All the devices I saw that provide 4-20ma output signal do so over the same wires as the ones powering the whole circuit .
How does that work?
And why doesn't this limit the total power going into the circuit?
Also wouldn't this mean that the whole setup needs to be able to work under 4ma of current?


I am trying to include 4-20ma output in my project .
I am fairly new to this and don't really understand the concept.

Any help and explanation is appreciated.
Thanks

 

hi idr,
Read through this PDF and link data for the 4mA to 20mA transmitter circuit operation.
E
https://www.analog.com/en/resources...nt-a-420ma-transmitter-with-the-max12900.html

 

The basic premise in post #1 is incorrect!! The 4 to 20 MA is an isolated analog instrument loop, it does not connect to power anything else. That is important! The same power supply can be used to power multiple sensors feeding separate inputs, but no parts of the loops should be shared. (This only applies if correct operation is required).

 

provide 4-20ma output signal do so over the same wires as the ones powering the whole circuit .
---------------------
And why doesn't this limit the total power going into the circuit?

It does limit the power.

Also wouldn't this mean that the whole setup needs to be able to work under 4ma of current?

Yes.
The voltage driving the 4-20mA loop powers the remote circuit so the maximum current consumed by the circuit can be no greater than the minimum current of 4mA.
So, for example, if the loop is powered by 12V, the circuit can use no more than 12V * 4mA = 48mW of power.

 

Long ago Burr Brown had a quite complete line of dedicated ICs for 4-20 loops. Later, IIRC, TI took charge and later, maybe AD.

Variety was impressive.

 

Usually the open circuit voltage in a 4-20 mA system is greater that 12 volts. It is the difference voltage drop that powers the electronics in the non-powered end of the loop. So often the voltage supply is 24 volts or more.

 

Hi,
After i read the replies, i got my hands on a temprature transmitter that uses HART. It had an ADC ic, an STM32 and a 50 mm lcd display.
Now my question is:
How Can all these operator on such low amp at once.

Thanks for all your answers they have been very useful.

 

Ultra low power STM32 processors consume about 80 μA/MHz @ 1.8 V.
LCD modules consume less than 1 μA.

 

Hi,
After i read the replies, i got my hands on a temprature transmitter that uses HART. It had an ADC ic, an STM32 and a 50 mm lcd display.
Now my question is:
How Can all these operator on such low amp at once.

Thanks for all your answers they have been very useful.

The short answer is that they are powered by the voltage drop. So if the supply voltage is 24 volts and half of that is used as the control system power then 12 volts x 0.004 amps is 48 milliwatts.
The fact is that 4-20 mA systems do not run on the very low voltages. Sneaky but brilliant.