Hi,

I have been trying to calibrate the 7758 and then read energy, voltage and current values. I have several problems, so I will start with the most basic.

I seem to be missing some piece of information on how to convert the register values into real-world watts. My understanding is that you simply compute a Wh/LSB constant from your design, read the WATTHR register, multiply by the register value by the constant and convert from kW to W.

If I do the Wh/LSB computation, I get ~.0002820

Applying 7 amps at 166.6 volts to the circuit, I read 7956d from the WATTHR register after 5 seconds of accumulation. This is a resistive circuit (PF=1), so the result should be basically V * I. 116.6 * 7 = 816.2W.

I do not see how to use the constant to get the correct result. My guess is that there is some factor involving accumulation time that I am not taking into account. I did use 5 seconds as the time when computing the Wh/LSB constant.

What am I missing? Can someone explain how to get the correct watt value based on the above numbers?

Hi pico20191

Firstly, I would like to say that getting WATT information from WATTHR register reading might not be the best method, as power is mostly an instantaneous measurement. It changes with time based on V and I and the integral of power over time gives energy which is stored in the WATTHR register. So dividing the result by time, will just give the average power over that period. You can get the instantaneous WATT reading by using the WAVMODE register.

With respect to the calibration math that you have:

1. If the conversion constant is 0.0002820 Wh/LSB and the WATTHR register reading is 7956d, then it means 2.243592 Wh is the energy accumulated.

2. Equation 60 of page 50 shows the expected WATTHR reading based on ideal inputs.

But make sure that you have computed the constant correctly using the calibration procedure as shown in the datasheet. Does your RMS calibration constant match with ideal inputs that you provide?

Regards

hmani