User's Manual

Table Of Contents
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REX Meter
2 Product Description Technical Manual
2-2 30 November 2006
Figure 2-2. REX meter block diagram (Form 12S meters)
General Theory of Operation
Power Supply. The single phase (Forms 1S, 2S, 3S, and 4S) REX meter can
accept a line voltage of 120 VAC to 240 VAC ± 20 % (96 VAC to 288 VAC). The
Form 12S meter can accept a line voltage of 120 VAC ± 20 % (96 VAC to
144 VAC). Using this input voltage, a power supply generates a 6.4 V and 3.3 V
supply. On Form 12S meters, phase A voltage must be present to power the
meter circuitry.
Current and Voltage Sensing. Power line currents and voltages are sensed
using specialized current sensors and resistive dividers, respectively.
Multiplication and other calculations are performed using an integrated circuit
(called the meter engine).
The meter receives the current through a precision-wound current transformer
that reduces the line current proportionally. The meter engine samples the phase
current to provide accurate current measurements.
The meter receives the voltage through resistive dividers to ensure that a linear
logic level voltage is maintained. This also serves to minimize phase shift over a
wide dynamic range. The meter engine samples the scaled inputs provided by
the resistive dividers to provide accurate voltage measurements.
Meter engine
EEPROM
Microcontroller
LCD
900 MHz radio
6.4 V power supply
3.3 V power supply
3.3 V
16-pin
header
Optical port
20.48 MHz
CT
Phase A
current
Resistive
divider
Phase C
voltage
6.4 V
SPI bus #1
Wh rec
Wh del
fref = 20.48 MHz
fref = 20.48 MHz
SPI bus #2
Tx data
Rx data
CT
Phase C
current
Resistive
divider
Phase A
voltage