop across it in order to operate. As more and more receivers are added to the loop, it leaves less and less voltage across the transmitter. Most transmitters need 9 volts or more to operate. So, if you have 4 receivers in a loop and a 24 volt loop power supply, there will be a total of 5volts x 4 = 20 volts drop across the receivers. That leaves only 4 volts across the transmitter and it will not work. Interestingly, the loop will probably still work at low milliamps because the drop across the receivers will be small. But as the loop current increases it will reach a critical point where the voltage drop across the transmitter is too low.
Long winded, but I hope this helps.
Mickey
Lifetime Supporting Member
rwy34ils,
Nice write-up, but I will play devil's advocate here on just a couple of points.
1-Check the arrows on your drawing. ( if you are an engineer, I'm sorry to hear that.
http://www.allaboutcircuits.com/vol_1/chpt_1/7.html
2-You can have more then one power supply ( for redundancy or extra power connected in parallel)
Nice write-up, but I will play devil's advocate here on just a couple of points.
1-Check the arrows on your drawing. ( if you are an engineer, I'm sorry to hear that.
http://www.allaboutcircuits.com/vol_1/chpt_1/7.html
2-You can have more then one power supply ( for redundancy or extra power connected in parallel)
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rwy34ils
Member
Yes that debate has been around since electricity was discovered. Whether you say potatoes or potatos, the important thing is that the electricity moves around the loop in one direction. Fortunately I am bi-polar, ambidextrious and dyslexic so I can read it either way!!!
As for the power supplies, yes we can have multiple power supplies in parallel or even in series, but from an electrical point of view, they could all reduce to a single source. The important point is that there IS a power supply in the loop somewhere. Often it is 'hidden' inside the transmitter or receiver and that can cause all kinds of problems if the designer or installer adds an external power supply where there is already one built into the transmitter. I think this is where the original poster is getting confused.
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Roy Matson
Member
Don't assume that the VFD input will be the standard 250 Ohms, there is no reason for a VFD manufacturer to use that. The signal will normally go into an Op Amp which requires very little voltage variation so the input impedance could be any odd value. The best thing to do is think in mA or Percent (4mA=0%, 20mA=100%)
Roy
Roy
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