So after receiving my hardware a few months ago, getting the Raspberry Pi sorted and checking the radio transmission side of things, after a long hiatus I’ve finally managed to install the power monitoring side of things.
Because I didn’t have a socket outlet in my switchboard or any other power source available to use a USB supply, I ended up with a tiny control power transformer, 240V - 12V. Open circuit voltage is about 14.1V, within the overall limits for the onboard voltage regulator for the EmonTX.
However, since the turns ratio and OC voltage is higher than the design 9V plugpack, I need to update the setpoint in the EmonTX to resolve the incorrect readings. The firmware source indicates that the resultant scaling factor is something like (Vin * 13) / Vsec * 1.2, is this correct, or do the two numbers represent something else and need to be changed?
Also, since I intend on upgrading to the continuous firmware anyway, and I need a serial interface to update the scaling factor, does anyone know if its possible to power the device with a USB supply, and to use a standard USB-Serial TX/RX on the header pins, or is it mandatory to use the designated USB programming interface?
I’ve got a couple of older FTDI USB-Serial devices and a bunch of DB9 connectors at my disposal, but the FTDI units don’t allow for supply of power to the connected device.