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DEVELOPING AN AC CURRENT GENERATOR

by Ernesto Gradin & Aubrey Kagan

Start ý Features ý Magnetics ý Primary and Secondary Turns ý Primary and Secondary Winding ý Primary Inductance ý Hardware ý Firmware ý Analog to Digital Conversion ý User Interface ý Sources and PDF

HARDWARE

Figures 5aýd show the complete schematic of the AC current generator controller.

The microcontroller is of the 8051 family. The only requirements are that it has 256 bytes of RAM, a serial port, and sufficient program memory for the firmware, which is currently at 7 KB. We selected the Atmel AT89C55 simply because, with 20 KB of EPROM, there would be plenty of room for expansion. The controller has no particular need for graceful recovery from a soft error, so for the sake of consistency with other designs, an external watchdog is employed, both for its watchdog and reset capabilities.

The crystal frequency for the microcontroller (11.0592 MHz) is selected to provide a convenient divisor for a standard data transfer rate. The RS-232 level shifting is achieved using the industry-standard MAX232. The 5-V supply is derived through a simple linear regulator from the 24-VDC input. Originally the project was going to be a standalone instrument, so an EEPROM was added to maintain the constants. This functionality was retained despite the fact that it can all be stored in the PC and downloaded. All peripheral devices use serial interfacing to simplify the printed circuit board.

The AC waveform source is a dedicated chip, the Micro Linear ML2037. This device is a sinewave generator with programmable frequency and gain with the output centered around 2.5 V. The output frequency is generated by dividing down a crystal oscillator, and the crystal frequency was selected to give integer values for the divider. The device also has a shutdown input that allows the output to be turned on and off and start at the same wave phase, which is a desirable feature for output step waveforms.

The output of the sinewave generator is passed through a programmable resistive divider (DS1267). This allows for 512 discrete values, a resolution of 0.2%. Based on the results of the control loop, if we were to rework this project, this is the area we would focus on.

The wiper of the digital potentiometer is AC-coupled to the input of the power amplifier (LM12 from National Semiconductor). The op-amp has a fixed gain of 1 + (R7/R6), chosen for the maximum input (ý1 V) to reach ý20 V, allowing for op-amp "headroom." The output of the op-amp drives the external transformer directly, but the return of the transformer is AC-coupled through C18 and C19 to prevent any DC magnetizing current. The return transformer wire is biased to zero volts by the resistive divider (R9, R10), whose values are high enough to make a negligible contribution to the DC current.

The returned analog signals are measured by a 12-bit A/D converter from Linear Technologies. The conversion from 4 to 20 mA is achieved through a resistor to convert the current to a voltage. This resistor need not be accurate because the firmware will allow the system to "learn" the voltages associated with specific AC currents. Additional channels are available, so the host PC may use them for any other purpose (e.g., measuring variables during a test procedure).

You can see a close-up of the ACCG controller in Photo 2.

Photo 2ýNote that the fan is mounted directly on the heatsink above the power op-amp. Directly beneath the unit are the 100-A transformer (mounted vertically) and the shunt. Mounted horizontally on the lower right is the 10-A transformer.

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