[SOLVED] Why 4 to 20 mA Current Conversion

[vijay s]

i came across some RPM , Temperature and flow meter.. which outputs value in 4 t0 20 mA current range... is there any standard to transmit output in this range and why it is transmitted as current instead of voltage or frequency

 

[IanP]

One can only list the advantages of the 4-20mA current loop, so let’s forget for a while about it’s small drawbacks and name some of them (the advantages, that is):

-Data can be transmitted over considerable distances ..

-Signal voltage at any load is (Isignal * Rload), which is independent of supply voltage variations and line resistance ..

-Random induced loop noise voltage at any load is reduced by the factor: (Rload)/( Sum all Rloads + Rline +Rsignal) ..

-Supply voltage variations are reduced at any load by the same factor as shown above ..

-Multiple loads can be series connected in a transmitter loop, providing considerable control and display opportunities ..



Current loops (incl. 4-20mA) are well documented on the web ..
Follow these links and you'll find answers to all your questions:

https://www.murata-ps.com/data/meters/dms-an20.pdf

**broken link removed**

...
Rgds,
IanP

 

[FvM]

They are standarized according to IEC 381-1 (current) and IEC 381-2 (voltage).

 

[ali_aku_suka_telo]

'why it is transmitted as current instead of voltage' Because if we use Voltage, it will corupted by parasitic resistance in wire that we use; specially if the transmission distance is long
But if we use current; we must understand that current transmission is bounded by voltage suply of the op-amp that become the current source, so it still restriched in long of distance
Final summary : the best (for me) if we use fibre optic

 

[raiya_23]

The 4-20 ma range is a "standard" adopted by tradition and also ANSI 50.1, and most process instruments and controllers use it.

It does have several advantages:

The 4 ma "bottom of span" signal allows the receiver to detect a broken wire or failed instrument.

Since the loop current never falls to zero, the instrument can be loop powered, allowing for two-wire devices.

20 ma is low enough to be made intrinsically safe, making it easier to apply in hazardous locations.

The constant-current feature of a current loop cancels out voltage drop errors due to long wiring runs (of course this would also be true if you selected different current values for zero and span).

The 4-20 ma signal dropped across a 250 or 500 ohm resistor creates a very convenient 1-5V and 2-10V, respectively, also fairly standard ranges in the industry.

 

[vijay s]

Folks thanks for your reply..

can we use resistors to convert current to voltage or we have to go for IC's....

 

[FvM]

The question has just been answered in the previous post...
Generally, you can refer to ohms law.

 

[vijay s]

@FvM
what i meant is, resistor will have some tolerance. so small change in resistor produce large change in its drop volatge.. in such case what type of resistor we can go for..
or
is there any IC which produce perfect result

 

[FvM]

small change in resistor produce large change in its drop volatge

Why? It's supposed to be proportional, you really may want to study ohms law. Also current measurement in an
IC is involving a sense resistor...

Basically, every step in a measurement chain is introducing errors, from sensor til digitizing. You can get cheap, limited
accuracy or expensive, high performance devices for each of them. Current sensing in a 4..20 mA loop isn't a particular
critical job among others.

 

[vijay s]

Thanks FvM..

I'm not meaning it is critical,, i'm looking for accurate measurement....

what i meant with large change in drop voltage is

if R = 250 when I = 4mA
V = 1V

if the resistance is not perfect and if it is R = 255
then V=1.020V...

This is proportional... but i'll be measuring in a sense resistor is 250E.... and end-up with uncorrect result

anyhow i'll be using resistor with good tolerance

thanks for ur reply...

 

[karesz]

Thanks FvM..
...if the resistance is not perfect and if it is R = 255, then V=1.020V...
.. but i'll be measuring in a sense resistor is 250E.... and end-up with uncorrect result... ...anyhow i'll be using resistor with good tolerance...

Sorry Vijay s,
if your 250 Ohm is a 255 Ohm resistor, then means only that its a 2% typ, and not that "it is not perfect"!
If you wish to apply 0.5% types, and if their are not "defect"; off course your real resistor value will be never more as 251.2 Ohm...
Also the problem is only theoretical.
Than; if "Current-loop" was as standard defined_ the most "precise resistors" are not better as 1 or 2%... Tu and this is originaly a standard/system for industrial apps & not for research data aquisitions.
regards
K.

 

[frankliner]

This depends how accurate you need to be. Can you loose 20% of the ADC range and meet the requirements. Generally you must also remove the 0-4mA from the output. The best IC I know is RCV420 from TI(former burr Brown).

 

[vijay s]

HI I'm again here...

i got a Spec, where Temperature Sensor Module Outputs Current in 4-20mA.. but No details on its temperature range... i want to convert this current in to Temperature... Any comment on this

 

[raiya_23]

if there are no details abt the temp range then how can you assome 4ma stands for what? otherwise you could have easily used an ADC