How to wire and read 4-20mA transducers - Page 1
Forum section: electronics / beginners

author: igendel
author_rank: Frequent Contributor
timestamp: April 24, 2025, 11:19:58 am
content: Hi all,Intro: It's my first time dealing with 4-20mA "Current loop" sensors, there seem to be more variations than I assumed in terms of wiring schemes and naming, and so far I couldn't find clear information on the web - I'll appreciate any help in wrapping my head around this. Ultimately, the sensors will be read by some MCU, probably through shunt resistors, opamps and ADC ICs as required.I've seen three types of wiring:- Two wires: this makes the most sense to me; the transducer acts as a variable resistive load, I provide it with power and measure the voltage drop on a shunt resistor in series. Right? A certain model has the wires marked "A (E+)" and "B (S+)"... which one connects to positive and which to GND?- Three wires: for example, on a Sentec PM4233 vacuum transducer, some of the models have "V+", "S+" and "GND" wires. So is "S+" the current driven by the transducer? Can I put a shunt resistor between that and "GND" to measure the current?- Four wires: For instance, in a "4-20mA Signal Generator" from Aliexpress... this module has two wires for external power supply, and two for the current loop. So how do I measure this current safely?

author: tszaboo
author_rank: Super Contributor
timestamp: April 24, 2025, 11:50:40 am
content: Did you get the installation manual for these two sensors? Or just the datasheet?

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 24, 2025, 12:54:08 pm
content: Quote from: igendel on April 24, 2025, 11:19:58 am

author: igendel
author_rank: Frequent Contributor
timestamp: April 24, 2025, 02:34:46 pm
content: Quote from: tszaboo on April 24, 2025, 11:50:40 amOnly datasheets (which are not too informative either). But these are just examples, at this point I have no idea if I'll be asked to use these models or others.Quote from: WattsThat on April 24, 2025, 12:54:08 pmOh, ok.Quote from: WattsThat on April 24, 2025, 12:54:08 pmCoolQuote from: tszaboo on April 24, 2025, 11:50:40 amI see, so as a more general question: I have wire through which there's a current of 4-20mA, but I know nothing about its voltages or ground reference. Is there asimpleway to read this current using my own circuit? The cheap/common Hall effect current sensors I've seen are used for higher currents; an LED brightness response (e.g. in an optocoupler) is not linear*; differential opamps still need some common voltage reference if I understood correctly...*Edit: well, the brightness response is pretty linear... reading it accurately is more difficult

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 24, 2025, 02:40:50 pm
content: First the output isn't like a resistor but rather a current source that is the ouput current isn't affected by the load resistance as long as the load resistance is less than the maximum. For example the Sentech sensor would deliver the same current if the vacuum level is the same regardless of the load resistance or supply voltage as long as they are within the limit. Also if you use a shunt resistor make sure the resistor value is equal or less than that the sensor can accept (often 250 Ohms max).for the Sentec the maximum load resistance depends on the supply voltage. If the supply voltage is 12V then it's 600 Ohms maximum.

author: igendel
author_rank: Frequent Contributor
timestamp: April 24, 2025, 03:01:52 pm
content: Quote from: BeBuLamar on April 24, 2025, 02:40:50 pmYes, but if my own circuit is supplying the power - hence, the current - for the whole loop, then isn't the transducer itself considered the "load"?I know these sensors are not really variable resistors, that was just my simplistic mental model of them for the purpose of reading the data.Quote from: BeBuLamar on April 24, 2025, 02:40:50 pmYes, I did not mention this point but it's worth emphasizing for other beginners who might be reading this. The electronics in my circuit will get 5V, so an absolute maximum of 250R (voltage drop at 20mA == 5V) makes sense.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 24, 2025, 04:52:24 pm
content: "Yes, but if my own circuit is supplying the power - hence, the current - for the whole loop, then isn't the transducer itself considered the "load"?I know these sensors are not really variable resistors, that was just my simplistic mental model of them for the purpose of reading the data."For example if you apply a vacuum of -7PSI and the transducer is supposed to output 12mA then it would output 12mA regardless if you power it with 12V or 36V and regardless whether you terminate it with a 0 Ohms or 600 Ohms shunt resistor. If your system is 5V you must have another power supply to supply 12V to the Sentech sensor. For other sensors I don't know what is the requirement but most would require at least 10V. Most of them are powered by 24VDC because that's a commonly used voltage in the industry.

author: igendel
author_rank: Frequent Contributor
timestamp: April 24, 2025, 05:45:30 pm
content: Quote from: BeBuLamar on April 24, 2025, 04:52:24 pmOf course - I just include the pressure as part of the system ("Gas pressure based variable resistor")Quote from: BeBuLamar on April 24, 2025, 04:52:24 pmIndeed. I'll supply what voltage is required for the sensors per their specs, but the electronics (MCU and other "delicate" components) are 5V, they'll have their supply/LDO/step-down/buck/whatever.

author: TimFox
author_rank: Super Contributor
timestamp: April 24, 2025, 06:55:54 pm
content: Quote from: igendel on April 24, 2025, 05:45:30 pmA "variable resistor" should obey Ohm's Law (to be a resistor), and the loop current would depend on the excitation voltage and load resistance in the normal way.The sensor model should be a controlled current source, vaguely analogous to a JFET or other active device with a high output impedance.This model is very useful, especially when dealing with poorly regulated power supplies, long cables, or variable loads.

author: langwadt
author_rank: Super Contributor
timestamp: April 24, 2025, 07:24:20 pm
content: Quote from: igendel on April 24, 2025, 05:45:30 pmand don't forget to use a large sense resistor or some form of current limiting for when the sensor wires invariably gets shorted and dumps the full supply on you sense resistor

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 24, 2025, 09:27:53 pm
content: Quote from: langwadt on April 24, 2025, 07:24:20 pmYou can not use a sense resistor larger than the maximum in the specifications. For the Sentech it's 600 Ohms with a 12V supply. You can use very small sense resistor almost like a short circuit and it would work just fine. For example you can simply put an amp meter at the output of the sensor and it would work perfectly. There is no problem at all when the output of the sensor is shorted. The sensor will still put out exactly the current that is corresponding to the pressure.

author: TimFox
author_rank: Super Contributor
timestamp: April 24, 2025, 09:37:45 pm
content: Traditionally, 4 to 20 mA sensors were wired to d'Arsonval moving-needle milliameters with appropriate scales and very low resistance.

author: Smokey
author_rank: Super Contributor
timestamp: April 25, 2025, 02:47:12 am
content: Lots of app notes (page 28):https://www.ti.com/lit/ds/symlink/ina826.pdf

author: langwadt
author_rank: Super Contributor
timestamp: April 25, 2025, 07:57:54 am
content: Quote from: BeBuLamar on April 24, 2025, 09:27:53 pmby large I mean wattage, if you have say a 200R sense resistor for slightly less than 5V fullscale, shorted sensor wires with a 24V supply will dump almost 3W in the sense resistor

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 25, 2025, 09:35:18 am
content: Quote from: langwadt on April 25, 2025, 07:57:54 amI never see system that use large wattage shunt resistor for a 4-20mA sensor. I do use them a lot. Never had the problem.

author: igendel
author_rank: Frequent Contributor
timestamp: April 25, 2025, 08:17:29 pm
content: Quote from: langwadt on April 24, 2025, 07:24:20 pmYes, I was looking at resettable fuses (e.g. MICROSMD005F-2) along the line to protect the power supply itself, plus a Zener diode to limit the voltage my components "see" from the voltage drop on the shunt resistor. But of course I'm open to any other robust solutions.