How does 2-wire instrument work?

[aljamri]

Hi,
I've been long time dealing with Instrumentation issues, one of many questions always in my mind is regarding the power and signal share/divide in instruments.
I’ve encountered with two types,
4-wire-instrument where power and signal connections are separated, and
2-wire instrument where power and signal shares the same conductors, and some times called Loop Powered.
While the constructions of the first type (4wire) is clear enough for me, I’ve cloudy idea of How the “2-Wire instrument” works?

 

[ericgibbs]

hi,
This datasheet gives the internals of a 4to20mA loop IC, it may help,

 

[aljamri]

Hi Eric,

I've printed the file. I'll study it and come if any question arise.

Thank you

 

[KeepItSimpleStupid]

Basically, the tramsmitter always draws 4 mA + a 0-20 mA additional for it's signal. Some portion of the 4 mA is used for power.