0-20mA current loops and where load is? - Page 1
Forum section: electronics / beginners

author: implor
author_rank: Regular Contributor
timestamp: July 01, 2025, 06:49:25 am
content: Really noob question here. Mostly based on I have never designed a 0-20mA output stage before and I'm unsure what is "standard".I have looked at some reference designs based on a OPA + BJT transistor with current feedback resistor below emitter.(see image). This works fine as long as loop load is on high side. Due to voltage divider feedback to OPA is reference to GND this falls apart if loop load is on low side.You can make this agnostic with another OPA with differential connection over shunt resistor but I have seen many reference design with the setup above.So here is my question. Is there a convention that loop load will always be on high side (loop+, basically working as output impedance from voltage source)?Or can it be placed anywhere in the loop?

author: Jeroen3
author_rank: Super Contributor
timestamp: July 01, 2025, 07:25:01 am
content: Most devices have multiple analog outputs on a power domain, so using the low side is complicated and there could be only one common wire.Outputs are not guaranteed to always be isolated either.High side is then the most suitable way. If yours is isolated, you can use low side.https://www.ti.com/tool/TIPD102Low side may also be subject to nonlinearities in the lower regions of the amplifiers.The 61131 standard deprecates 0-20mA in favor of 4-20mA, the maximum output impedance is 600 Ohm, and it of course should be able to withstand any overload.

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2025, 07:30:20 am
content: Firstly the standard is 4-20mAHigh side or low side typically doesn't matter because these sensors are almost always just 2 pin devices, so it can be anywhere you like, it always passes current between its 2 pins and has no idea of its common mode voltage as it is floating with no reference to real ground.But what about the power input? That has ground!... Well you typically don't have one. The point of it being 4-20mA (and not 0-20mA) is that the 4mA down at the bottom can be used to power the sensor. At the same time it provides detection of broken wires if current falls below 4mA. This is why 4-20 current loop is so popular in industrial applications, it minimizes the number of wires, no need for a power supply, has fault detection built in, is not affected by long wires, immune to ground loops etc..The actual limit is the requirement of how much voltage a 4-20mA sensor needs between its two terminals in order to function correctly. Typically they work down to around 5 to 10 volts so that they can be used in a current loop with a 12V supply on the high side, but in industry supply is typically 24V so that allows for very long cables.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2025, 07:37:31 am
content: Current loops normally don't go to zero, but 4mA. This is for error detection i.e. 0mA = open circuit which is a fault.Is this loop powered?Are the power supplies isolated?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 01, 2025, 07:39:00 am
content: Quote from: implor on July 01, 2025, 06:49:25 amThere is no rule about that, can be on either side, or on both sides at the same time, doesn't matter. The 4-20mA is supposed to be floating, isolated from earth/ground, and it usually powers itself from the 4-20mA loop/wires. The video linked in this reply happens to have some classic 4-20mA examples:https://www.eevblog.com/forum/projects/precision-current-sink-i-found-a-gem-of-a-circuit/msg5925196/#msg5925196

author: implor
author_rank: Regular Contributor
timestamp: July 01, 2025, 07:56:41 am
content: Quote from: Berni on July 01, 2025, 07:30:20 amYou are correct of course for 2 pin devices. This is for an HVAC and they are generally powered by 12-24VDC/VAC. in that case it may or may not be isolated ground references as both transmitter and receiver (Building BMS) may have the same power source in the setup (and common GND).

author: implor
author_rank: Regular Contributor
timestamp: July 01, 2025, 08:09:07 am
content: Maybe I have been mixing up 2 and 4 wire 4-20mA transmitters. What I have is a 4 Wire transmitter. Looking at this TI presentation then the sensor supply voltage to they loop and not the receiver in 4-wire designs.https://www.ti.com/content/dam/videos/external-videos/de-de/7/3816841626001/6213906106001.mp4/subassets/amplifiers-current-loop-transmitters-configurations-presentation-quiz.pdfIs it common knowledge among installers that 2-wire should be power and 4-wire should not?

