Why 4- 20 mA?

I would not generalise the use of 4-20 mA because there might be transmitters out there that use more and some less.

Also if you quote Ohms Law you might want to think about the resistance and Voltage.

What do you make out of it?

I'd also look the what a transmitter actually does. It basically an information gathering system.

So if you think about it in terms of how much you .... need to gather that information and if you want to spend that much.... Then you have the answer.

Fill in the blank and have fun!

oh ! so its depends on how much information need to be gether and how much mA need to be used .. isn't rite ?

Nope!

still can't figure it out ��

That's like asking why traffic lights are green for go, amber for caution, and red for stop. There is no particular reason for those choices other than historically someone used them and they caught on. There is a bit more reason for 4-20mA, because it is a decent compromise among voltage and power needed to operate the system, wire size necessary to accommodate current flow, and achievable precision. From there, it caught on and became a common standard. Those numbers are fairly convenient for arithmetic, but there is no absolute reason why they couldn't be 3-30mA.

Not all "why" questions are meaningful.

ok. so no such things as "MUST " it can be variable. just follow the range tht must be used in certain instruments.

Maybe something like that, but your language is not expressing it well.

A uniform system such as 4-20mA can accommodate many ranges of input. For instance, a typical 0-100 psig pressure transducer might give an output of 4 mA for 0 psig, and 20 mA for 100 psig. Or a 0-1000 psig transducer might give 4 mA for 0 psig and 20 mA for 1000 psig. The inputs are different, but the signals transmitted are the same (and thus can use similar circuitry, wire sizes, PLC input modules, etc.) On the other end, instruments/software can restore the original psig value--but in between the protocol stays the same. On the instrument end, you need to do some calibration, but it's just y = mx+b simple algebra.

if full range is set at 20mA,what will be the current when over voltage occurs,current will exceed 20mA and will transducer get damaged.

Those numbers are fairly convenient for arithmetic, but there is no absolute reason why they couldn't be 3-30mA.

Really? A 16ma range is convenient? Have you ever tried to set up a PLC using that range of numbers? I've had experienced programmers bog down trying to get the correct engineering units entered to cover that weird range. In RS5000, dividing 32767 by 16 does not work out very nicely. Then you have a pressure instrument with a 100psi range to also divide by 16 and things start to get nasty.

One reason for the offset from zero is that low signals (e.g., <2mA) can then be flagged as out of range, as can high signals (such as >22mA).

There are other ranges that can be used, such as 1-5 VDC, 1-10 VDC, etc. Some interconversion is possible by having a resistor in the network. (For instance, 5V/20mA = 250 ohms (which is where the V=IR comes in).

Current-loop signaling (you can Google that) is relatively insensitive to electrical noise and interference, which is another reason for its use.

You may either use 4-20mA or 1~5V (its reversible) analog signal for industrial instrumentation. Analog is the simpliest way to go for a switching(on or off) and proportional communication only(0~100%).

Least complicated, robust and can not be easily affected by RFI (at minute level), but it's only limit is distance of transmission.

1) The most important reason for 4-20mA is that 2 wire loop-powered transmitters operate on ~3.6mA of current, so that separate electrical wiring is not needed.

The 2 wires running out to the transmitter carry both the power needed to operate the transmitter (3.6mA), the signal (4-20mA) and the capability of fail-safe fault signaling (3.8mA Low or 21.5mA high)

The 2 wire factor is a huge factor in process plants using hundreds (electrical power generation) or thousands (steel mills, paper mills, refineries) of field transmitters. This is what put 4-20mA on the map.

Officially, 4-20mA is a standard, ANSI/ISA SP50, (revised as ANSI/ISA-50.00.01-1975 [R2002]), but the adoption of thstandard just recognized what had become the defacto commercial standard.

2) The 4:1 ratio of signal span or signal range to the live zero range just seems to work for both pneumatics and electronics.

3-15psi signal = signal:live zero = 12psi:3psi = 4:1
5-25psi signal = 20psi:5psi = 4:1
6-30psi signal = 24psi:6psi = 4:1
10-50mA signal = 40mA:10mA = 4:1

3) As others mentioned, current is far more noise resistant than a voltage signal, and a live zero is has its uses.

