Loop powered 4-20 mA transmitter

[scoobyrollz]

I need help!!!!!!!!!!!!!! I am struggling with the idea of designing a loop powered transmitter. I have the circuit designed for the V-I transmitter portion, but I need power for the op-amp IC I am using. I know I can get this required power from the 4-20 loop, but how without effecting the signal? Any help would be greatly appreciated.

 

[Warpspeed]

You measure the loop current at the transmitter and compare it with the incoming signal in a current sinking feedback amplifier.

The idea is that the nominal total internal operating power consumed by the transmitter will always be less than 4mA. The transmitter feedback amplifier then deliberately forces more current than that, to set the actual real final loop current.

The feedback amplifier is then scaled to set the loop current to 4mA at zero input signal, and 20mA for maximum full scale input.

 

[itsmoked]

This of course requires a minimum voltage across the whole loop because the transmitter gets only:

4mA x Vacross transmitter.

 

[onelett]

If the input does not need to be galvanically isolated from the 4-20 ma loop, then a simple solution is the Burr-Brown XTR115 IC.

 

[fsmyth]

Another possibility, if you have enough headroom
(voltage)-- add a resistor in line, and power a
DC-DC converter.
A few years ago, I would never have considered
this; I always used an extra pair to power the
remote.
<als>

 

[itsmoked]

fsmyth it's done all the time now. I did it in 90 and ran a power sucking 8051 that way. Now days with these power sipping micros it's easy.