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2025, 09:49:50 am
content: The 4-20mA current loop is mainly designed to be used for remote sensors that inherently float and scavenge power from the current loop itself, so it is a 2 wire sensor with no power supply needed, the floating 2 wire transmitter simply pulls the appropriate current from the loop by modulating its own power consumption. This hugely simplifies things.However when you want to use current loop to communicate between powered equipment then things get tricky and you basically need to generate an local isolated floating power rail inside your equipment to run the current loop (Like it is shown in that TI document). This is the only way to make sure there are no ground loop problems. Otherwise running a long cable on the other side of a large building could easily put an extra 10V of 50Hz AC onto your ground, this can cause all sorts of problems and completely mess up the signal.Robust long distance communication via copper is hard.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 01, 2025, 09:51:11 am
content: Quote from: implor on July 01, 2025, 06:49:25 amIt is not clear for me if you are asking about analog current loop with all current values between 0 (or 4) and 20mA being possible or about digital communication 0-20mA current loop.In 90s we were using 0-20mA current loop to communicate by 2 wire between PC and our access controllers at the distance up to 1200m.AT PC side there was up to 64 current loops switch (with 4kV isolation of each loop) connected to COM port (and powered from its pins - 5mA was enough). The 20mA for current loops were supplied by controllers.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2025, 10:00:16 am
content: It's not clear what the OP is trying to do.How about using a pre-built module?It will cost more in parts, but it saves in design hours and will be tested. If you choose a galvanically isolated module, then you don't have to worry about whether or not the output ground shares the input ground or not.https://www.phoenixcontact.com/en-gb/products/signal-conditioner-mini-mcr-2-u-i4-pt-2902030https://uk.rs-online.com/web/p/signal-conditioners/4662258https://www.prelectronics.com/filearkiv/pdf-ftp/3100_series/3105/manual/arkiv/3100v100_uk.pdf

author: implor
author_rank: Regular Contributor
timestamp: July 01, 2025, 10:42:42 am
content: Quote from: PGPG on July 01, 2025, 09:51:11 amQuote from: Zero999 on July 01, 2025, 10:00:16 amOriginal question was if loop load always was in series with Loop+ on collector side. Answer to that seems to be No.After a good lunch i figured out that you could get this isolated simply with an optocupler for the pwm signal and power the rest (opa, LDO, bjt) from the loop.I was stuck in my head with the problem to need a expensive isolation DC/DC to get the current loop power. But it's not needed.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2025, 11:52:49 am
content: Yes, there's no standard.If the output shares the same positive, or negative rail as the input, then you could use an op-amp in Howland configuration to give a differential input and output.https://www.ti.com/lit/an/sboa436/sboa436.pdfU1 needs to be a single supply op-amp i.e. work when its input is near the negative rail.This circuit will work as either a current source or current sink. +out can be connected to +24V with the load connected between -out and 0V, or -out can be connected to 0V, with the load between +out and +24V. I've shown two load resistors, RL1 and RL2, for the purpose of the simulation.There obviously needs to be enough voltage headroom for the op-amp to drive the MOSFET.I've not done any transient/AC analysis. A compensation capacitor might be necessary. You'll probably get away without one, if you use a small, low power MOSFET and a slow op-amp.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 01, 2025, 12:56:00 pm
content: Remember that this circuit also needs to survive at least 1kV fast transients according to IEC 61000-6-2.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 01, 2025, 02:43:57 pm
content: There are devices that output 0-20mA as well as devices that take input of 0-20mA.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2025, 03:38:02 pm
content: To be be pedantic the circuit I posted above is for 4mA to 20mA.If you want to convert 0 to 5V to 4mA to 20mA, then a voltage reference and biasing resistors are required. See the circuit linked below for an idea of how to implement this.https://www.eevblog.com/forum/projects/2-wire-and-3-wire-4-20ma-transmitter-designs/msg5923116/#msg5923116Here's another variation of the above circuit, but using an instrumentation amplifier. It has more voltage drop, as the minimum gain of the IA is 1, rather than the 0.1 I used before. It's much more accurate than the previous circuit and has a higher input impedance, but uses a more expensive instrumentation amplifier and a dual op-amp.