in This picture of the CT input in the emontx schematic, after the burden resistor (R7 = 22Ω) there is R19 (1K) and R5 (22) as part of the converting current to voltage (i.e.: the job of the burden resistor) why is there R19 and R5? I was expecting just R7…then onto R6 and R1 to raise GND for Arduino measurement VPP.
The other challenge I have with the schematic: the input on the stereo jack notes (pin 4) AGND. Then AGND is used on the voltage divider that biases the VPP. Shouldn’t the bias be on Arduino’s GND?
I am learning. I apologize if these questions are difficult to understand and/or “dumb”. Thank you.
If you look at the pcb itself and the bill of materials, R5 is not fitted. It is there to force a pair of holes for a wire-ended resistor to replace the fitted SMD burden, when a different value is required. The 1 kΩ R19 is there to limit the current into the ADC input, and normally plays no part in the circuit operation (or slightly more accurately, it has no significant effect in normal operation).
Pin 4 is a “break” contact, as is pin 3. It grounds the input when no plug is inserted, which allows the sketch to detect that condition (it’s only tested at power-on or reset) and thereafter that input is ignored.
The analogue and digital grounds are kept separate as far as possible, to prevent current in the digital ground affecting the input. There’s no Arduino associated with the emonTx V3.4, it has it’s own on-board processor, the ATMega328.
THank you. Your explanation was very helpful. I appreciate the time you took to answer.
1 Like
