On Sun, 09 Dec 2012 05:43:40 -0500, Spehro Pefhany
wrote:
On Sat, 08 Dec 2012 22:35:19 -0800, the renowned Jeff Liebermann
wrote:
Yes. The conspiracy theory is that components are selected to survive
no longer than the warranty period.
I think it's more a design spec than a conspiracy.
My first clue of a conspiracy was about 15 years ago, when I was
fixing a Sony CRT monitor (in the days when Sony made complete
monitors). It suffered from the usual electrolytic capacitor
failures. On any other brand, I could replace one or two caps, and it
would work. However, on this Sony, I discovered that a rather large
number of caps failed an ESR test, making repair uneconomical.
Inspecting the components, I noticed that there was a wide range of
rather strange voltage ratings on the electrolytics. In the past, I
would see dozens of electrolytics in a small number of standard
values. In this Sony, I saw a rather wide assortment of values and
voltages. I didn't do any lifetime calculations, but my impression
was that the voltage ratings were calculated for some target lifetime,
and no more. The result was near simultenous failure of all the caps.
Looking at a typical specs of the internal parts of a linear wall
wart- at full rated current, the winding temperature rise is 70°C.
At a 30°C average ambient, if we assume the temperature in the wall wart
interior rises only 15°C (say half rated current), and 10°C internal
cap heating from the ripple current, we'll have the cap at 55°C. With
a 105°C/2000 hour rating, we should get
Life = 2000 hours * 2^((105-55)/K) where K is 10-15
Assuming K is 15, we get a life of 20,000 hours. Plugged in all the
time (24/7) that's about 2-1/3 years.
Run it at full rated current in a high ambient and you'll be at a
small fraction of that.
That sounds a bit on the low side. Let's see how it compares with one
of the online capacitor lifetime calculators:
http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx
For the wall wart, I used:
L1 = 2000 hrs Load Life Rating
Vr - 16VDC Maximum voltage rating of capacitor
Vo - 12VDC Operating voltage of application
Tm - 105C Maximum temp rating of capacitor
TA - 30C Ambient Temp.
which results in 483,000 hrs or 55.1 years.
Sheesh, no input box for ripple current. Plugging the same numbers
into the above equation:
x = (Tm - Ta + Tr) / 10 = (105 - 30 + 10) / 10 = 8.5
L2 = L1 (Vr/Vo)*2^x
L2 = 2000hrs*(16/12)*2^8.5 = 2,666*362
L2 = 965,000 hrs = 110 years
Here's yet another formula:
http://www.low-esr.com/endurance.asp
L2 = L1 * 2^(T1-T2/10) = 2000 * 2^((105-30/10)) =
L2 = 362,000 hrs = 41.3 years
Assuming that I'm not making any of my usual math errors, there seems
to be a discrepancy between the various methods of estimating
capacitor life.
There are caps that will last longer, but they're too expensive to use
in an AC adapter.
There are extended life capacitors, with up to 10,000 hrs rated life
for commodity components. I don't have a clue on pricing. Example:
http://www.niccomp.com/help/presentations/AlumE-CapExtendedOperation0809-revA.pdf
http://www.low-esr.com/endurance.asp
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558