ECN Forum
Posted By: Cinner Why is 4-20ma used.. - 02/07/07 05:54 AM
Why is the range of 4 -20ma used for signaling in PLC applications
Posted By: gfretwell Re: Why is 4-20ma used.. - 02/07/07 07:06 AM
That is current switching as opposed to voltage switching. It doesn't matter what the voltage is, they only look at current. That used to be a jumper selected option on the old serial port cards in PCs. It is better in "noisy" environments since a common mode transient won't be seen at all.
I believe RS422 is the protocall
Posted By: n1ist Re: Why is 4-20ma used.. - 02/07/07 01:56 PM
20mA current loop was the old serial signaling scheme (RS232 on ASR33 teletypes come to mind).

4-20mA is an analog signal where the output of a sensor varies between 4mA and 20mA. Current loops are often used when you have long cables since as long as it is constant and not too high, the series resistance of the cable is not important. If you have a voltage source, the series resistance would form a voltage divider with the input impedance of the receiver/meter.
Posted By: JBD Re: Why is 4-20ma used.. - 02/07/07 02:22 PM
As was mentioned by using a current signal the resistance of the line is not important, therefore conductors lengthes can be very long. Most 4-20mA devices have an acceptable voltage range of 15 to 35 volts DC.

The reason that they start with 4mA is for "loss of signal" monitoring. If the PLC sees 4mA is knows that it is at zero value, but if it sees 0mA it knows the signal has been lost and can generate an error.
Posted By: JBD Re: Why is 4-20ma used.. - 02/07/07 02:26 PM
oops, duplicate post.

[This message has been edited by JBD (edited 02-07-2007).]
Posted By: SolarPowered Re: Why is 4-20ma used.. - 02/07/07 04:13 PM
Quote
The reason that they start with 4mA is for "loss of signal" monitoring. If the PLC sees 4mA is knows that it is at zero value, but if it sees 0mA it knows the signal has been lost and can generate an error.
Also in many cases, the 4mA is used to power the sensor.

I believe that the reason for "4-20mA" is that that is what the first major product of this sort used, and everybody after that just followed suit because there were "existing products we can leverage." And the reason that the first product did it this way is because the two engineers who were designing it decided over lunch that it would be a really cool way to do things.

As an engineer, I often see things done this way.



[This message has been edited by SolarPowered (edited 02-07-2007).]
Posted By: jay8 Re: Why is 4-20ma used.. - 02/07/07 04:54 PM
One thing to keep in mind with the 4-20 mA signals is you often have to use an external power supply for loop power, so sometimes if you have a choice between 0-10 volt signals and 4-20mA signals, you may want 0-10 volt since you wont need external loop power for your device.
Posted By: LarryC Re: Why is 4-20ma used.. - 02/07/07 06:49 PM
Quote
One thing to keep in mind with the 4-20 mA signals is you often have to use an external power supply for loop power

Many of the displays and transducers I use, have internal supplies that you can switch in or out with DIP switches.

The 0-10V signals are OK in areas with little or no EMI, but in machines with multiple VFD's or high power loads, I prefer the current loops. The low impedance of the inputs makes it a lot less susceptible to picking up spurious signals. One disadvantage of the current loops, if you want to add another display, you have to open the loop to insert the new display in series. With the voltage signal, it is easier to parallel with any available signal.

Larry C
Posted By: rad74ss Re: Why is 4-20ma used.. - 02/07/07 06:51 PM
4-20mA is great for long distance runs as mentioned above. It also gives you better resolution when you scale an input or output. On a simple scale like a reset signal of ten degrees you have 16mA of range to scale it too instead of 10V.

Another problem with 4-20mA is that you have to get 'in' the circuit to test it. With 0-10V you just put your probes across the terminals.

So to make it easy on guys troubleshooting in the field I go for 0-10V unless distance or resolution comes in to play.

