I'm looking at the 4-20mA circuit below and trying to understand it. For some reason I can't get this to click in my mind.
Pressure sensor is a Metallux ME751 with a supply voltage of 9..35 VDC and it outputs 4-20mA.
From my point of view (please correct me where I'm wrong!):
LM317 together with the R1 creates a current limiter at 1.25V / 56Ohm = 22.3mA. This current then goes through Pres sensor which change it based on the pressure. 4mA for its lowest point and 20mA for its highest point. The R2 resistor I don't understand the purpose of, but R3 will give a voltage to the ADC of 0.88V @ 4mA and 4.4V @ 20mA.
- The pressure sensor needs a supply voltage of 9-35VDC, how do I know what voltage the pressure sensor gets? i.e. what is the voltage on the lowside of R1??
- What is the purpose of R2?
- If I removed the LM317 and just supplied 24V to the Pressure sensor I would know it got the voltage needed. Would it still produce a 4-20mA current based on the pressure or does it need a constant current to work?