Hi, I need to design an accurate and durable 4-20mA and 0-10V digital signal transmitter. I found many examples on the web and on this forum, but I can't get them working properly with 0-3.3V input signal. Device's requirements:
- Rpi zero/nano as a MCU
- Powered by a Li-Ion battery, through 24V step-up converter for the current loop and 5V linear voltage regulator for the RPI
- grounded load, typically 600R
- input control signal from PWM or DAC (0-3.3V)
- R1, R2, R5 are crucial for the precision of the device, but it's hard to find resistance values matching needed ratios. Problem can be solved with trimmer potentiometers but maybe there is other, simpler solution.
- Most of the circuit comes from the "Art of Electronics" rev 2 (U6 and U7 circuits, photo in the attachment); I don't understand how R3 and R4 affect output current and therefore can't calculate correct resistance values for them, it seems like it's the main cause of the malfunction of the transmitter
- I found a great example circuit here, but I can't get it working with 0-3.3V properly (with 0-5V either btw), won't describe the attempt here since I am curious how to get my own circuit working. However, it uses only 2 Opamps, thus my previous complaint about complexity of my design
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