Electronic Design

Bob's Mailbox

Hi Bob:
Many years ago, I read about a subject called "Floobydust" in a National Semiconductor book. I've been searching around and asking people, but I haven't found any good answers. What does it mean and where does it come from?
Per-Anders Sjöström
via e-mail

Floobydust just means "miscellaneous." This phrase apparently came from a customer visit between Dennis Bohn, Tim Isbell, and a customer of NSC, about 30 years ago. It may even have come from the customer's daughter. Lost in the murk of history. Best regards.—RAP

Dear Bob:
I wanted to thank you for all the good, practical, and usable advice that you have given over the years. I don't understand why no one else talks of these things. I know good info can be had in some tech tips, but it's hard to find. I'm one of those self-taught audio designers that you have come down so hard on in times past, and I see the merit in your arguments. I would like to discuss some of this if you like. (I plan to study some audio circuits soon. /rap)

The main subject I wanted to comment on was your mentioning of typical signal diodes' sensitivity to light. I had an amplifier in which I couldn't tame the 60-Hz noise. It had vent holes in the top lid, and an incandescent light was shining down on it in its usual place of use. (Incandescent lamps tend to be bright at dc, with only a small 120-Hz component of noise. That's because the large thermal mass of the filament doesn't let the light intensity fluctuate much. It's the fluorescent lamps that make a lot of 120-Hz hum! And at 360 Hz. Not much at 60 Hz. /rap)

When I moved it to the test bench, removed the cover, and focused my shop high-intensity light on it, it measured very high noise. (I haven't studied halogen lamps' emissions, nor noises, very much. /rap) I was lucky as the light generated enough heat to cause other problems. When I moved the light away from the unit, the noise stopped. This was when I remembered reading of this phenomenon in your article. Taking an ultra-black magic marker to the input limiting diodes solved the problem. Thanks again and please publish more practical engineering stuff. I love it!
Roger Sheker
via e-mail

Isn't it funny how removing a noisy source correlates well with the noise going down a lot?—RAP

Hey Bob:
I got and used my new $150 GPS receiver for the first time in Panama two weeks ago—a Garmin Etrex. It's nifty to be able to push a button and record (with 20 ft of inaccuracy) the latitude, longitude, and elevation of land for sale, then take it home and tell (almost) exactly where I was, within a milli-minute of the readout.
Dennis Feucht
via e-mail

I was just flying from Inchon, Korea, to home. I got one reading at 179° 59.5 minutes east—and a couple of seconds later, one at 179° 59.5 minutes west. So I sure straddled the International Date Line! I was sort of hoping we would fly far north, and see Mt. McKinley, but we never went appreciably north of 43° north. So I guess that's where the jet stream was that day.

I didn't bring my GPS around the Annapurna Circuit this time—no need to. But I took odometer readings all the way around, and it was very close to 146 miles from BesiSahar to Beni. Then we rode an additional 150 miles before and after the trek.—RAP

Dear Bob:
Re: Shunts. Take a sheet of high-chrome stainless steel. Cut out a dog-bone shape with big ends for unipotential areas. Put holes in the ends for the force and sense connections. Screws and star washers worked for me. Using a pin punch and grinding, the dimple beats chewed up a good drill. Stainless work hardens when being drilled. Trim with file. The result: a near-zero-tempco shunt that can run to a red heat under overload. Of course, you have to characterize the resistivity of the particular alloy and thickness you're using.
Bill Schmitt
via e-mail

Really? Maybe less tempco than 3000 ppm/°C, but less than 1000, or 100?? You have to watch out for the thermocouples, too! Working at such temperatures, don't expect the thermocouples at the ends to cancel really well! I tried to look up the tempco of stainless steel and found no info.—RAP

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

More Knot Stuff: Many people wrote in to recommend various knots that they thought were better than my square knot with half-hitches. But Mr. Ashley's tests (p. 16 to 19 and 273) showed that most of these knots weren't trustworthy enough to take 100 jiggles. Definitely not the square knot or sheet-bend (without half-hitches). Not even the single or double Carrick bends. Not even the fisherman's knot (Ashley's #1414) or the water knot (#1412). Maybe the barrel knot (#1413) or "Ashley's Knot" (#1452) are safe. I'm confident that a square knot with half-hitches would pass the test.

Mr. Tom Moyer has done some meticulous tests on knots that are usually trusted. See www.xmission.com/~tmoyer/testing and click on "water knot testing." Some knots fail—they slip or "spill"—at low fractions of the ultimate breaking strength. /rap

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