Making a Tiny 4-20mA Current Loop Tester 

S Banergee, maximum wire length in a 4-20mA loop is mostly a function of wire resistance, and larger diameter (gauge) wire will have less resistance. For long wire distances, you need to calculate the total wire resistance and then use Ohm's law to calculate the voltage drop in the wire, and subtract that from the available loop power supply voltage, and then see if there is enough voltage remaining to meet the minimum voltage specification of the transmitter (after first subtracting the maximum voltage drop of the receiver or receivers). As I said in the video, cable shielding is not very useful in attenuating magnetically coupled noise, but in most 4-20mA loop environments, magnetic sources tend to be low frequency (50 or 60 Hz) and all loop receivers are designed to reject such frequencies (and higher ones), leaving only the very slow changing process variable signals. Cable shielding can be useful if there is a lot of high frequency electrostatic noise, but then again most receivers are designed to filter that out. Nevertheless, it is common pactice to use shielded cables for 4-20mA loop wiring that runs for long distances; just make sure to leave the shield unterminated at the transmitter end, and terminate the shield to ground near the receiver end.

@@youtuuba Thanks a ton for ur detailed reply! This means distance is really not a prob as long as the supply voltage condition is met and LPF is there. I have seen max distance spec of 5000 ft for a Mccrometer 4-20mA transmitter (available in Google), and even 10000ft is mentioned in Wikipedia on this topic. However wanted to listen from you as you have worked on this for decades. Your video will help me putting up an idea relating to this. So thanks again. I have kind of similar hobby of playing with circuits, so sharing a video of mine.

ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Bss2zhME33U.html

I can't tell what you are talking about, or specifically asking. If you set up your single current loop, or multiple current loops, correctly, then there should be no problem with ground loops. One caution is that, due to some industry standards that require all low voltage wiring to be in shielded cables, sometimes a ground loop will exist between multiple panels, if the shield on both ends of a cable running between panels is terminated to ground at both ends. Correct practice is to ground a cable shield at only one end, and leave the other end of the shield either unconnected, or connected to a 'tie point' termination that itself is not connected to ground or anything else. This will solve that particular ground loop issue. You can in fact have multiple power supplies in the same current loop, but they must be in series and preferably right next to each other and not at opposite points on the loop. You would do this only if you found that due to many receivers/loads in the loop, that the existing loop power supply would have insufficient voltage to compensate for all those voltage drops in the loop, and then you could add a 'booster' power supply. For example, if you had a loop with a 24VDC power supply and three receivers/loads, each with a 250 Ohm shunt resistor inside, that would drop 15 volts right there in the receivers, leaving only 9 volts to power the transmitter. If you then needed to add another receiver/load in the loop. there would be 20 volts dropped by loads and the transmitter probably will not work with only 4 volts for itself (and some of THAT might be dropped in the wiring). In this case, you might add a 6V or 9V or 12V power supply in series with the original 24V supply, so the extra load's voltage drop does not swamp the system. I wonder if you are having issues with putting a 4-wire transmitter into a loop that also has its own power supply. That could be a problem. Or, you might be trying to connect together two different loops. Those situations can he handled by adding a 'current loop isolator' to one of the loops; it bridges between the two loops, essentially making a copy of one loop current and then acting as the transmitter in the second loop. This is too much to try and cover here.

I have made PCB circuit design kindly have a look and give your opinion is it looks ok to go for production?

drive.google.com/file/d/13a9p-g2PTveN9CRWJZ4hEIZbT3_0MQV7/view?usp=drivesdk