Hello Mr. Pease:
Thanks for writing all of these interesting tidbits of information over the years. I am tempted to purchase an old VW and wire up the sound system with some of that newfangled oxygen-free copper at $6/ft so that it will sound better. (With an old VW, you can't even hear the sound system, unless you turn off the ignition! /rap)
In your article, "What's All This Shunt Stuff, Anyhow?" (Electronic Design, April 29, p. 94), you mentioned that you soldered nichrome wire using a strong HCI flux. What exactly is that? (That's not HCI, but HCl or hydrogen chloride. Hydrochloric acid is a very strong acid. Go to any good plumbing store and buy a little bottle. But be careful to wear goggles when soldering, and don't touch the stuff, and wash off all flux. /rap) The only way that I have been able to solder nichrome wire is with silver solder. If there's an easier way to do that I would like to know!
I didn't know that silver solder is one way to do it. But that takes very high temperatures. So, just try the HCl method mentioned in my column.—RAP
I always enjoy your "Stuff." When do you find time to read and write all the e-mail? I could use some. I used to make up all kinds of drills that I didn't have as a kid. So when I need a special drill, I do as you would.
I once owned a house in New England, built in 1925. It had an iron water-feed pipe from the street (~50 ft) that had almost rusted closed (~2 gpm). I was determined to "fix" it before spring when the ground would thaw and I could dig it up. I soldered a hole saw to a 1/2-in. copper pipe and made interlocking sections of pipe to make the length. I then unscrewed the water meter and elbow to get a straight shot down the pipe and proceeded to drill. I put a PVC angle with a close fitting through-hole to channel the water away from the drill, and I wore rubber boots and rubber gloves as water was everywhere with the 120 V ac.
When the water started to flow heavy, I stopped drilling and pulled out. With everything back together, life was good—until I noticed the next day that the wall was getting wet from behind. I guess the drill broke through the side of the weak pipe, and water was leaking against the house. Afraid that it would push the basement wall in, I drilled a drain hole through the concrete down low. With a hose connected to that drain we were okay until spring when I dug it up and put in copper. That was my next most exciting drill job.
Hello, Steven. Thanks for sending in some of the wildest comments on drilling that I have received so far.—RAP
I love your columns and books. Maybe you can help resolve something that's been nagging at me. While fixing up a few dead power supplies, I found they had blown (among other things) the current-sensing transistor. In the designs, there was a low-ohm resistor in the hot lead, with the base-emitter junction of the current-sensing transistor directly across it. The collector, of course, goes back to some point to inject the shutoff current.
Now, if you do a little thought experiment: If you short the output, then there's full output voltage across the current-sensing resistor for an instant, and base to emitter. (Quite right! /rap) In a few microseconds, the series-pass transistor will turn off. (Well, in theory. /rap) But for a few microseconds, at a first approximation, there's an awful lot of overdrive to the current-sensing transistor!
Is it just me, or would it be a good idea to have, say, a 1-kΩ resistor to limit the worst-case base overdrive? (Pretty much right. I would have said 100 Ω. But somewhere between 1 kΩ and 100 Ω would probably work with the best safety margin. To define any safety factor, you might apply a vicious short to the output with various R values between 47 Ω and 2 kΩ. I think if you put in 2 or 1 kΩ you will tend to destroy the main pass transistors, whereas anything below 47 Ω will likely rip out the sense transistor. That's a guess. I learned that in 1968. /rap)
One design was in a shakily designed telemetry scope. But the other two were in multikilobuck Fluke and Hewlett-Packard equipment, usually noted for good, conservative designs.
George R. Gonzalez
Yeah, well, they're usually good, conservative designs. You should complain to Fluke and HP, and tell them I'm with you. Do you think Spice says that 0 Ω is okay? I bet that's a major part of the problem. If Spice says it's okay, then you don't have to test it, do you?—RAP
All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
Mail Stop D2597A
P.O. Box 58090
Santa Clara, CA 95052-8090
A Note On Ginger Stuff: Is Ginger fun to drive? I did a brief test-drive of a Segway, under the watch of inventor Dean Kamen. Yes, it's easy and fun. The words that come to mind are intuitive and user friendly. I watched a couple dozen people also learn quickly as they took a brief test-ride. Obviously, we all loved it! To make this interface work so naturally certainly shows world-class engineering. I still don't think it will stop really fast, but it stops okay. (So does my bicycle, and considerably faster.) More later. /rap