1-5VDC to 4-20mA Converter? (PLC stuff) - Page 1
Forum section: electronics / beginners

author: robobeamer
author_rank: Newbie
timestamp: May 17, 2016, 11:12:54 pm
content: Hello, I work in and am new to the industrial control world. One of my coworkers pointed out to me that while converting a 4-20mA signal to a 1-5VDC signal is fairly easy, we typically don't convert voltage signals to current. He challenged me to figure out why. Hoping to get some help here.

author: uncle_bob
author_rank: Unknown
timestamp: May 18, 2016, 01:15:18 am
content: HiOk, first, what has to be isolated from what?I would *assume* that both the ground reference and the voltage signal need to be fully isolated from the current loop.Is this correct in your case?Bob

author: robobeamer
author_rank: Newbie
timestamp: May 18, 2016, 01:44:09 am
content: No. Interestingly, even the return of all DC buses are connected to earth on everything I work on. I come from the electronics industry, and I'll tell you what, the industrial control field is a very strange beast. Everything that can be grounded is grounded.

author: jeroen79
author_rank: Frequent Contributor
timestamp: May 18, 2016, 02:14:59 am
content: Quote from: robobeamer on May 18, 2016, 01:44:09 amJust to rule out any surprises?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2016, 09:44:32 am
content: Quote from: jeroen79 on May 18, 2016, 02:14:59 amThere are numerous reasons for earthing the DC PSU, such as EMC and to ensure it doesn't float at a high voltage, with respect to earth, due to a fault.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: May 18, 2016, 09:57:06 am
content: Quote from: robobeamer on May 17, 2016, 11:12:54 pmtechnically there is no reason why you couldn't. The unique reason i can think of is voltage drop across long wires in industrial environnement that will effect measurement precision. but to locally convert 1-5VDC to 4-20mA signal that will be transmitted somewhere else there is no reason why you can't ! 4-20mA sensors do that !! wth he's talking about.

author: Ammar
author_rank: Regular Contributor
timestamp: May 18, 2016, 10:11:36 am
content: Could be that an analog to digital converter deals with voltage? Weird question. Where there is smoke, there is fire. Hence where there is voltage, there is current. The two sort of go hand in hand. Not sure what the person is talking about. Good luck with it!

author: Jeroen3
author_rank: Super Contributor
timestamp: May 18, 2016, 10:19:58 am
content: You can chain 4-20 mA receivers.The receiving end is often isolated.

author: retrolefty
author_rank: Super Contributor
timestamp: May 18, 2016, 10:57:30 am
content: I suspect your co-worker was trying to convey one of the main advantages of loop powered 4-20 ma control loops. That is that no matter how long (up to a 1,000 ft or more) the wires are in a specific loop are there is no requirement to 'calibrate' for the length of the loop wires, as the signal is a constant current proportional to the measurement value and will measure the same value no matter where in the loop you make the measurement.Attempting to measure a 1-5 vdc value on loops of long lengths would be more difficult and less accurate in real world environment.A second advantage of current loops is that the field end can utilize some of the loop current (< 4ma) to self power the field device (transmitter).

author: nowlan
author_rank: Frequent Contributor
timestamp: May 18, 2016, 12:31:07 pm
content: "we typically don't convert voltage signals to current"who is we? you being an electronics guy, or the industrial control tech? I was under the impression everything is either pwm or current loop in PLC land. The advantage is that current loop doesnt pick up noise, well probably does, but the transceiver adjusts.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 18, 2016, 02:03:50 pm
content: To answer your original question.Converting 4-20mA to 1-5Vdc requires a resistor.Converting 1-5V to 4-20mA requires a more sophisticated circuit.

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 18, 2016, 04:12:51 pm
content: Voltage in/Current loop out functionality is available in a single chip. An old Burr-Brown part now sold by TI:http://www.ti.com/product/XTR115

author: uncle_bob
author_rank: Unknown
timestamp: May 19, 2016, 12:10:46 am
content: Quote from: robobeamer on May 18, 2016, 01:44:09 amHiOk, so if your current loop is grounded to the same ground as your voltage signal the conversion process is quite easy. In 99.99% of all current loop setups, that's not the case (the loop floats).Bob