Measure 16 different 4-20mA - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 11, 2016, 04:03:29 pm
content: 1. task is to measure 16 different 4-20mA transmitter outputs without shorting each other ground reference.Receiver side controller is microcontroller is powered by 3.3V.2. I have attached a pdf, having two different circuits. First page shows a circuit in which transmitter has been individually enabled by CD4067(16:1 mux demux). 150 ohm resistor cmake 150x20mA = 3V. First both mux are inhibited then combination is formed & output is taken. Its equivalent diagram is shown in rectangular box. CD4067 has 125 Rdson & 25mA max current capability. Problem in this circuit that on return path the ground is different, a additional 125ohm is added between two. Although much current will not flow from it, but it would create offset.3. Second page, uses only one 150 ohm resistor. Its equivalent is also shown. Although groung problem is corrected in it but it allow entre 20mA to flow from CD4067, & it has max specified limt as 25mA. I dont know how safe it would be here?4. Is there any better method to do where there is isolation also?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2016, 07:19:13 pm
content: The CMOS mux has a limited voltage range - the voltages must be within the supply of the chip. So no more that than about 10-16 V betweeen the inputs. So the inputs are not connected, but can not float very far.Normally no current should flow over the MUX - otherwise the measured voltage will be wrong. If there is some possibel current to ground flowing one should have a thrid MUX für the GND connection - having one channel at a time connected to GND.So it may work. if the 20 mA inputs are really separated, but it may fail both with wrong results or damages chips (if large currents flow at to much voltage for the MUX).Isolation with DC is dfficult. One may end up with 16 completely separate channels and optocoupler to transfer the digitized signal. It really depends on how good the isolation needs to be.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 11, 2016, 07:50:23 pm
content: The most space efficient way would be to use linear optocouplers. If you can find those for 4-20mA range.I often use AMC1200 digital isolators. But you'd either need to make a isolated power supply, or loop powered circuit to make it work.Because of the need for measuring 16 channels (PLC?) there will be no 10$ budget to do it, I guess?Not interfering with the ground reference on the transmitters requires real isolation, as with opto's, digital isolators or relays. Not a semiconductor mux.There are also pre-made DIN-rail mount signal isolators from phoenix's MCR family. (expensive)

author: Rerouter
author_rank: Super Contributor
timestamp: January 11, 2016, 07:51:50 pm
content: For isolated or atleast semi isolated inputs you have 3 main choices that spring to mind,1: use an optocoupler with a resistor inline and a zener in parallel with both parts, so that if excess current flows it will be shunted by the zener,this method may require a more involved calibration, and over time the parts will drift, so if you go this way try and get from the same batch, and use an extra one as a reference to compensate over time (measuring input vs output ratio)2: Use an isolated op amp, for each input, preferably one with a built in power supply so that you do not need 16 isolated regulators $$$, this will be the most flexible approach,3: Use op amps whos inputs can greatly exceed the supply voltage, they exist, there just harder to find, e.g. an older project used a 15V supply, and its common input range was +-110V or something along those lines, it wont be truely isolated, but for the normal 48V max for these loops as long as the supplies are referenced it shouldn't matter,

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2016, 07:55:43 pm
content: Use 16 resistors and the CMOS mux to select the voltage to measure.

author: Marco
author_rank: Super Contributor
timestamp: January 11, 2016, 10:42:14 pm
content: Quote from: Vindhyachal.takniki on January 11, 2016, 04:03:29 pmWhy exactly? 4-20ma sensors/transmitters are supposed to be ungrounded at their end aren't they? If there is a ground loop it can still screw you over (you're still grounding the currently connected loop at the receiver end in your schematic) not as badly as with all 16 connected at the same time, but still.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 11, 2016, 11:00:45 pm
content: How/where are the current loops powered? Critical parts of the circuit seem to be missing from your diagrams.If each loop had a 250 ohm resistor from the "bottom" of the loop to ground, then there would be 1-5 volts across the resistor correlating to the 4-20mA loop current. Of course, you could scale the resistor for higher or lower desired voltage.Then you can use simple multiplex methods of selecting each loop to sample the voltage across the resistor.i.e. Figure 7 in this document....https://www.acromag.com/sites/default/files/Acromag_Intro_TwoWire_Transmitters_4_20mA_Current_Loop_904A.pdf

author: SL4P
author_rank: Super Contributor
timestamp: January 12, 2016, 07:46:55 am
content: This link from NI has some good insight into current loop implementationhttp://www.ni.com/white-paper/6940/en/