Searching for the Elusive Diode Forward Voltage COLUMN BY NATHAN UNGER Hobby Harold was reading through some old issues of the ERE paper in the archive section of the Electrical Rebuilders Association (ERA) Web site. He had typed the words “forward voltage” into the search box, and found several articles discussing how to measure the forward bias voltage when grading diodes and rectifiers. “That,” he said to himself, “is what I want to get into.” The first thing he needed was to get a voltmeter. He scomed the thought of get- ting just an ordinary one. His was going to be a deluxe handheld digital scope with multimeter all rolled into one. He did a Google search and before long discovered something that made his eyes get big and wide. All the specs! All the functions! And what an incredi- bly low price for all that! He scrolled to the “find a dealer’ sec- tion, parted with a chunk of his hard- earned credit, and rubbing his hands together with glee, settled down to wait for the delivery truck which came a few days later. His trembling hands ripped open the package and with something not unlike awe, he pulled his new tester out of its bubble wrap. This thing would set him apart in the world of technicians because he could now accurately measure a diode’s for- ward voltage. He could now observe the diode ripple waveform of a charging alternator. He could test those new Ford PWM regulators properly. He could even set the timing on his car because it could read rpm and dwell from its handy little ignition wire clamp. It also had a handy little diode test function in which it put a small current through a diode and displayed the volt- age. Cool. He took a common automotive rec- tifier out of his pile of cores and went to work. The first thing he noticed was that the voltages were all slightly different. He went to his bin of new diodes and checked a dozen and discovered the same thing. “So just because the voltages are all different doesn’t mean the diodes are bad,” he thought. “I guess that means that if you want to build a rectifier that has matching diodes you would have to do that by actual testing, not just by part number. Hmmm.” He further mused that in the real world, you would have to go by a limit- ed range, not just an exact specification, something which you establish for each type and rating of diode. After working on this all afternoon, Harry discovered that the voltages were getting higher and higher. He retested some of his original diodes and the read- ings were higher than before! Then the low battery icon in his tester display started to blink. When he changed the batteries, the diode voltages he retested were back to normal. “Oh great,” he thought, “now I have to watch out that my voltmeter bat- teries don’t get low since it affects the calibration.” His excitement over the new tester got tarnished, just a bit. Then he noticed that one of the 10SI rectifier diodes he tested that seemed good actually had a cold solder joint with some of the solder having dripped out. Obviously his test method wasn’t catching all the problems. He went back to the ERE archives, reread those articles and concluded that maybe his meter test was not load- ing the diode enough. After giving it some thought, he decided on another approach. He would hook the diode up to a car battery. “But of course,” he murmured to himself, “one needs to have a way of protecting the diode.” He poked around in a bin of miscel- laneous junk and found an old 20A fuse. “Perfect,” he exclaimed to no one in particular. When he touched the positive wire to the fuse, which he’d connected to the diode, which in turn he’d connected to the negative terminal, nothing at all happened. “Oh, of course, I have the diode in backwards!” When he turned the diode connec- tions around and touched the positive wire from the battery, he saw a brilliant flash of light. The fuse seemed to have sort of a darkish, smoky-looking appear- ance. A quick continuity test of the fuse verified that there had been a fatality. He scratched his head, and tried a 30A fuse. Same problem. About that time his wife phoned, “Supper’s on!” He remembered that the hockey playoffs were about to start and got in his car, mulling over his experiences of the afternoon. By the next morning, he had it figured out. “The fuse cannot limit the current; it is only there to interrupt the circuit if the current gets too high,” he thought aloud. Casting his eyes around his shop, he spied a box of old ignition ballast resistors. “Tf it works for the ignition coil, it will work for my diode, too.” Setting his ammeter on the 20A scale, he soon figured out a combination of resistors that would permit only 20A through the diode when powered by the 12V car battery. He started checking diode voltages again, but this time the readings were at least 0.3V higher than with his diode test function on the meter. Furthermore, as he not so patiently watched his meter blinking and thinking, he noticed that the