Nuts & Volts - July 2005

Q&A

years ago, I wired up a large metal working machine with all DC motors and control. The motor that moved a table back and forth had to stop and change direction about every three seconds. I tried several relay arcing fixes — including using capacitors. But what finally cured it was connecting a light bulb across the motor leads. When the motor was energized, the lamp came on; when the motor was stopped, the lamp brightened considerably as it absorbed the inductive decay. All I had to do was try different wattage lamps until the arcing was suppressed.

I was about 23 at the time, out of college with an EE, and working for my father-in-law. One thing he pushed into my head was that simpler was better. So that was why I tried the light bulb trick. The owner of the machine shop was quite impressed. About five years ago, I went back to my home town and visited the owner. He was close to 90 and still active. He had sold the business but retained the right to use the machinery. He took me on a tour of the shop and said that he wanted to show me something. He fired up the old planning mill and there was the same light bulb going on and flaring up before going off.

— Robert E. Robinson

Dear TJ,

Your answer to the question about relay contact resistance measurement (in the May 2005 issue) was interesting, but I believe has a serious flaw. If you had computed the power dissipated at the contacts under the specified conditions [ 50 milliohms], you would have realized that 1.5 to 3 volts across contacts carrying 30 amps dissipates 45-90 watts. That much power dissipated in such a small space would raise the temperature of the contacts higher than the temperature of the filament of a light bulb, destroying the relay and likely starting a fire.

— Howard Mark

Suffern, NY

Response: I didn’t invent these numbers. They come from datasheets. But your advice is well taken, and a reason why relays fail when contact resistance exceeds a certain limit. And, yes. I have seen my fair share of melted relays. — TJ

Dear TJ,

About your “Needs Stereo Chips” answer in the April 2005, your answer is perfectly sound. But your reader must not know about Google, because a search was not “fruitless” for me when I was looking for the

exact same thing he was seeking.

http://electronickits.com/kit/ complete/ampl/ k100.htm is a kit for a pre-amplifier that does the exact same thing as the LM1036 does, but is based on the Philips TDA1524A.

www.ramseyelectronics.com/ cgi-bin/commerce.exe?preadd= action&key=UAM2 is the Ramsey UAM2, a “class D” audio power amp that works like gangbusters. You CAN still build what your reader wanted from kits.

— Kenneth Tindle Elect. Technician

Univ. of Kentucky Language Lab

Cool Websites!

NASA Deep Impact will rendezvous with comet Tempel 1 on July 4, 2005. The objective is to shoot a “bullet” into the comet and analyze the debris. http://link.abpi.net/l.php?20050505A9

Are you fast on the draw? Simply “shoot” five darting sheep with a tranquilizing dart and score your reaction time in milliseconds. www.bbc.co.uk/science/humanbody/ sleep/sheep/

NASA World Wind — satellite view of the big blue marble. http://worldwind.arc.nasa.gov/ features.html

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