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4-20 mA current loop regulation

199 ratings | 128392 views
This video demonstrates how the current output by a 2-wire transmitter is a function of the process measurement and not electrical parameters such as supply voltage or loop resistance.
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Text Comments (46)
Shamekh Power (2 months ago)
What happened if we connect an ammeter upstream from the transmitter? What would be the current? I am thinking it will be 20 ma as long as we supply the appropriate voltage (45-13.5 vdc in some models. Can you please let me know what would be the current upstream from the transmitter?
S C (3 months ago)
Is that a 250 ohm resister in series with common lead?
Shamekh Power (3 months ago)
Thank you for sharing your knowledge.
Horace Greeley (1 year ago)
chegotti (2 years ago)
what would happen with the current if you take a higher resistance? will the transmitter compensate this to hold the 12mA? please answer. thanks.
podotresno (9 months ago)
every transmitter have a maximum resistance loop allowed, if it's in spec nothing happens, if the value higher the current may will drops
Christian Marquez (2 years ago)
Thank you so much for your videos. I'm taking classes for Instrumentation Tech and it is helpful to hear more explanations from different people. You are good at explaining things.
Good day all for a maintenance purpose what would be a good 4-20 ma tester to use, could you supply me make [druk,fluke] model number , i need to purchase one. Thank you Johann
Humberto Cavazos (2 years ago)
sunil k (3 years ago)
which model of Fluke meter are you using
Altash Khan (3 years ago)
I can't hear ur voice
KissMy HonkyDonkey (2 years ago)
+Altash Khan I can not hear him either. What a dildo!
Merlis Soto (3 years ago)
Very interesting, very instructive, very easy to understand. Muchísimas gracias BTCInstrumentation.
Michael Brady (4 years ago)
Very helpful, thank you.
David Ramirez (5 years ago)
If you don't have a plug-in power supply or are in the field, three 9 volt batterries plugged into eachother in series will supply enough voltage to power your transmitter. Very good video. I have been doing this three years now and I always appreciate when someone passes on the knowledge.
PimiPetty (5 years ago)
Hello BTCInstrumentation! I am a very new on this and I would like to ask your help. I am testing one noise sensor, according to this and what your book says, it is 4 wire: It has 2 wire for +12V source and other 2 wire for the current loop. But I have one big question, do I have to supply (another) voltaje to the loop to read the output current? I connected the sensor to +12V but read nothing at the output. I need to confirm the sensor is good, or bad. I hope you can help me. Thank you!
tsmith35 (5 years ago)
It's at this location now: ibiblio (dot) org (slash) kuphaldt (slash) socratic (slash) sinst (slash) book (slash) liii (dot) pdf
Ricardo Pacheco (5 years ago)
BTCInstrumentation (5 years ago)
The resistor serves no practical purpose in this demonstration circuit. However, most 4-20 mA loop circuits contain a precision resistor to generate a measurable voltage drop based on that 4-20 mA current. For example, many loop controllers require a 1-5 volt DC signal, which you can get by passing the 4-20 mA current through a 250 ohm resistor.
triskelion taugammaphi (5 years ago)
what is the use of that resistor???pls answer.tnx
mukesh kumar (5 years ago)
good explanation
BTCInstrumentation (5 years ago)
Current is always guaranteed to be the same value at all points in a series circuit, whereas a voltage signal may degrade over long distances of wire due to voltage drop. We use a "live zero" like 4-20 mA instead of 0-20 mA because the transmitter is actually powered by the current signal. If it went all the way down to 0 mA, the transmitter would stop working. The "live" value of 4 mA ensures the transmitter always gets power.
adoinstru (5 years ago)
this video is very helpful as i was always scared of these transmitter thing. I am studying these things and have some questions. Why do we transmit currents? Can viltage not be transmitted? And why do we use 4-20 ma , instead cant we use 0-20 or 2-20 range?? Pls answer...i am very new and i want to learn more from you..
oliver jessel (6 years ago)
hello sir, your video is so helpful. i already work for this 4-20ma current loops but still need to understand more. i really need that power supply that gives 4-20 ma if i want to test some instruments. do you have any diagram for that power supply? tnx GOD BLESS...
neil_dufc (7 years ago)
Hi, I plan to connect my a/d chip to a 4-20 ma sensor (a Honewell sensepoint). As my a/d is 0-5v input, I assume I would also connect the 0v from the power supply to my 0v on my board, as it has its own power supply. Would this be correct? Thanks Scott
Jack Nay (7 years ago)
Thanks for your upload and book.
rpaul247 (7 years ago)
Great stuff, your videos are the best on here.. thanks alot.
Godwin Zh (7 years ago)
@BTCInstrumentation Thanks a lot for your answer. And thank you for your book too. It 's a wonderful book for me to learn industry instrumentation. Do you offer a online course for this?
BTCInstrumentation (7 years ago)
@sfacp If the sensor is 4-20 mA DC, then you may safely assume it's got either a current regulator in it (2-wire) or a true current source in it (4-wire). Read the chapter in my book on analog instrumentation signals: openbookproject (dot) net (slash) books (slash) socratic (slash) sinst (slash) book (slash) liii.pdf
Godwin Zh (7 years ago)
Great video! I am not new to electronics but it is my first time to work with 4-20mA current loop transducers and don't know much about the industry instrumentation standards. Your video helps a lot! Now my question is, may I assume that all transducers with 4-20mA current output have constant current source built-in?
