ML0030 August 2009 Copyright 2009 Bitronics, LLC
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otherwise occur during the measurement of distorted waveforms with non-true RMS
instruments. Voltage of a given phase is sampled first, followed by the current of the same
phase. Phases A,B and C are sampled in succession, providing the MCU with
instantaneous measurements of all voltage and current inputs. Samples are accumulated
for 0.5 seconds, at which time the MCU calculates the WATTs and VARs for each phase.
The VARS quantity for each phase is derived from a power triangle calculation where the
WATTS and VAs are known. This technique provides a "true" measure of VARs even with
distorted waveforms. Zero offset is also adjusted for each signal channel every 0.5
seconds by the MCU. These per phase quantities are then summed to form the total three
phase WATTS and VARS. Calibration constants stored in both the Power Supply
EEPROM and the EEPROM located on this board provide drift-free calibration, and
complete interchangeability of Analog Processing boards. Checksums are incorporated
into both EEPROMs which are read periodically by the microcontroller to check the integrity
of the calibration constants and the CT and PT setting (See section 4.3 ). A "Master Gain"
trimpot is also located on the AP board to provide the user with fine tuning capability if it is
necessary to match other devices on the power system. Once the total WATTS and VARS
have been calculated, the MCU scales the values by the external PT and CT ratios which
have been selected by the user, and displays the values.
A second function of the AP board is to provide the 0 to +/-1 milliamp transducer output(s).
This is accomplished with 12-bit digital to analog converters (DAC), one for each output
channel. Constants stored in the EEPROM on the analog board are read by the MCU and
used to compensate for gain and offset of each output channel. This technique provides
stable calibrations for the output(s). Additional circuitry converts the output of each DAC to
the 0 to +/-1 milliamp current. This additional circuitry is powered from a +15volt and a
-15volt supply also located on the AP module. The outputs have a +/- 10volt compliance,
and can drive a +/-1.0mA into a 10k ohm load, with a reduced load the output can drive up
to +/-2.0mA. An indep5 volt power supply which provides an electrically "quiet"
supply for all the analog circuitry, and a high precision low drift reference are also present
on the AP board. Both the reference and 5volt supply are shared by the ADC and DAC
circuitry.
2.6 Microcontroller Board (MCU Board)
The microcontroller board consists of an Intel 80C51FA microcontroller (MCU), address
latch, EPROM memory and a watchdog timer. All the data acquisition, signal processing
and display manipulation are controlled by the microcontroller. Communications to the
other boards is accomplished via a serial data link comprising a set of three lines common
to all the other devices (ADC, 2 DACs, 2 EEPROMs, 2 Display Drivers). Individual select
lines for each individual device, allow the MCU to communicate with one device at a time.
The watchdog timer prevents the MCU from "locking up" in the event of a transient or other
type of interference. The watchdog timer also provides a reset on power-up or when
resuming from a brownout (low supply). The watchdog timer can be triggered manually, by
entering the CT/PT set mode (See section 4.1) and holding down the select push button for
approximately 1.2 seconds. In the unlikely event of a microcontroller failure, the watchdog
circuit will continuously attempt to restart the processor. A positive indication of this
condition is provided by having the watchdog flash both of the LED displays on the front
panel.
