F EATURE
ARTICLE
by Kevin Gorga (USA)
AC Tester Testing, Troubleshooting, and Debugging Safety is a top priority when working with electronics and circuits. The AC Tester design is an isolated variable voltage power source that includes an electronic circuit breaker for testing and debugging equipment. An mbed controller displays voltage and current, and it controls the breaker’s trip point and response time. In addition, this inventive design can display power factor, VA, and VAR.
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June 2012 – Issue 263
or many years, I have had a variac transformer plugged into an isolation transformer with a lightbulb in series and a pair of alligator clips on the end of a line cord. This is my most useful piece of test equipment for repairing and testing line-operated equipment. It provides safety from shock and short circuits. The isolation transformer provides the safety for equipment that has one side of the line as a common. With the isolation transformer in place, the scope probe can be ground-clipped to one side of an offline power supply with safety. The series lightbulb provides for current limiting when a short occurs or your probe slips. The variac provides a means to slowly apply power to a piece of equipment or to verify proper operation under fluctuating line voltage. In this article, I’ll describe my NXP Semiconductors mbed-based AC Tester design (see Photo 1). I use it for prototyping, testing, and repairing equipment that is powered off the AC line (see Figure 1). The AC Tester provides power line isolation to enable the safe use of an oscilloscope on nonisolated equipment. It provides adjustable line voltage to test proper operation at all line levels. It displays on both analog (for quick recognition) and digital (for accuracy) volt and ammeters. It also has a series incandescent lightbulb to provide protection and indication of overcurrent conditions. If a repair or prototype has a high-power short, the series bulb will illuminate and limit the power to the device. A variable response time solid-state circuit breaker is also incorporated for safely cutting off power at any set current and response time. Peak surge current is also displayed. An optional energy meter (E meter) is also provided for displaying of watts, VA, and VAR. Power factor is also available.
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DESIGN OVERVIEW The design is partitioned so that it can be built in three forms. The simplest version is the variac, isolation transformer, and analog meters. The next level up incorporates the microcontroller to enable a more accurate digital volt-
age and current display. It also adds the variable response time electronic circuit breaker. The ultimate version adds an electronic E meter for watt, VA, and VAR display. I used the I2C, analog-to-digital (A/D), digital-to-analog (D/A), SPI, and interrupts functions of an NXP Semiconductors LPC1768 microcontroller. I2C is used to expand the I/O capabilities. Four NXP Semiconductors SAA1064 LED drivers
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Photo 1a—The AC Tester powered up and running. The E meter is in the plastic case on the top of the Tester. The series current limit bulbs are on the top. b—Take a look at the inside of the AC Tester. The power transformers are in the back on the bottom. The triac assembly is above on the left. The power supply and processor board are above on the right. c—This is the prototype before I put it in a case. d—The bottom board is the power supply. The top board includes the LPC1768 processor and analog electronics. CIRCUIT CELLAR®
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