Electronic Design

Bob's Mailbox

Mr. Pease:
I was shopping at a thrift store in Willoughby, Ohio, yesterday and found a Black & Decker 1/4-in. drill, No. 7004, type 1, with an orange $6.50 price sticker on it. Recalling your recent article on the scarcity of new power drills with quarter-inch chucks, and not having a good one in my collection, I immediately took it to the wall outlet in the appliance department and gave it a quick power test.

It was in great condition, except for some surface rust on the chuck. I popped it into my cart and headed for the checkout counter. It was half-price day for items with orange price
Martin Eich
via e-mail

Hello, Martin. You were wise to grab a valuable and rare item. Take good care of it and appreciate it for years! Best regards.—RAP

Dear Bob:
Roger Sheker's letter about the photosensitivity of input-protection diodes, which appeared in your recent Mailbox column (Electronic Design, Aug. 19, p. 66), reminded me of a trick that I started using many years ago when working on low-level circuits. In the days before SMT components, I always found it very frustrating that signal diodes, including low-leakage diodes, were inevitably in clear glass packages. (Not all are. Fairchild made many low-leakage diodes in black-painted packages. But when the paint gets scratched, it's almost as bad. /rap)

As such, the photocurrents often far exceeded the actual leakage of the diode, particularly when the board was out on the bench for calibration, etc. Putting black heatshrink tubing over the diode packages improved the problem, but was a pain in production. Then I realized that I could use an inexpensive small-signal transistor (2N3904, etc.), in a completely opaque TO-92 package, as a diode. The C-B junction is fairly low in leakage, has a high breakdown voltage, and the transistor may actually cost less than special low-leakage diodes of lesser performance.

But if you can tolerate the modest reverse-breakdown voltage (such as in back-to-back diode applications), the B-E junction makes an exceptionally low-leakage diode. Although transistor data sheets caution against breaking down the B-E junction, I have found that in protection or clamp applications this isn't particularly harmful if the current is limited to reasonable values. Hence, the B-E junction can make a useful low-leakage bidirectional input clamp, providing a forward drop in one direction, and a clamp of about 6 to 9 V in the other—without any problems due to photosensitivity.

By the way, amazingly, some small metal-can transistors DO show photosensitivity because the ceramic/glass header and epoxy seal at the bottom are translucent. Of course, today with SMT technology, you can finally buy signal diodes in opaque packages! (Yeah, but many of those junctions are still LOUSY in terms of conduction and leakage. /rap)
Eric Kinast
via e-mail

Eric, I wrote this all up in my book, about 12 years ago (on p. 66 and 67). I largely agree. The B-E junction is low in leakage. But in terms of fast-turn-ON and fast-turn-OFF, which are often quite important, the diode made of (C plus B) versus E is much faster—as long as you can stand a lousy 4-V working rating.—RAP

Dear Bob:
I've enjoyed your series on knots (March 5, 2001, p. 142, Dec. 3, 2001, p. 88, and Aug. 5, p.76). It's good to remember that even in this age of ROM, RAM, bits, and bytes, sometimes nothing can replace a good knot. Some years ago, I was in charge of flying a gigantic aerodynamic kite/balloon called a dart, which was used at the White Sands Missile Range to calibrate radars. We were taught a number of knots designed especially for use with cored nylon line. We used your Dutchman when we transported the dart on a 40-ft flatbed truck. You brought back fond memories.
Al Lacey
via e-mail

I'm impressed that somebody else really knew it and was teaching it! Thanks for the positive comments. I'll tell my Uncle Roger.—RAP

Dear Bob:
I think it's important whenever we discuss classical knots to remember that while many of the old knots worked well with hemp rope, they are not very secure with modern synthetic ropes. For example, I wouldn't trust my life on anything from the Boy Scout Handbook. I don't think I've seen any reference that spells out that warning, although I haven't yet looked at the Web sites you mentioned.
Malcolm Chamberlain
via e-mail

You are quite correct to point out that nylon ain't polypropylene ain't hemp. Knots that were pretty good and safe, with some materials and rope sizes, are NOT so good with "modern materials." As I said in my first column, the square knots and bowlines from the old books are NOT trustworthy, as shown. But they are safe and reliable when you secure them with a couple of half-hitches. That was true for hemp. Now with nylon or synthetics, I'd still trust these basic old knots if secured with two or three half-hitches.—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

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