Analog signal 4-20mA

And how do you distinguish no signal from 0mA?

Bob

 

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?

 

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

 

Good explanation about the reason.

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!

 

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.

 

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.