Electronic Design

Bob's Mailbox

Dear Bob: When looking at datasheets or application notes that have the schematics of an IC in them, I often see very strange components; transistors with two, three, or even four collectors or emitters, and a component that looks like a capacitor where one plate is a resistor. The LM675 datasheet on page 5 has both of these. For example, check out Q12, Q15, Q16, and R23. Application Note 446B on the LM12 internal design also has these. On page 3, Figure 3, Q3 and Q4 have two collectors and a short line through an extension of the base that seems to be shorted to itself. And, R32 in the lower right has one of these resistor-capacitor hybrids but nothing connected to the plate. I'd like to learn more about what these mean and was wondering if you could write a column about them and/or point me at some references that talk about them.

  • Len Fischer (via e-mail)
  • Pease: Hello, Len. Okay, I finally found a few minutes to peek at the LM675 datasheet. It is perfectly logical. The pnps that seem to "have four collectors" really do have four collectors. Whatever current is sent into the emitter winds up at the four collectors, split into about four equal parts. I've been doing that for many years. My LM331 has done this since 1977, and the LM301 and LM741 have done this since the early '70s. So has Tom Frederiksen's LM324. If you wanted to see more about this, you could look at www.designarrays.com, which is Hans Camenzind's Web site. The resistors with the bar above them are pinch resistors. You can learn about them also at Hans' site. They have lousy tolerances and lousy tempcos, but they're better than nothing. This is nothing new. You're just starting to notice them, but they have been around over 30 years. And, I finally got a chance to look at the LM12 datasheet. The resistors with bars are pinch resistors. The bar across the collector of a transistor represents a ring that collects current when the transistor saturates. I had to do some digging around to find the answer to that one.

Dear Bob: Have I had fun. I recently took up a course on arc welding (one needs to stay entertained), and wow! This is something I think every electronics engineer should experience. Just you, a 25-V, 150-A arc, a mask, a stick, and a little voice in the back of your head going "PULL THE POWER! PULL THE POWER! IT'S BAD! VERY BAD!"

  • Dan Williams (via e-mail)
  • Pease: Yup, there sure are a lot of of femtoamperes in a 150-A arc! "Billions and billions!" I welded up a tandem bike, starting from two bicycles, when I was a kid. I rode it several hundred miles, and it was stolen just about the time the main welds began to go. Yeah, welding is a useful art—like baking or cooking.

Dear Bob: (Forwarded to Bob by Ted Hardin, senior applications engineer at NSC) Could Bob Pease comment on "the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS)"? Are there that many old lead-filled scopes going into landfills? The new nolead solder really sucks. My technicians having to use this stuff are cursing me out. It is too stiff, doesn't wick well into thru holes, and always looks like a cold solder joint. Are there any good choices for no-lead solders?

  • Dan Mohr (via e-mail)
  • Pease: Hello, Dan. I agree, it is unfortunate that we have to use inferior solder. I'm not sure exactly where it says you can't use solder containing lead for experiments. But exactly where that crosses the line to "not using it for production," or for material that is to be shipped out, is something I have not heard. I'll look into that. But why ask me? Ask Dear Abby. Ask Kester or other people that make solder. I don't sell any solder. Let me know if you find anything good. Meanwhile, get a hotter soldering iron!

Dear Bob: I just finished reading "What's All This Bivouac Stuff, Anyhow?" (ELECTRONIC DESIGN, April 13, p. 20). So, what's all this snowshoe stuff got to do with Electronic Design, anyway? (If I froze to death, I couldn't write any more columns. That's what. /rap) In short, planning and preparation. That's the key to success in any venture, and the lack thereof can be the key to failure. Works the same for manufacturing electronic products or climbing mountains. Thanks, Bob!

  • George Gerwick (via e-mail)
  • Pease: Check! /rap

Comments invited! [email protected] —or: Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

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