A general answer is that a current loop has good noise immunity, especially if a twisted shielded cable is used. The lower limit is not zero to allow for detection of an open circuit. Using current to indicate measurements means that the resistance of the wire loop has no effect on accuracy.

https://en.wikipedia.org/wiki/Current_loop

I think that you should easily be able to answer your own questions, simply by using Google for example. Never heard of it???

I got 66,000,000 hits just by posting "current loop". I would guess that one or even two can answer all your questions.....

One of the main reasons for using current in a loop is very simple, at all points of the loop, the current is still the same, for quite long distances. Generally twin telephone cable is usually enough...

Whereas a voltage signal will generally be affected by distance and extra calibration will be needed.....and to reduce those effects over long distances, would need far thicker cables.

I quite liked this link myself:-

Current_loop

The second paragraph gives some really simple insight!:-

Given its analog nature, current loops are easier to understand and debug than more complicated digital field buses, requiring only a hand held digital multimeter in most situations. Using field buses and solving related problems usually requires much more education and understanding than required by simple current loop systems.

Is this a "Homework" question for school?

Hmm. I missed that. I thought the OP was only asking why the 4-20ma limit? NOT why a loop, or why current controlled vs voltage controlled.

Subtle, Andy. Good catch.

Just a comment on a defective current loop.

If the loop is open (like a broken wire) the current in the loop goes to zero which is an invalid state.

If the loop is shorted, the current will go to a very high value, which is another invalid state.

If the loop current is 4 - 20 ma. then the loop is good.

Bill

Two reasons. But first ... net signal = measured/generated signal + noise. 1. To make negligible the noise's contribution to net signal, minimum m/g signal value should not be zero. 2. Instruments measure variables that have square-root relationships with other variables. For example, fluid flow is very often proportional to the square root of pressure drop across an obstruction in a pipe. (For example, doubling the flow across the obstruction multiplies pressure drop by four). Useful pairs of numbers for reason 2 are: 2-4; 3-9; 4-16; and so on. After that, it becomes a case of choosing which non-zero minimum signal value to use and which pair of numbers to use for reason. "Elegant" possibilities include 1-5 (1 + (2x2)); 1-10 (1 + 3x3); 4-20; 5-30. A minimum value of 1 was deemed too low, so the people who set the standard decided on 4-20 mA. Voila! And good thing they didn't choose 4-13 mA!

why transmitter using 4- 20 mA ., isn't related with v=ir ?

What these guys are trying to say is the resistance really does not come into play with milliamperes. We are talking 4 thousandths of an amp to 20 thousandths of an amp. If you do add resistance, then it will change to volts instead of amps. Hence the IxR, so, it's not applicable.

http://www.allaboutcircuits.com/textbook/direct-current/chpt-9/current-signal-systems/

This will be a better explanation.

Excellent, hit the nail squarely on the head. If fact that was more like the Karate kid scene about driving a nail completely with one blow.

(No offense to Jackie Chan, but pat Morita will always be my 'Mr. Myagi.' besides, that remake was just Will Smith buying a 'staring role' for his kid.)

Thank you, sometimes these guys really over analyze simple questions.

I remembered that 4-20mA was a current loop system, but it's been so long since I've actually worked with one, I couldn't remember the details.

All the stuff with PLCs tend to run on 24vDC, with 0v or Open Circuit being 'logic zero' and +24v or Closed Circuit being 'logic one.'

Current loops are great for long distance reporting, since they'll maintain the current, and therefore the signal integrity, regardless of the added resistance of long wire runs. Up to the maximum voltage the current source can run at, of course.

it has two reasons,

1. If you increase the cable length then there will be more voltage drop so voltage signal strength will decrease but it's current value will be almost unchanged. So 4-20mA is independent of cable length and its resistance.

2. its easy to troubleshoot the signal error with this. At zero signal value you will get minimum 4mA but if your signal cable has broken from anywhere in field then you will get 0mA so you can distinguish that either your signal value is zero or signal wire is broken from somewhere.