AN2288

Overview

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Overview

Power meters / Energy Meters are also known as kiloWatt-hour meters. As per definition,

energy consumed is a measure of work that is done over a known time period. Suppose, a

heater of 2kW is ON for half an hour, the consumed energy will be 2000W * 1800s =

3 600 000W-s (Watt-second), which is 1kWh.

The Active Energy Pulse output of 50% duty cycle is an indication of active power

consumed, as measured by the power meter. If the active power is higher, the frequency of

the pulse will also be higher. The pulse count gives the active energy measured by the

meter. The greater the number of pulse counts, the greater the amount of consumed active

energy. The pulse output frequency is easily configurable by software. The current software

has 3,200 impulses per kiloWatt-hour.

All the measurements can be calibrated by software, so there is no need for any trimming

components. With the firmware, phase difference created between voltage and current due

to current transformer can also be compensated. Because only one ADC is used to convert

both analog voltage and current signal into digital form and there is no dual sampling feature

available in the ST7FLITE20 microcontroller, the shift error (the sampling difference

between voltage and current) can also be compensated by the firmware. The calibration

procedure can be automated, which removes the time-consuming manual trimming required

in traditional, electromechanical meters. Digital calibration is fast and efficient, reducing the

overall production time and cost. Calibration coefficients, accumulated kWh and tampering

information are safely stored on the external EEPROM. This is more secure than internal

EEPROM since data would be lost in the event of a burnt microcontroller. Internal EEPROM

can be used in place of external EEPROM by changing only the microcontroller (without any

change in hardware design), if the consumer or supplier prefers.

The most important part of the meter is the firmware which includes tampering detection

functionality in a single-phase meter. The firmware can be modified and updated at any time

by using In-Circuit Programming, even when the meter is installed and running. The

firmware is entirely written in C except some time critical routines which are written in

assembly, which makes modifications easy to implement.