AN3322 datasheet(21/38 Pages) STMICROELECTRONICS

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AN3322

Watt-hour meter using the STM32

To keep energy resolution up to 1 mW, we need power to be specified by 25 bits or more. In

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

are computed in µWs.

1.10.6

Pulse output

In order to reduce jitter when generating the pulse output signal, the following algorithm is

used:



The energy is computed once per period of mains (i.e. every 0.02 s) EWh



The energy computed is divided by 44. EfragmentWh = EWh / 44



This fragment is added to the accumulator every time the ADC is triggered (44 times

per one period of mains) that is equal to 2.2 kHz



Whenever the accumulator surpasses a certain level, ETHR, impulse threshold, the

pulse LED is toggled and the ETHR is distracted from the accumulator



In order to reach desired precision in the pulse generation, ETHR and all the energy

values are computed in µWs.

The pulse constant (the number of pulses generated per consumed kWh) is set by the

manufacturer.

Example 4

Impulse constant K is set by the manufacturer to:

K = 3200 impulses / kWh

Relation between joules and watts per second is the following:

W x s = J

W x h = 3600 J

kW x h = 3.600.000 J

1 kW x h = 3.600.000.000.000 µW x s

In order to be able to set correctly ETHR, it is necessary to know that 1 impulse means:

3200 impl. ...........................1 kWh

1 impl. ...........................X kWh

3200 impulses = 1 kWh = 3.600.000 J

The energy of one impulse EIMPL is:

x = EIMPL = 1 kWh / K [imp / impl/kWh] = 3.600.000.000.000 / K [µWs]

EIMPL = 1125 J = 1125 Ws

EIMPL= 1.125.000.000 µWs

Because the LED is toggled by reaching the ETHR value (impulse-threshold), the generation

of one complete pulse (rising and falling edge) requires two events (two times reaching

ETHR within EIMPL step). It is necessary to divide the threshold constant for 1impl EIMPL by

two:

ETHR = EIMPL/ 2 = 1.125.000.000 / 2 = 562.500.000 µWs

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