This application note describes a low-cost, high-accuracy watt-hour meter based on the AD7751. The meter described is intended for use in single-phase, two-wire distribution systems. However, the design can easily be adapted to suit specific regional requirements, e.g., in the United States power is usually distributed to resi- dential customers as single-phase, three-wire. The AD7751 is a low-cost, single-chip solution for elec- trical energy measurement. The most distinctive feature of the AD7751 is that it continuously monitors the phase and neutral (return) currents. A FAULT condition occurs if the two currents differ by more than 12.5%. Power cal- culation will be based on the larger of the two currents. The meter calculates power correctly even if one of the two wires does not carry any current. AD7751 provides an effective way to combat any attempt to return the current through earth, a very simple yet effective way of meter tampering. The AD7751 is comprised of two ADCs, reference circuit, and all the signal processing necessary for the calculation of real (active) power. The AD7751 also includes direct drive capability for electrome- chanical counters (i.e., the energy register) and has a high-frequency pulse output for calibration and com- munications purposes. This application note should be used in co-operation with the AD7751 and other related devices to construct an electrical energy measurement system.
This document details the design of a tamper-resistant watt-hour energy meter intended for single-phase, two-wire distribution systems. The design utilizes the AD7751 single-chip solution to monitor phase and neutral currents, calculating power accurately even if one wire carries no current or if tampering attempts to return current through earth.
The note outlines the design goals based on IEC1036 standards, providing design equations for signal scaling and calibration. It includes a schematic for a simple implementation using two current transformers and an electro-mechanical counter, along with a two-step calibration process to adjust line voltage attenuation and gain mismatch.
The meter shown in Figure 1 is designed to operate at a maximum current of 40 A.
The meter shown in Figure 1 is designed to operate at a line voltage of 240 V.
The basic current for this meter is selected as 10 A.