Minimum start recording power

Hi, Bob

I remade a new board, uses 2.5V reference voltage and 1.25V bias voltage, and a simple RC(1kohm & 10nf) low-pass filter is set on the ADC input.

In testing, it was found that the power reading was 0 when the current was small.
The two transformers of input 1 and input 2 in the figure below are connected to the same wire, the measured current is 0.13A, and the current reading is stable, but the power reading is 0.

So, what is the minimum recording power of IoTaWatt? How to modify it.

kind regards.

Minimum power is around 2 Watts. There are a couple of mechanisms that contribute to that. With little details, I don’t think that’s why you see zero power. Without getting into specifics, changing the input circuit can have a major impact on accuracy. You are essentially designing a whole new device that is outside of any parameterization in the IoTaWatt firmware.

When you introduce additional resistance you affect the charge time of the ADC sample and hold, and when you introduce capacitors, you phase shift the signal. I’m afraid I can’t help you with that.

Thanks for your detailed reply.

Some try, just for the better. Because it is found that without low-pass filtering, if the channel is unloaded, there will be a current fluctuation of 0.02A. When filtering is added, the no-load current is 0A. I guess filtering is good for current measurement.
However, as you’ve analyzed, this was a mistake and I should remove the filtering.


You’re trying to make a diamond out of a piece of coal. Accurately measuring 20mA AC RMS is something you would do with a quality voltmeter. IoTaWatt is a power monitor. At the end of the day small +/- variations are filtered out by averaging.

The VT channel is calibrated and the reading is accurate.

For the two CT channels, remove the filter of one of the channels, and find that there is no change in the current reading, but there is a certain change in the phase shift.

If the same filtering is set on the VT channel and the CT channel, can the phase shifts cancel each other out?

Haha, a bit superfluous. IoTaWatt has done the design of digital filtering, it’s great.

I will replace the filter circuit with 0 ohm resistors.

Thanks for your correction,learned a lot.