On Sat, 29 Jun 2013 14:06:33 -0700, Jeff Liebermann
wrote:

What's the highest ambient temperature
you plan to operate the LCD monitor? Looking at various random spec
sheets, it looks like 85C is the common maximum for commodity
monitors. With an 85C capacitor, that means that you have absolutely
no way to make this work, because you can't put any voltage across the
cap. So, we go to 105C caps, which now allows you to self-heat the
capacitors with:
105 - 85 = 20C
rise in temperature. That should work, but only if you also take into
consideration voltage deration and expected lifetime.

Oops. I goofed. I was looking at storage temperatures. Maximum
operating temp is about 40C. Might as well do it right. See:
http://www.navsea.navy.mil/nswc/crane/sd18/Images1/Capacitors/CapacitorsDerating14b2.gif

Temp at 70% Temp at 40% Temp at 20%
of rated voltage of rated voltage of rated voltage
105C rated 75C 85C 95C
85C rated 55C 65C 75C

An 85C rated cap, with 70% of the rated voltage applied, can run up to
65C with a self-heating rise of:
65C - 40C = 15C
The relationship between watts dissipated internally and temp rise for
electrolytics is different for each package. For small Al caps, I use
about 5C/watt. So, this cap can dissipate internally:
15C / 5C/watt = 3 watts
Now, if we replace that with a 105C rated cap, with 70% of the rated
voltage applied, it can run up to 85C with a self heating rise of:
85C - 40C = 25C
This cap can now burn internally:
25C / 5C/watt = 5 watts
In short, the 105C cap can handle about 60% more ripple current for
the same internal temperature rise and:
(5 / 3)^0.5 = 1.29
or about 29% more ripple voltage than the 85C cap.

There's an example of the math he
http://electrochem.cwru.edu/encycl/misc/c04-appguide.pdf


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558