Category Archives: Stress Analysis

2nd Qtr 2011

(c) 2011 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Miraculous Emergency Landing on Railroad Track! / DESIGN MASTER TIP: AC Rectifier Bulk Capacitor Ripple Current / HUMANITARIANISM: Capitalism + Volunteer Engineering Helps Haitians / UNINTENDED CONSEQUENCES: Nanny Engineering / SHAMEFUL BEHAVIOR: Shanghai Euchips Industrial Co. Used Fake UL Label

NEWS BITE: Miraculous Emergency Landing on Railroad Track!

Ground-Effect Robot Could Be Key To Future High-Speed Trains” by Evan Ackerman, 10 May 2011 IEEE Spectrum

DESIGN MASTER TIP: AC Rectifier Bulk Capacitor Ripple Current1 

Pretty good estimates of capacitor ripple amps for full wave rectifiers driven by low source impedance can be obtained by using the equations below. Note that an exact solution requires iteration, which can be done automatically by the Design MasterTM worst case analysis software. If you don’t have Design Master, you can get some quick results by first estimating the ripple voltage and solving for tC. Then calculate Vripple to see if your estimate was close. After a couple of iterations you will zero in on good values for tC and Vripple, and then you can solve for total capacitor ripple amps.


1. tC = charge time, sec = ACOS(1-rRIP)/(2*Pi*f)
ACOS = inverse cosine function (COS-1)
rRIP = ripple ratio, Vripple/Vdc.
Vripple = ripple volts peak-peak = Idc*tD/C
Vdc = average DC output volts
Idc = average DC output amps
tD = discharge time = 0.5/f – tC, seconds
C = bulk capacitance, F
f = line frequency, Hz
2. Dc = charge duty cycle = 2*f*tC
3. Dd = discharge duty cycle = 1 – Dc
4. ICchg = ripple amps rms due to charge from full wave rectifier
= Idc*SQR(1/Dc-1)
5. ICdis = ripple amps rms due to discharge to load
= Idc*SQR(1/Dd-1)
6. ICload = rms content of pulsed load amps (e.g. input of switchmode regulator) if applicable. If load amps is purely DC, set ICload to 0.
7. ICtot = total capacitor ripple amps rms = SQR(ICchg^2+ICdis^2+ICload^2)

The great thing about analysis, as compared to simulators such as SPICE, is that you can learn a lot by reviewing analysis equations. For example, if you set the ripple voltage ratio to a desired amount (e.g. 15%), and rearrange the Vripple equation to solve for C, you can readily obtain a graph of the required ripple amps rating versus output current, regardless of input or output voltage. Now you’ve generated a general-purpose design guideline to use for numerous applications.

See the example graph below for the capacitor ripple amps requirement versus DC load currents from 100ma to 5 amps, for a 15% ripple voltage and a 60Hz source. For example, for 2 amps of load current, the capacitor will require a ripple current rating of 4.4A

1. Excerpted and revised from DACI Application Note, ” Why Power Designers Need More than SPICE to Avoid Blow-Ups.”

HUMANITARIANISM: Capitalism + Volunteer Engineering Helps Haitians

Non-governmental organizations operating on free-market principles can offer the most effective assistance to those in need. For an example click here.


Yes, the government does provide some essential functions. Unfortunately, it doesn’t have the self-control to restrict itself to those functions. The result is that we are plagued by governmental busybodies who like to justify their salaries by telling the rest of us how to behave, in areas that are none of their concern.

For example, as pointed out in Engineering Thinking, there have been numerous regulations passed that restrict our right to choose the products we may want, such as incandescent light bulbs. In that case, the government has deemed such bulbs unacceptable due to low efficiency. But if incandescent light bulbs are inefficient, that fact becomes evident in our electric bill; why do we need the government to tell us how best to spend our money?

Furthermore, perhaps some of us would, regardless of efficiency, prefer to use the incandescent type. But no, the governmental busybodies have decreed that you don’t get to freely choose. Forget about all of the other parameters that might be of more importance to you: short-term cost, color rendering, lifetime, reliability, and environmental hazards. Also, some folks in chilly climates might even appreciate the extra heat that incandescent bulbs provide. But none of these considerations matter to the one-solution-fits-everybody government.

Now, as typically happens following such governmental decrees, we find that they are rife with unintended consequences; e.g. the compact fluorescent lamps (CFLs) that the government wishes us to use instead of  incandescent bulbs have significant disadvantages:  (a) substantially lower lifetimes than expected, (b) may emit hazardous fumes (click here), (c) emit electromagnetic interference (EMI), (d) emit a color that can disrupt melatonin production and thereby cause sleep disorders, (e) sometimes create an irritating buzzing nose, (f) contain hazardous materials that pose significant environmental disposal hazards, and (g) will kill the domestic incandescent bulb industry, and replace it with products that are primarily foreign-made.

Some years ago the government illustrated similar brilliance by outlawing magnetic ballasts, again simply on an efficiency basis. It should be no surprise that the electronic replacement ballasts were more expensive, had shorter lifetimes, were less reliable, contained hazardous materials, and emitted a lot of EMI (so much so that some hospitals refused to use them because of their tendency to interfere with medical instruments).

Recently some smart engineers from China, unencumbered by the U.S. regulatory dictatorship, have created a magnetic ballast whose efficiency is better than electronic ballasts, at lower cost, longer life, higher reliability, using non hazardous materials. [1] Congratulations!

Sad to say, this is the sort of advance that was typically accomplished by U.S. engineers, back before the government decided to play Nanny Engineer.