BTCInstrumentation (8 years ago)
@Mart34 Most loop-powered transmitters have minimum and maximum terminal voltages ranging around 12 to 40 VDC. Specifications vary, so be sure to check the manufacturer documentation (datasheets, user's manuals) to get a definitive answer for your transmitter. Maximum loop resistance is a function of minimum terminal voltage. Loop resistance drops voltage, taking away from the power supply voltage, leaving the transmitter whatever remains. R_max = (V_supply - V_min) / 20 mA
Mart34 (8 years ago)
Excellant video. Whats the maximum loop resistance and less importantly, whats the max voltage you can apply ? cheers, mart
BTCInstrumentation (8 years ago)
@nugent5438 Sure thing: check out my free textbook online, with a whole chapter on analog instrumentation signals. Pardon the weird URL, as YouTUbe does not allow me to post it verbatim: openbookproject (dot) net (slash) books (slash) socratic (slash) sinst (slash) book (slash) liii.pdf
BigNuge (8 years ago)
Hello, I am working on developing a simple training curriculum regarding 4-20ma current loops. I work in the industrial burner/boiler industry. Our industry has just about completely converted to 100% electronic controls and the use of 4-20ma current loops is very wide-spread. I am trying to find a nice simple, easy to view graphic of a 2-wire current loop circuit to demonstrate with. Would you happen to know a good example of one on the net or perhaps have one yourself? Any help would be great
Camarillo (8 years ago)
@BTCInstrumentation Hi. Thanks for the information. About the sensor: we tested it and had 4.65mA at 0 PSI (indicator). We'll work on an ecuation for a microcontroller. I'll post information on it. 340 psiA is a lot!!!!!
BTCInstrumentation (8 years ago)
@mcr1981 It's sometimes helpful to imagine the 4-20 mA transmitter as a continuously-adjusting variable resistance, increasing or decreasing as needed to hold the circuit current wherever it should be. Thus, if some other resistance (load) in the circuit changes value, the transmitter senses the resulting change in current and immediately compensates by changing its resistance in complementary fashion to bring the current back to where it belongs.
Camarillo (8 years ago)
@BTCInstrumentation Thanks for your help. I'll check the sensor tomorrow at my friend's shop. If all goes well, I'll try to make a video of it. I said I'm new to electronics, that's wrong, I'm new to this sensor type electronics. Will look into the constant current thing in the sensor (change load but same current?). You got me there.
BTCInstrumentation (8 years ago)
Wiring detail #22 on page AP-10 says the white wire is nothing but a case ground, so it's really a two-wire transmitter: Red (+) and Black (-). Therefore it should work just like the transmitter shown in the video. Note that the "FMA" is an *absolute* pressure transmitter, so it will probably give an output greater than 4 mA at atmospheric pressure (i.e. it should output 4 mA at a perfect vacuum).
BTCInstrumentation (8 years ago)
@mcr1981 Google-search for "Sensotec 811 FMA" and go for the third hit. It's a catalog showing all the Sensotec products. They give wiring details on pages AP-8 through AP-11. The schematics show up on my screen pretty grainy, but the text spells out which wires do which functions. Hope this helps!
Camarillo (8 years ago)
@BTCInstrumentation Sorry, this is the last part of the explanation (500 characters is not enough). The sensor is a Sensotec 811 FMA pressure sensor. The data on the sensor states that it is a 4-20mA sensor output but it says it's two wire insted of three..... that is what I don't understand. I looked for a solution "how to connect three wire sensor" on the net but no luck. Until I landed here (great video by the way). Thanks for your help. If you can help me I'm be very grateful.
BTCInstrumentation (8 years ago)
@mcr1981 The output signal may very well be voltage, not current. Anyway, the only sure answer is to research the documenation for this instrument and see what the manufacturer tells you. Most instrument manufacturers provide downloadable user's manuals for their products online. Go forth and Google!
Camarillo (8 years ago)
@BTCInstrumentation Hi, thanks for the reply. I got a pressure sensor on a "garage sale" (company went bankrupt) from a friend. I'm starting with electronics and went to the machine shop of another friend to test the pressure. I connected the sensor to a power supply (15-50v) and I tried to measure the output (4-20mA). Three wires: red, black and white. Positive power supply terminal to red, negative terminal to black, and the white I used as the current terminal (output). I had no luck.
BTCInstrumentation (8 years ago)
@mcr1981 Typically three-wire transmitters take in DC power on two wires (typically red and black) and output an analog voltage signal between the third wire and DC ground (white and black). Can't be sure though until you consult the manufacturer's documentation. Every transmitter I've ever worked with has some sort of documentation in the manual showing how to hook it up.
Camarillo (8 years ago)
How do you connect a three wire transmitter? Black, Red and White
sixto espinoza (8 years ago)
exelente aporte, aun que esta explicado en ingles algo se logra entender saludos BTC instrumentacion
rayram55 (8 years ago)
You did a great job explaining the pressure transducer. I like your video lessons on you tube. I allways look for more knowledge on your you tube lessons. Thank You.

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