Figure 2 shows how three AD7755s-one for each phase-are used
with a microcontroller to make a three-phase energy meter.
Microcontroller-Based
Energy Metering using
the AD7755
SOURCE
A
B C N
POWER DOWN
POWER
SUPPLY
8
3
SEGM
RESET
CF
CURRENT
SENSING
COMM
AD7755
REVP
3-to-8 DECODER
VOLTAGE
SENSING
by John Markow
ENABLE
MCU
As the energy metering industry converts from electromechanical
meters to more-accurate solid-state meters, power-system designers
have a chance to incorporate new features that weren’t previously
possible. In demand now are solid-state meters that measure energy
more accurately than electromechanical meters, incorporate
multiple-rate billing, and are capable of being read remotely by
the utility company.This article describes how theAD7755 Energy
Meter1 integrated circuit could be used in three-phase energy
metering with power outage detection and measurement backup,
and remote, automated, multiple-rate metering.
CURRENT
SENSING
CLEAR
DISPLAY
CF
AD7755
AD7755
REVP
MODE
VOLTAGE
SENSING
ENTER
RESET
CURRENT
SENSING
CF
ENABLE
SCK
REVP
VOLTAGE
SENSING
EEPROM
SDA
TO LOAD
CLK
CLK
Figure 2. Functional block diagram of a three-
phase microcontroller-based energy meter
The microcontroller serves as the “brains” of the system,
performing all the required housekeeping tasks and interacting
with the other components—the energy meter ICs, the power
supply, the EEPROM, the display, and buttons to operate the
meter—to view energy or power, calibrate the phases, or clear the
reading. Besides low cost, the basic microcontroller requirements
are:
The AD7755 is an accurate (to 0.1%) single-phase energy-
measurement IC. It accepts a pair of voltage inputs that represent
the voltage and current of a power line. Internally, these signals
are converted to the digital domain with oversampling A/D
converters. A fixed-function digital signal processor continuously
multiplies the two signals; their product is proportional to
instantaneous power.After being low-pass filtered, the digital signal
is then converted to a frequency—scaled according to selectable
settings—to generate frequency outputs at terminals F1, F2, and
CF. The signals at F1 and F2 can be used to drive an
electromechanical counter (typically at full-scale rates from 0.5 to
5 Hz), while the higher-frequency CF signal is suitable for
calibration. The frequency (or rate) of the pulse outputs is
proportional to the instantaneous real power being monitored by
the meter. Accordingly, in a given interval, the total number of
pulses generated at these outputs is proportional to the energy
transferred to the load. A reverse-polarity logic signal indicates
when the measured instantaneous power goes negative (i.e., the
load is returning net power to the line).
*Sufficient I/O to drive the display. If an LCD display is used a driver
is required. If one is not incorporated into the MCU, an LED
display can easily be controlled with a 3-to-8 decoder.
*Interrupts. To avoid missing any energy-indicating pulses, the
system can be configured to trigger interrupts in the MCU. A
power supply monitor can generate an MCU interrupt when
it has detected a brownout condition and initiate an emergency
energy measurement backup.
*EEPROM Serial Interface. A simple serial interface can be created
using only two or three I/O lines. An MCU with a built-in
serial interface makes the design even easier.
*Timers. There are two main time intervals that need to be
maintained. First, a display update rate must be set at about 2
seconds. Also, if an LED display is used, a timer must cycle
through the digits at a sufficient rate to minimize on flicker.
Additionally, the calibration routine must be carefully timed,
but can be implemented with interrupt postscalers.
AV
DD
DV
DD
G0 G1
AGND
AC/DC
DGND
AD7755
POWER
SUPPLY MONITOR
PHASE
CORRECTION
SIGNAL
PROCESSING
BLOCK
...110101...
V1P
V1N
ADC
⌽
PGA
x1, x2, x8, x16
HPF
LPF
As an added feature, a second serial interface could be used to
communicate with a host system for remote/automated metering.
Also, either an external or internal clock could be used to
implement multi-rate metering.
MULTIPLIER
...11011001...
V2P
V2N
ADC
DIGITAL-TO-FREQUENCY
CONVERTER
RESET
4k⍀
2.5V
REFERENCE
Reference Design: A three-phase energy-meter reference design
(Figure 2) has been implemented to demonstrate how multiple
AD7755s can be interfaced to a microcontroller. It uses a
Microchip PIC16C67 microcontroller2, serial EEPROM, an 8-digit
LED display, current transformers for current sensing, and resistor
dividers for voltage sensing. Power is furnished by a transformer-
based supply incorporating power-loss detection.
CLKIN CLKOUT
S0
REVP
F1
REF
IN/OUT
CF
SCF
S1
F2
Figure 1. Block Diagram of the AD7755
The CF frequency output is a pulse train proportional to the F1,F2
outputs, with full-scale output rates of 21.76 Hz, 43.52 Hz, and
5.57 kHz, for ac inputs. It is well suited to interfacing to a
microcontroller that performs calculations and makes decisions.
The analog interface to the AD7755s is instrumented with voltage
divider resistors for the voltage channels and current transformers
1www.analog.com
2www.microchip.com
Analog Dialogue 33-9 (1999)
1
5秒后页面跳转
塔塔电子与力积电达成技术转让协议，携手建设印度首座晶圆厂
英特尔震撼发布：128核心256线程的至强6性能核处理器
SK海力士量产12层HBM3E，容量高达36GB
意法半导体传感器驱动Sphere，开启沉浸式电影新时代
微信公众号
抖音公众号
浙公网安备 33010502006866号 浙ICP备10014259号-119
营业执照ICP证
数据手册
如果您发现信息不准确，
