# Analog signal 4-20mA

Discussion in 'General Electronics Discussion' started by Sangari, Oct 16, 2013.

1. ### Sangari

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Oct 16, 2013
Why 4-20mA is used in Analog Signals? Why not 0-20mA? or else? please explain

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
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Jan 21, 2010
Consider what happens if a wire breaks...

5
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Oct 16, 2013
No signal...

4. ### BobK

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Jan 5, 2010
And how do you distinguish no signal from 0mA?

Bob

5. ### Sangari

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Oct 16, 2013
No signal ; no current

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
So what is different between 4-20mA and 0-20mA?

7. ### Sangari

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Oct 16, 2013
Ya i got it but i want to know what are the advantages of 4-20mA analog current loop over 0-20mA analog current loop? Reason behind 4-20mA for industrial analog communication!

8. ### shrtrnd

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Jan 15, 2010
Sounds like you didn't 'get it'.
I worked industrial electronics constant current for most of my life.
We started in the 1970's with 10 - 50ma, but everybody eventually went with the 4-20ma type.
You cannot use (as *steve* & BobK tried to help you figure-out) a zero ma signal as
your zero value point for the current signal, because if the circuit failed, you would also
read zero mA, and you would not know that the circuit failed, you would just think that
your circuit was alright, and you were actually reading a 0ma signal.
The circuits you're wondering about, use the 4ma value as the signal 'zero', because
you know the circuit is working when you read 4ma. If the circuit ever does actually
read 0mA, you KNOW the circuit is not working.
Does that explain it?

9. ### duke37

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Jan 9, 2011
Also, I thought the 4mA was used to supply the transducer with power.

10. ### eman12

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Aug 24, 2013

I myself think that zero "mA" is nonsense? So it can be the one of the main easons why they do not use zero mA for this job!

11. ### gorgon

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Jun 6, 2011
The reason for the 4-20mA system is the supply of the transducer. It is normal that they also need an voltage overhead of around 10V to function properly.

0-20mA is normally used when the transducer has it's own Power supply.

Many transducers use either <4mA(2-3) or >20mA (21-25) as a fault indication in addition to the normal range.

12. ### Sangari

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Oct 16, 2013
Yes clear explanation!

Moreover i got the following reply from one of my colleague that "most of the TTL circuits operate on voltage 5V or lesser than this , beyond which they need to handled with power transistor which is not economic as well handling heat dissipation

What value add do you point in increasing the loop resistance, in general practice, we try to keep the loop resistance as low as possible to reduce voltage drop and signal attenuations

13. ### gorgon

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Jun 6, 2011
Why you mix TTL into this picture, I don't know. (0)4-20mA is an analog signal in nature.

The max value of the loop resistance is purely dependent on the overall voltage overhead in your current loop. For a 4-20mA loop the tranducer will normally need around 10V of the loop voltage to function. If your loop is supplied with 24V you'll have around 14V, minus a safety margin, left to use in the loop resistance. This resistance will be a combination of wire resistance and measurement resistance. If your max current is 20mA, the max loop resistance will be less than 700 ohm. In the real world you'll always need a safety margin, so the practical value should be less than that, maybe around 500 ohm. If the max current is 25mA, for error indication, the max loop resistance will be even lower. The measurement resistance is the sum of all inputs connected to the loop. There is no attenuation of the signal, as long as you keep a free overhead voltage available for the driver.