Electronic Design

Bob's Mailbox

Dear Bob: I found Eric Kinast's observations (Electronic Design, Oct. 28, 2002, p. 84) about photosensitivity most interesting. I wasn't aware that small metal-can packages can have light leaks. Years ago, I resolved to always use metal-can packages, rather than plastic, when low leakage was required. I had had a bad experience with TO-92 packaged 2N5555s, which I found to be very photosensitive. I may have encountered a bad production run, because he has found TO-92s to be "completely opaque." (I am not aware of any TO-92-packaged devices that are sensitive to light! If you saw some that seemed to be sensitive, "a bad run" does not explain that. The input diodes on our LMC662 low-IB FET op amps are consistently better than 6 fA, and about 1015Ω. Not bad for a plastic-package device. /rap)

As for his discussion of breaking down BE junctions, I have had much the same experience. That is, there seems to be no harm if the current is limited to reasonable values. I have used reverse-biased BE junctions as makeshift 8-V zener diodes in breadboards. I have also seen equipment in volume production unwittingly designed with transistors in nearly continuous BE reverse breakdown. To my knowledge, none of those transistors ever failed.

  • John Connell (via e-mail)
  • Pease: I have used 2N2369s that were quite touchy. We had to replace some used as zeners at 4.5 V that had croaked. Some npns can be damaged and degraded if you zener the VBE, and the beta falls for a while. And it may creep back up partway in a year. Don't assume there is no damage, unless you know where to look. In particular, don't take 10 npns from your drawer—and zener their VBE junction to find their VZ—and if they are not what you need—don't just put them back! They may be degraded and drifty for months!

Dear Bob: The lowest-leakage diode I ever found was the gate junction of a JFET. Back in 1970, it worked fine for storing charge on a small capacitor for 15 or so minutes. A coworker told me about it.

  • Raymond Payne (via e-mail)
  • Pease: Yes, but was it in a metal can, or in plastic? Usually, the plastic ones are better!

Hi Bob: I was a PC-Write aficionado (full version). It came installed on the first 486 computer that I bought in I think around 1987. I used it for years. In fact, I purchased the last upgrade the day that Quicksoft closed their doors. They cashed my check but didn't send the upgrade. :-(

I continued to use it regularly until about 1997 when I switched to using Word. But I maintained a copy of PC-Write for reading old documents. I was constantly cutting and pasting into Word or completely retyping documents so I could convert them to PDFs. Last April, I ran across a program called "WordPort" (www.acii.com/) that can read PC-Write files and convert them to Word and other formats.

Then, in a spring-cleaning frenzy, I tossed PC-Write. I had everything—the Wizard book, the Macro book, the Font selector, the whole ball of wax. But it was occupying about 2 ft of bookcase space. With only 55 ft of available shelf space, and piles on the floor, ancient stuff had to go. (I even tossed all DOS stuff before version 6.22.) Sorry about that.

For nondocument editing, I have all of my technical programs default to using UltraEdit—a very nice alternative to NotePad, with no size limits, and with regular-expression capability.

  • Jim Thompson (via e-mail)
  • Pease: Hello, Jim. "Junk" is something you throw out two weeks before you need it. Fortunately, I DID find a copy of the Wizard book.

Comments invited!
[email protected] —or:

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

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