0-20mA isn't used very much and I don't know why. You will also see 2-10V on some things like Belimo actuators.
Posted By: iwire Re: Why is 4-20ma used.. - 02/07/07 09:32 PM
This is an interesting thread. [Linked Image]

I knew of the two systems but never knew why one was better than the other. As always it's all about trade offs, it seems neither is actually better or worse.
Posted By: JoeTestingEngr Re: Why is 4-20ma used.. - 02/07/07 09:43 PM
0-20mA isn't used because you wouldn't be able to discern between zero output and cut wires. 4-20mA is a good method to provide supervised analogs over long distances in a noisy environment. The wiring resistance doesn't even have to be constant as long as the full 20mA can flow through the loop within the source's supply capabilities.

Let's say that I have a 0-60PSI ---> 4-20mA pressure sender but no cut sheet. I do know that it works off of a 24VDC supply. If I hook this thing up to my bench supply with no load,(output leads open), there will probably be close to 24 volts on those leads. Let's say that I measure 23.0 volts with a 24.0 volt supply. 23V / .020A = 1150 ohms. This is the highest load (loop) resistance that would allow a full scale output current. It would be 1000 ohms max. with a 20 volt supply.

My problem is not loop resistance. My issue is that my 2 SCADA (Supervisory Control And Data Acquisition) analog input cards use 0-1mA with 1000 and 5000 ohm load resistances. 1mA will develop either 1V or 5V across each of my analog input points with a full-scale output. So in my case, I need to shunt 19mA, at either 1 or 5 volts, around my input. 1V / .019 A = 52.6+ ohms. 5V / .019 A = 263+ ohms. Now the transducer outputs 4-20mA, but my SCADA sees .2-1mA through its input resistance. Master computer scaling and offset numbers are programmed in on the other end.

Finally, if I were crazy enough to want to do it, I could probably series my 4-20mA sensor output through several different analog inputs, and get accurate readings on every one. But again, the cumulative full-scale current voltage drops plus drop across the wiring, can't exceed the available supply. This strange connection would also require isolation on all of the inputs. This has never been a problem for me since the more I talk theory, the more isolated i seem to end up.
Joe

[This message has been edited by JoeTestingEngr (edited 02-07-2007).]
Posted By: JBD Re: Why is 4-20ma used.. - 02/08/07 12:48 AM
Joe,

In the old days it was very common to run multiple devices off of a single 4-20mA circuit. I have done sensor to chart recorder to digital display to loop controller. The only typical problem is if mutiple devices have one side of their signal tied to "ground".

rad74ss,
0-20mA is very common as the analog output of new generations of VFDs and "small" PLCs.
Posted By: JoeTestingEngr Re: Why is 4-20ma used.. - 02/08/07 02:13 AM
JBD,
Yes, we could have all matter of fun finding that not-quite-perfectly-isolated input, couldn't we. I should clarify something with the 0-20mA. I would never design around a 0-20mA output UNLESS my normal, non-alarm, operating point was well above zero. For instance, I wouldn't mind using a 0-100 degree C ---> 0-20mA transducer to monitor a process that ran between 20 and 100 degrees C. Any current < 4mA would be in the alarm range anyway. My programming would be very easy because I would be dividing a real world full-scale value of 100 degrees, by my full-scale A-D count of 2047 to come up with a scaling factor. There would be no offsets to program in. With 4-20mA, you end up dividing your real world range by 80 percent of the maximum count number, and using an offset value to slide back down to a zero reference. It gets a little confusing at times though. I have one RTU mfg. where I have to offset by 2047 X scaling factor, just to get my zero, Adding 4-20 to that mix is not something I can handle before my second cup of coffee.
Joe
Posted By: Rich Thomas Re: Why is 4-20ma used.. - 02/08/07 06:14 AM
4-20mADC process variable transmitters have the advantage over other types of signal transmitters that they are "2-wire" instruments. They get their power from the same two wires used for the 4-20mADC signal. In otherwords, they do not require a separate power input. This makes them convenient.

As mentioned by others, the current analog signal is less susceptible to interference. However, the only signal available is the process variable. The newer digital type transmitters can send a lot more information, such as, the process varible say pressure, the temperature, the tag number of the transmitter, the transmitter's health, etc.
Posted By: mhulbert Re: Why is 4-20ma used.. - 02/08/07 04:51 PM
How does HART factor in to 4-20mA control? Does it run on top of the loop or is it a seperate interface?
© ECN Electrical Forums