Note 1: “A ‘Class-A2’ Ultra-Low-Loss Magnetic Ballast for T5 Fluorescent Lamps — A New Trend for Sustainable Lighting Technology,” Hui, Lin, Ng, and Yan, Feb 2011 IEEE Transactions On Power Electronics.

SHAMEFUL BEHAVIOR: Shanghai Euchips Industrial Co. Used Fake UL Label

Details here.

1st Quarter 2010

(C) 2010 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Small Alien Spacecraft Seen Landing In Australia! / NEWS BULLETS ALERT! Toyota Sudden Acceleration Update: It’s Not Just Floor Mats / TECH TIP: Preventing Output Dips In Linear Regulators / TECH TIP: Be Aware Of Aging Capacitors / DM 7.4 RELEASED: Tuned, Tanned, and Ready To Roll / HUG AN ENGINEER TODAY: Millisecond Response to Nerve Gas Threats / PECULIARITIES: The Mysterious Google Town That Doesn’t Exist

NEWS BITE: Small Alien Spacecraft Seen Landing In Australia!

(“The Tech Behind the Winning Solar Car” by John Boyd, Nov 2009 IEEE Spectrum)

NEWS BULLETS ALERT! Toyota Sudden Acceleration Update: It’s Not Just Floor Mats

The following excerpts are from “Toyota’s runaway-car worries may not stop at floor mats,” by Ralph Vartabedian and Ken Bensinger, 18 Oct 2009 L.A. Times:

“Toyota has blamed the incidents — apart from those caused by driver error — on its floor mats, asserting that if they are improperly installed they can jam open the accelerator pedal. A month after the Saylor crash, Toyota issued its biggest recall in company history, affecting 3.8 million vehicles in model years as far back as 2004. But auto safety experts believe there may be a bigger problem with Toyota vehicles than simply the floor mats.”

“…One remedy being considered by Toyota implicitly acknowledges what critics have been saying for almost 10 years: that the company’s highly computerized engine control system lacks a fail-safe mechanism that can quickly extinguish sudden acceleration events, whether they are caused by floor mats, driver errors or even unknown defects in the electronic control system, as alleged in some lawsuits.”

“…One obvious line of defense is to simply shut off the engine, a step that may not be intuitive on the ES 350. The car has a push-button start system, activated by the combination of a wireless electronic fob carried by the driver and a button on the dashboard.

But once the vehicle is moving, the engine will not shut off unless the button is held down for a full three seconds — a period of time in which Saylor’s car would have traveled 528 feet. A driver may push the button repeatedly, not knowing it requires a three-second hold.”

[DACI’s comment: Requiring three seconds to shut off a raging auto engine is wildly inconsistent with safety concerns, and indicates a case where a software feature (3 second turn-off) was not appropriately reviewed by the Toyota engineers responsible for safety.]

TECH TIP: Preventing Output Dips In Linear Regulators

Functional Analysis: Regulators/Linear

The design of linear regulators must consider the valley of input ripple voltage, not just the average minimum DC value. Failure to consider input ripple can result in unacceptable output ripple. (Design MasterTM subscribers can download a sample analysis here: LM317T.)

TECH TIP: Be Aware Of Aging Capacitors

Stress Analysis: Capacitors

Capacitors that are operated beyond their rated lifetimes will exhibit a significant reduction in capacitance, and suffer other parameter degradation as well. This can easily happen to electrolytic capacitors when they’re subjected to higher ambient temperatures, or to increased internal temperatures due to ripple current. To avoid trouble, be sure to check each capacitor’s lifetime rating against your application’s requirements. (The LM317T sample analysis mentioned above shows how DM’s Electronics Part Analysis Wizard was used to import part files for a full stress analysis, which includes a capacitor lifetime calculation.)

DM 7.4 RELEASED: Tuned, Tanned, and Ready To Roll

Design MasterTM V7.4 (Major Upgrade) has just been released.

Building on more than a decade of successful applications worldwide, the latest version of Design Master rolls into 2010 tuned, tanned, and more ready than ever to help you provide bulletproof designs.

We improved the text formatting and toned down the bright screen colors, added a handful of small but useful enhancements, and then went through and tidied up a few little quirks that are invisible to the average user, but may slow a power user down a bit.

Last but not least, for improved efficiency and flexibility we made one of our powerful under-the-hood algorithms accessible to the user via a new Calculate/Auto Scan option.

For more details, please click the link below. If you’re a subscriber, you can also download your update and the latest manual:

Design Master V7.4

Design Master Manual

If you haven’t started using Design Master yet, you can learn about our free fully-functional 10-day trial version here:

Using Design Master To Create Bulletproof Products

Many thanks to all of you who have helped us improve Design Master over the years!

HUG AN ENGINEER TODAY: Millisecond Response to Nerve Gas Threats

“A new molecule that detects and destroys lethal nerve gases has been developed by researchers in the US. It is hoped that the research will help develop new early-warning systems against chemical weapon attacks, and possibly give rise to an effective antidote.”

From “Nerve gas detection in a fraction of a second” by Lewis Brindley, Chemistry World, 24 Sep 2009

PECULIARITIES: The Mysterious Google Town That Doesn’t Exist

As initially reported in the London Telegraph, a town on Google Maps called Argleton, Lancashire, England, does not exist, even though details such as businesses and photos are provided. A traveler following Google’s instructions will be rewarded with a visit to an empty field.

According to the Telegraph, “Google and the company that supplies its mapping data are unable to explain the presence of the phantom town and are investigating.”

(As reported by Robert Charette in the IEEE Spectrum)

4th Quarter 2009

(C) 2009 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced sources is prominently displayed with the copied material

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.” (, 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?”