Application of energy metering IC ADE7757.
Use of octocoupler IC MC2TE.
Application of voltage regulator IC 7805.
The Low Cost Energy Meter employs the ADE 7757 electrical metering IC for electrical energy measurement . The circuit can count upto 9,99,999 units and displays them on an 8-digit LCD calculator with a character display height of 0.56 inches. A fool proof feature of the circuit is that it has battery back-up so that when power fails the display will not be reset & unit-counting will not be interrupted. This meter can also be configured for automatic meter reading (AMR), remotely.
The Low Cost Energy Meter uses Analog Devices ADE7757 chip for metering of single-phase, 2-wire (phase and neutral) systems used in households. Its salient features are:
1. It can read up to 999,999 units (kWh) with a resolution of 0.01 unit.
2. It is designed for nominal 230V AC, 45-65 Hz and maximum line current of 30 amps. (The metering IC can be used with a maximum current of 120 amps.)
3. The dynamic range is 400 (i.e., 75 mA to 30A).
4. The meter count is 100 impulses/kWh, i.e., 100 impulses will be required to register one unit.
5. The accuracy level is better than Class 2 defined in international standard IEC1036 (1996-09).
In operation, the ADE 7757 chip directly interfaces with a shunt resistor (used as the current sensor) and AC analogue voltage sensing input and operates off the AC input. The only analogue circuitry used in IC ADE 7757 is that in the sigmadelta ADCs and reference circuit. All the other signal processing is carried out in the digital domain.
The two analogue-to-digital converters (ADCs) used in the chip digitise the output of current and voltage sensors. The ADCs are 16-bit, sigma-delta type with an oversampling rate of 450 kHz. These work with oversampling so that the bandwidth of the input signal is much less than fs/2, where ‘fs’ is the sampling frequency. In its most basic form, the sigma-delta converter contains a one-bit ADC and DAC. It produces a higher-resolution digital word output by averaging several one-bit samples.
The real power is derived from the instantaneous power signal. The instantaneous power signal is achieved by a direct multiplication of the current and voltage signals. In order to extract the real power component (referred to as the DC component), the instantaneous power signal is low-pass filtered. This scheme correctly calculates the real power for sinusoidal current and voltage waveforms at all power factors.
The power supply for IC ADE7757 is derived directly from mains using the capacitor divider network comprising C13 and C14. Most of the voltage is dropped across C13 (0.47µF polyester capacitor rated for 630V), while resistor R13 (470-ohm, 1W) is used as a current limiter. The output across C14 is limited to 15V DC, which serves as an input to regulator 7805 (IC2). The regulated 5V is fed to IC1 at its VDD pin 1.
