Electronic Design

Bob's Mailbox

Dear Mr. Pease:

I have been able to tolerate your vilification of Spice as that of an uninformed user.  However, your publication of a letter from another such user forces me to respond.

Check the words leading to the acronym Spice sometime.  A review of the original treatise by Vladimirescu and Liu might be informative.  See anything about board design?  If you are going to use a tool for a purpose other than what it was intended for, you had better understand how to apply it.  The custom Integrated Circuit designs that I have done performed as well or better than indicated by Spice simulation.

There is no reason Spice can't be used for board design, if you pay attention to how the program converges, and why it converges to a particular solution.  Any professional who uses public domain versions gets what they deserve.  The program was developed for those of us that can't breadboard a design, not those so foolish not to.

Kevin J. McCall, Custom Analog IC Design Engineer, Leominster, Mass.

What irritates me is when people say I am uninformed and Spice has to be telling the truth-when the truth turns out to be vice versa.  I'm delighted Spice works for you.-RAP

Dear Bob:

I particularly enjoyed your article last year about your calibrated cardboard boxes.  I work for a well-known temperature-controller company.  We were having customer problems with C-type thermocouples and chasing every last tenth of a degree.  Needless to say, it wasn't the customer's fault or our fault-the compensation cable didn't match the thermocouple table.  It turned out that the errors of an earlier instrument matched the bad cable better than the new one that 0had smaller errors!

...However, my fingers started to rattle the keys after reading one letter in your mailbag.  Namely the guy from Boeing looking for some strain-gauge power supplies.

Well, "my own bit on the side" just happens to be precisely that.  I sell them, at the recommendation of our National Physical Laboratory, to people wanting very high stability for precision force measurement.  Now, I took a lot of care to develop a circuit that would give Kelvin buffering to both the positive and negative outputs, and add minimal errors to the basic reference.

I haven't taken that much care over the noise immunity and earthing that Mr. Iverson is looking for.  However, I've checked a few details and can quite easily get shielded transformers specially for him.  So if anyone else is after stable 10-V power supplies - mine will put out up to an amp, making it a good laboratory reference, too.  Please pass them my address.

And now while I'm writing to you, here's a poser that bemuses me.  If I, as an engineer, wish to start up my own business at home while working for someone else, it causes all sorts of chaos.  "Company loyalty, design rights, conflict of interests" - you name it and it's there.  But now, if having left that company, I apply to a new company for a job, and openly declare my own private business, I'll be greeted by enthusiasm.  "This man has initiative..."

Jim Edwards, Quiet Electronic Designs, 60 Clayton Rd., Selsey, Sussex PO28 9DF, Tel:  (0243) 602132

That's a good conundrum.  Thanks for the info on good supplies.-RAP

Dear Mr. Pease:

I enjoyed the Pease Porridge about splices in speaker wire.  It reminded me of the philosopher Paul Thagard's essay about astrology.  He says the thing that really marks astrology as a pseudoscience is not that astrologers don't know why it should work; it's that they don't care why it should work.

The same could be said for the speaker-wire cult.  If these people had really found an empirical correlation between splices and poor audio quality, they'd be scrambling to find a physical explanation.  But no.  Like Peter Pan, "ya gotta believe" (and, in this case, pay money).

Michael A. Covington, Assistant to the Director, The University of Georgia, Artificial Intelligence Programs, Athens, Georgia

A good point, nicely worded.-RAP

Dear Bob:

Re:  What's All This Reflex Response Stuff, Anyhow?  ...This is in response to dropping a very small important item (also valuable in terms of inconvenience if lost), such as a 1-mm left-handed screw from a "grungle plunger."  Don't try to catch it or deflect it.  Just watch where it goes!

The reason?  Small items often bounce and roll a long way.  If you lose sight of it, it can take a frustratingly long time to find because it's small.

John Fleming, Research technologist, Department of Midwifery, University of Glasgow, Queen Mother's Hospital, Glasgow, Scotland

In your business, when you drop something, you darned well better watch to see where it rolls!-RAP

Dear Bob:

Sometimes I have a hard time getting my colleagues to take a simple path to an end.  But if I remember correctly, you are currently collecting other people's common-sense stuff for your next book.  I know I will have more luck getting my cohorts to try this trick if it appears in your book first.  So please accept this widow's mite.

People who design electronic projects often need to be concerned about temperature.  Hot spots accelerate degradation of materials and thereby cause premature (whatever that means) failure.  So how do we find hot spots?  If we really care about temperature uniformity, for example in a precision analog instrument, we set up a tangle of thermocouples on a sacrificially modified prototype, and wait.  A long measurement period in which we collect lots of data is followed by hours of disagreeable evaluation, culminating in dubious conclusions.

If all we want to do is find hot spots, though, we can use a simpler approach - liberal use of mercury-in-glass fever thermometers.  Fever thermometers have a number of favorable characteristics for surveying hot spots in electronic systems.

First, remember how you have to initialize a fever thermometer?  It's a peak recorder.  You have to shake it down to the arrow points before you use it.  You can shake down a few thermometers and then leave them in the system for as long as you like.  You can operate the system in all it's various modes, some of which certainly generate different temperature distributions than others.

At the end of the exercise you simply take the thermometers out and look at them, and learn what the worst peak temperatures were in all of the spots you monitored.  There is no excess data to discard.

Second, the fever thermometer's temperature range, 94º to 106º F, is near ideal for surveying commercial equipment.  By testing your equipment at room temperature ambient, you can learn about an internal 25º to 35º rise.  If it turns out to be less than 25º, it's safe and you don't care to know any more about it.  If it's more than 35º, you need to understand it better and will want to investigate it in more detail.

Third, a fever thermometer is self-contained, tiny, and (unless you break it) electrically insulative.  You can insulate it right where you want it and secure it with sticky tape.  Compared to the average thermocouple wire, a glass thermometer is an easily tamed instrument.

Fourth, the mercury bulb of the fever thermometer offers a small, well-focused sensitive target.  If located in a convective plume above a heat sink, a fever thermometer will give a surprisingly high (but accurate) reading.

If you think you want a larger target to average over a larger area, wrap the bulb end of the thermometer with several thicknesses of 1-in.-wide aluminum foil, and insulate with clear tape before installing in the system.  Or, survey the plume with three or four thermometers all at once.

Doug Raymond, Teradyne ATWC, Walnut Creek, Calif.

The temperature of a hot spot is the room ambient plus its rise above ambient.  If you want to resolve large rises, work in a cold room.  To resolve small rises, start in a warm room.  So, your thermometers are even more useful than at first glance.-RAP

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