Electronic Design

Bob's Mailbox

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!
Steve Danielewicz
via e-mail

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.
Steven Smith
via e-mail

Hello, Steven. Thanks for sending in some of the wildest comments on drilling that I have received so far.—RAP

Hi Bob:
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
via e-mail

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
[email protected]—or:

Mail Stop D2597A
National Semiconductor
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

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