4th Quarter 2009

(C) 2009 Design/Analysis Consultants, Inc.
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This Issue: News Bite: Giant Spider Attacks Liverpool / News Bullet ALERT: Exploding iPods and iPhones / Tech Tip: Preventing Capacitor Meltdown / Sightings: When Managers Pretend to be Engineers

NEWS BITE: Giant Spider Attacks Liverpool!

arachnoid(“Arachnoid Affair,” IEEE Spectrum, Nov 2008)


What, Me Worry? According to Apple Computer, Inc., a rash of exploding iPods and iPhones are “isolated incidents.” (Breitbart.com, 18 Aug 2009)

Our View: The claim of “isolated incidents” is believed to be highly improbable. First, the term “isolated incidents” is an oxymoron; how can a scattered group of incidents be considered isolated? Second, over a period spanning hundreds of projects and problem investigations, we have never seen a case that resulted in a root cause of “isolated incidents”; there has always been a specific cause, usually related to insufficient design margin. In this particular case, an industry history of battery explosions suggests a likely hypothesis: the batteries used by Apple are fabricated in a manner that is not compatible with their charge/discharge stresses.

Until this is cleared up, it would be wise to contact Apple and insist that they guarantee that your particular iPods and iPhones will not explode or catch fire, or provide a refund.

fire TECH TIP: Preventing Capacitor Meltdown

Stress Analysis: Capacitors


Operating within a capacitor’s core temperature rating is generally assured by confirming that the capacitor’s rms ripple current rating is not exceeded. (Vendors often specify ripple current ratings because ripple currents are a good predictor of core temperature and can be directly measured, whereas it usually isn’t possible to measure the core temperature.) For some applications involving very high ripple currents, capacitor mounting and heat sinking may be critical. In those cases it may be necessary to perform a complete thermal analysis to compute the core temperature:

Tcore = Ths + Rchs * Irms^2 * ESR


Tcore = capacitor core temperature, C

Ths = heat sink temperature, C

Rchs = thermal resistance from core to heat sink, C/W (as provided by the capacitor vendor)

Irms = capacitor ripple current, amps RMS

ESR = capacitor equivalent series resistance at ripple frequency, ohms

BugAlert Whenever possible, be sure to mount capacitors away from local heat sources, such as power devices and their heat sinks.

bozo SIGHTINGS: When Managers Pretend to be Engineers
Sightings is a collection of true experiences as reported by credible sources

Charlie the consultant was on the phone. His client, for what must have been the tenth time, was once again directing Charlie to implement yet another trivial product change.

As the client droned on, Charlie leaned back, stretched his legs, and then silently chuckled as an old adage popped into his head. The Peter Principle, from a book of the same name by Dr. Laurence J. Peter and Raymond Hull, said that “…every employee tends to rise to his level of incompetence.” That observation particularly applies to engineering administrators, thought Charlie, ones like his client, Mr. O, who lacked the benefit of an engineering education or any related experience. It all seemed so trivial to Mr. O, whose latest hot flash was that a front panel LED should be changed from green to yellow.

Charlie knew from experience that there were literally dozens of key issues to be addressed in his client’s product design, and the color of LEDs was not among them. There were critical performance concerns such as efficiency and thermal management, as well as immutable compliance mandates, including safety and EMI. In fact, compliance requirements dictated the color of the LED, so changing the color was not really an option.

Mr. O’s shrill tone snapped Charlie out of his reverie. “Well then, Charlie, what about it?”

“Um, I’m sorry Mr. O, but we can’t change the color due to safety regulations.”

“Well then,” barked Mr. O, “if you can’t change the color, then make it blink. That should be easy.”

“Well,” replied Charlie, “making it blink would not be difficult, but that would still violate the safety requirement. For its function the LED has to be on continuously; it can’t blink.”

“Can you change the name of the function?”

Charlie paused, momentarily nonplussed, and then said, “Uh, no, sorry. That would still be a violation.” Charlie sighed and waited silently for Mr. O to process the fact that the LED should not be changed.

“I’ve got it! I’ve got it!” shouted Mr. O, forcing Charlie to pull the phone away from his ear. “Just add another LED, one that’s not related to a safety function!”

“Um — well — um,” stammered Charlie, “Um, we don’t really need another function…”

“And make it pink!”

Charlie felt his gut tighten, girding himself to once again patiently overcome Mr. O’s stubbornness. But suddenly Charlie relaxed, remembering that Mr. O was paying for all of this nonsense.

“Sure,” said Charlie. “We’ll put in a function that can use a pink LED.” After a pause, he smiled and then added, “Would you like it to blink?”

Posted on September 25, 2009, in Capacitors, Stress Analysis, Temperature. Bookmark the permalink. 1 Comment.

  1. During my corporate America days, which fortunately are over, I had a similar experience. Mine was with an extremely uptight bureaucratic “security” employee. All development projects had to be approved by this guy. Ugh. One of his strange rules was that nobody could participate in his mandatory conference calls on a cell phone. This became sort of a joke as these calls were scheduled at his convenience in his time zone, so they often occurred during the lunch hour. The calls themselves were silly because he never really did understand what it was we were trying to accomplish, resulting sometimes in ridiculous edicts. Anyway, one day I became completely exasperated with this egghead and asked him WHAT COLOR he wanted the font in. He was temporarily silenced, which pleased me to no end (phone on mute at this point due to hysterical laughter), then he stated, very primly, that the color did not matter. Absolutely no sense of humor, this guy.


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