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7758 : Converting Raw WATTHR Register Values to Watts

Question asked by pico20191 on Mar 25, 2013
Latest reply on Apr 16, 2013 by hmani


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?