I've recently received my first emonTx 3.4, and I'm currently tinkering with the calibration and the sketches.
One of the things that is not quite clear in the documentation/calibration (or that I couldn't find), is what you are supposed to do with the power supply jumper while calibrating. The goal is to power the emonTx using the AC-AC adapter, so that implies calibration should be done with the jumper in place.
On the wiki however is stated that the emonTx is also powered by the USB/UART bridge, and it is not clear what you should do with the jumper if the UART dongle is connected.
So: can I power the board safely with the AC-AC adapter while the UART dongle is connected?
On a related note: is there a calibration pad on the print that allows me to easily measure the voltage that is being sensed by the ADC for voltage, or do I need to probe the miniscule MCU pin? (Good that it's not a BGA ;-)
Re: Calibration procedure emonTx 3 - power supply jumper?
The emonTx V3 (both V3.2 & V3.4 variants) does not normally use the internal reference, and as the supply voltage is always regulated (even when batteries are being used for the power supply), there is no real need to calibrate the voltage reference. Simply ignore that step and continue with the calibration procedure.
The presence or absence of the link won't affect calibration at all.
You're not supposed to run the V3.4 with both power supplies, but I've done it many times (though for only a limited time) with no apparent ill effect.
Re: Calibration procedure emonTx 3 - power supply jumper?
We might be talking about different things here; I'm talking about the voltage waveform delivered by the AC-AC adapter, and I have the impression you are referring to the voltage connected to the AREF pin on the MCU?
As far as I understand it, the tolerances of the AC-AC adapter as well as the circuit for dividing/biasing the waveform for the ADC input implies that a proper calibration is required for accurate results. I would also expect a (slight) voltage drop when the AC-AC adapter is powering the emonTx, so the calibration should (ideally) be performed in the same circumstances.
I'm just getting up to speed with the emonTx, and sometimes it's not always very clear what hardware part/version the documentation is referring to.
Re: Calibration procedure emonTx 3 - power supply jumper?
That wasn't clear from your original post, and my crystal ball failed me. The load on the adapter does slightly affect the calibration of course, but the power limit (e.g. the number of temperature sensors, when the LED flashes etc) has been set so that the effect is minimal and in practice it is undetectable. My simulations, with the standard ac adapter, show that the dip on the positive peak reduces the rms value of the voltage by less than 0.34% at maximum supply voltage to less than 0.25% at minimum supply voltage. My mains supply is not stable to anywhere near that, I don't have a stabilised power supply so it is impossible to verify that figure.
Here is the simulation at minimum supply voltage. The red trace is the input sine wave, the cyan is the voltage to the analogue input, the blue is the difference × 100.
Re: Calibration procedure emonTx 3 - power supply jumper?
Sounds perfectly sensible. Thanks for the explication :-)