AN3322 datasheet(20/38 Pages) STMICROELECTRONICS

20 / 38 page

Watt-hour meter using the STM32

AN3322

20/38

DocID18304 Rev 1

Even the CP is theoretically equal to (CI x CU); CP is set during the calibration process as

a constant between a supplied instant power PTheor and the instant power PW measured by

the STM32 power meter.

Equation 17

PTheor = CP x PW

The difference between CP given by the calibration process and (CI x CU) is given by the

phase shift that, even when carefully compensated, does still not have a negligible influence

on the computation of the power.

The instant power over one period of mains PW can be computed also from the energy E

taken over one period of mains equal to 20 ms:

Equation 18

Equation 19

where tS is the sampling period for u (t) and i (t).t is the period of mains equal to 20 ms. In

order to reach the desired precision in the pulse generation, PW and all the energy values

are computed in µW.

1.10.5

Energy

Energy is calculated as a product of current IIN, voltage UIN, time Ttimer and power

calibration constant CP (Ttimer - time in seconds).

Equation 20

EWh = CP x IIN x UIN x Ttimer

In order to save the microcontroller computational power, the previously computed instant

power PW can be used to get energy.

Equation 21

EWh = PW x Ttimer

Equation 22

Equation 23

E

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===

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--------

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fragment

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------

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tP



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