Single-Phase Energy Meter for Light Loads
Created: Sep 03, 2014
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Single-phase energy meter is commonly used for residential electrical distributions in monitoring electrical usage and other related applications. The PIC18F87J72 single-phase energy meter reference design is a fully functional single-phase meter based on the highly integrated single-chip energy meter IC. This low-cost design does not use any transformers and requires few external components. The PIC18F87J72 directly drives the LCD, and includes both an isolated USB connection and a non-isolated RS-232 interface for meter calibration and access to the device power calculations. The system calculates active energy, active power, reactive energy, reactive power, RMS current, RMS voltage and other typical power quantities.
The PIC18F87J72 single-phase energy meter reference design comes populated with components designed for 220V line voltage. At the bottom of the main board are the high voltage line and neutral connections. There are four connections that are made from the PCB to the meter casing. They are labeled LINE, NEUTRAL, SHUNT1, and SHUNT2. The shunt sits on the high or line side of a two-wire system and the meter employs a hot or "live" ground. The wires going into the shunt to SHUNT1 and SHUNT2 should be twisted together. The wires going into the LINE and NEUTRAL side of the meter should be twisted together, and also kept away from the SHUNT1 and SHUNT2 wires if possible. The neutral side of the two-wire system goes into a resistor divider on the voltage channel input. Anti-aliasing low-pass filters will be included on both differential channels. The voltage channel uses two 332kΩ resistors to achieve a divider ratio of 664:1. For a line voltage of 230VRMS, the channel 1 input signal size will be 490mV PEAK. The current channel of each phase uses current transformer with a turn's ratio of 2000:1 and burden resistance of 56.4kΩ. The resulting channel 0 signal size is 340mV PEAK for 20A, or twice the rated maximum current of the meter, still within the input range of the A/D converter of the PIC18F87J72.
The Microchip energy meter single-phase software is used to calibrate and monitor the system, and can be used to create custom calibration setups. For some accuracy requirements, only a single-point calibration may be needed. The energy meter software offers an automated step-by-step calibration process that can be used to quickly calibrate energy meters.