Electronic Design

Bob's Mailbox

Dear Bob:
In "What's All This Algebraic Equation Stuff, Anyhow? (Part 3)" (electronic design, Nov. 5, 2001, p. 94), you mention: "C1 should be a pretty good capacitor, mylar or metalized mylar." I haven't been able to find mylar capacitors in my usual catalog sources, such as Mouser Electronics or Digi-Key. Have mylar capacitors become scarce like the polystyrene cap? What's a possible equivalent? Polyester? I suppose the polypropylene caps could be used, but they're quite a bit more expensive.
Chuck Olson
via e-mail

Okay, Chuck, I guess I confused you. Sorry, it's my fault. Nobody buys a capacitor called "polyethylene teraphthalate." But that's what we're talking about. If you buy it from DuPont, they can call it "mylar." Nobody else can because DuPont holds the registered trademark on that name. Everybody else has to call it Polyester. But we're talking about the same stuff, okay? Note: I still say "Mylar" even when I shouldn't because (A) Electronic Design doesn't insist that we say "Polyester," and (B) it's too many syllables otherwise!—RAP

Bob:
Sorry to bother you with this trivial question, but it needs some real-world experience. I have a power-supply hysteresis circuit using a 3-MΩ feedback resistor. There are no special precautions in layout. Order of magnitude: what parasitic (leakage) resistance am I likely to get in parallel with this from a dirty 10-year-old board? This is a purely digital system (to the uninitiated), and it has this analog problem. Keep up the good work with your column. It's fun to read.
Fred U. Rosenberger
via e-mail

Hi, Fred. In many cases, 300 MΩ or 3000 MΩ, if it were just ordinary dust. Maybe even as high as 30 kMΩ with glass-epoxy, best case. But a board that's soldered badly and has bad handling of the flux could be as low as 30 kΩ or 3 kΩ. I've even seen it as low as 300 Ω!—RAP

Hello Mr. Pease:
I have a very fundamental question, and you're the first and foremost person that I had in mind to ask. How did Solid State Electronics get its name? I mean, what exactly is "Solid State" in the electronics?
Vijay Shanker
via e-mail

Germanium, silicon, gallium arsenide, and so on are "solid" materials. Semiconductors are mostly working because of what goes on inside of solid materials. That never happened in the past. Vacuum tubes, which were the only previously known active devices, worked only in a vacuum. Because a vacuum is inherently quite different from a "solid" state, the difference in naming is clear-cut to me —RAP

Dear Bob:
I'm a graduate student at Caltech, working on a mass spectrometer. Recently, we've been having a charge problem, and the suspected cause is a teflon block. I just read some of your online comments about Teflon (electronic design, Feb. 14, 1991, p. 115) and thought you might have some advice. The Teflon piece is upstream of the mass spectrometer. It's in direct contact with a metal lens used to steer ions in the system. The lens is held at +4 V. Could the Teflon be building up a charge simply by being in contact with the applied voltage? (I don't suspect the 4 V is nearly big enough to be a problem. /rap) Or, is the only possibility that the Teflon is gaining a charge when ions collide with it?
Rebecca Washenfelder
via e-mail

Yeah, that sure sounds like a likely suspect, just as with my friend Arnie's test fixture. I suspect that could charge the block's voltage sky-high. Put some kind of metal over the Teflon and see if you don't get some improvement. Then, find out why the Teflon was chosen in the first place, and get somebody's approval to re-engineer it. Some suitable resistive or conductive coating might be a good first-help.—RAP

Robert:
If I wanted to know the tightness of resistor ratios on a particular IC, or how closely matched the VBE for an npn and pnp on the same device would be, could you recommend a reference source?
Denton Conrad
via e-mail

Hello, Denton. In general, there's no way to be sure. For example, many OP-07s have a VOS of 75 µV, which would either mean 75 µV of VOS or 0.3% matching of R's. But these get trimmed, so you can't tell from that. In any given op amp, it may be hard to tell what amount of VOS is caused by the transistors, and which the R's cause. Not only can't I tell you which is which, but I usually don't even know myself, most of the time. This doesn't mean that I can't make well-matched resistors, but I don't usually know how well.—RAP

Hey Bob:
When are you going to do some electronic stuff? It seems you've been off on tangents for some time now. How about giving us some of your good old circuit insight—say on ADCs, DACs, or op amps?
Fred Bandy
via e-mail

Mix and match, Fred. More electronic stuff is in the mill. But realize that not all circuits worth analyzing (and learning from) are electronic.—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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