ECN Forum Forums General Topics of the Trade General Discussion Area Why is 4-20ma used..

Why is the range of 4 -20ma used for signaling in PLC applications

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

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.

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.

oops, duplicate post.

[This message has been edited by JBD (edited 02-07-2007).]

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).]

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.

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

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.

This is an interesting thread.

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.