MX844 View Datasheet(PDF) - IXYS CORPORATION
Part Name
Description
Manufacturer
MX844 Datasheet PDF : 21 Pages
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MX844
IXYS
APPLICATION EXAMPLE - POWER MONITOR
This design example illustrates the application of the MX844 as an AC line power monitor.
In this design the MX844 operates as a serial bus master, reading instructions from an external serial
EEPROM.
Following application of power, the first 16-bit word read from the EEPROM is 0x8200 (using "C" notation
to indicate a hexadecimal number) which in conjunction with the connection of terminal M1 to VCC,
causes the MX844 to set it's internal dividers for 488 A/D samples per second and an asynchronous baud
rate of 38.4 Kbaud when using an 8 MHz clock. The first measurement is made from input channel IN1
on the +/- 50 mV scale. For the power meter this is a measurement of the AC line current flowing through
sense resistor R1. A value of 0.002 Ohm for R1 results in a +/- 25A current scale.
The second instruction is 0x83A0, which results in a measurement from input channel IN2 on the +/- 250
mV scale. For the power meter this is a measurement of the AC line voltage through the voltage
attenuator consisting of resistors R2, R3, and R4 which provide a full scale of +/- 407 V.
The MX844 cycles through the first eight addresses of the EEPROM. Therefore the first two instruction
codes are repeated three more times in order to continuously alternate between AC line voltage and
current measurements.
This is how the EEPROM contents would appear in a typical programmer buffer editor:
00 82 A0 83 00 82 A0 83 00 82 A0 83 00 82 A0 83
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Note that the 16-bit word is broken up into two bytes with the least significant byte first.
The power monitor is powered directly off the line using resistors R14 - R20, diodes D1 and D2, and
capacitor C1 to produce an unregulated DC voltage for the MX844.
Opto-isolator U3 provides isolation between the AC power line and the computer serial port. Dual
transistors Q1 and Q2 and related circuitry amplify the photodiode signal into the required RS-232 signal
for the computer serial port. Power for the photodiode amplifier is very small and is supplied by the serial
port DTR and RTS terminals.
By analyzing the data stream from the MX844 the computer software program can determine the RMS
line voltage, the RMS line current, the RMS power consumed, and the power factor of the load.
Each digitized sample is transmitted to the computer as two 8-bit "characters" in asynchronous RS-232
format. Embedded in the two characters are the 12 bits of data and a 3-bit EEPROM address.
MX844
Drawing No. 084423
11
08/25/06
www.claremicronix.com
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