This is what I'm trying to build.
I think this circuit would work fine, I think most of the reason you're getting bad feedback is that the circuit isn't drawn according to the normal conventions. In particular, the signal (or, the "information") should flow from left to right in a circuit. However, you've got your sensing element at the very right edge of the circuit, and the thing reading your circuit at the left edge. In short, just flipping the entire schematic left-to-right would make it much easier to read, as strange as that sounds (see langwadt's post as an example of the broad arrangment). Also, the box on the right should be drawn to have visually strong horizontal lines for the GND node and the power pin of the opamp, so that R3 dangles very conspicuously off the bottom, making it clear that this circuit is measuring the current it's consuming as a whole. Also:
- You can see the confusion that has been generated by drawing a big box and labelling it with "X + my circuit". That box should be labelled "4-20mA sensor", and V2 should be labelled as "Any type of sensor that provides 1-5V as its output".
- V2 should be labelled 1-5V (it's 1-5V if the big box on the right is going to be a 4-20mA sensor.)
I'm not sure why R4 needs to be there? Edit: to stop the MOSFET being on at power-up, I get it now!- As mentioned a couple of times before, you don't need two separate op-amps, this can be done with just one, but that's up to you I suppose (although are you doing some trick here where the inverting input of IC1B is held at zero due to virtual earth principle, and therefore R6 is effectively in parallel with R3, therefore this circuit can more precisely measure the current it is consuming as a whole? Because if so, that's a lovely bit of attention to detail!) (Edit: Although now that I think about it, even this little detail can be achieved with a single op-amp too.)
But everything I've said is just pedantic style nits. Looks like it should work just fine.
{Edit: Everything I say above assumes R7 is a 20k resistor (e.g. two 10k resistors in series). Because it's 22k, the actual voltage range of V2 should be 1.1 to 5.5V, which seems odd.}
This is what I'm trying to build.
If the sensor is on the right then you do not understand the concept. The sensor is not a 0-5 V voltage source, it is a 4-20 mA current source. That is the basis of the whole idea.
I think you've missed the point. He has some hypothetical sensor that outputs 0-5V, as represented by V2, and his circuit (everything else in the box to the right), turns that into a 4-20mA sensor (all passively powered by the 4-20mA current loop, although how V2 is powered is left as an open question). Aside from the fact that V2 should be 1-5V, the circuit does that correctly AFAICT.