On Sat, 29 Jun 2013 12:08:41 -0700 (PDT),
wrote:

Could this possibly be an operating temperature? Or is it a storage temperature? Or perhaps it's an internal temperature?

No, No, and no. They're the absolute maximum operating temperature,
at which the maximum safe applied voltage hits zero.

The 85/105C temperatures are actually the "knee" on the derating
curve.
http://www.navsea.navy.mil/nswc/crane/sd18/Pages/Capacitors/CapacitorsDerating.aspx
Aluminum electrolytics are near the bottom. See Fig 13:
http://www.navsea.navy.mil/nswc/crane/sd18/Images1/Capacitors/CapacitorsDerating14b2.gif

The temperature also has a big effect on the life of an electrolytic.
http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx

It would seem like it would have to be a very high frequency component
to ever cause an electrolytic to ever approach anything like this.

Wrong. A capacitor only draws current when the voltage across the
leads changes. The capacitor only dissipated power, and converts it
to heat, when the voltage changes. Pure DC across a capacitor does
nothing to produce heat. The AC voltage change might be nothing more
than a few millivolts of ripple in a switching power supply, but since
the current in such supplies is huge, the heat dissipated is quite
large. Ignoring frequency dependent effects, the power dissipated is:
Power = Ripple_voltage^2 / ESR
or
Power = Ripple_current^2 * ESR
where ESR = equivalent series resistance. For example, the CPU filter
caps are typically 1000uF/6.3V electrolytics (0.12 ohms ESR). With a
current probe, I can usually see at least 4A average ripple current on
the CPU power leads. While trying to keep the voltage constant over
such large current variations, each cap would smoke:
P = 4^2 * 0.12 = 2 watts each.
There are typically about 10 caps in the string, dissipating a total
of 20 watts. That's gonna get hot.

Could one of these parts rated as such actually get this hot and
remain operational?

Sure. I've burnt my fingers on electrolytics and the machine
continues to run. I just replaced an ATX power supply on a server
that was running 24x7. Every cap in the PS had the top blown out by
boiling electrolyte. Yet, the only indication of trouble was that the
speedtest.net performance was erratic. When I finally turned off the
power to look inside, things cooled down. Then it wouldn't turn on
again, so the problem was obvious.

Would this actually be within prudent design parameters for the device?

No. However, there are many design parameters. The one that should
be a concern is the expected lifetime. See the formula at the top of:
http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx
One can easily calculate when the capacitor is going to being giving
problems. If you're designed for a 1 year warranty and a 5 year
target life, it makes no sense to use more expensive parts where the
caps might last 30+ years, when you can save a few pennies making it
blow up just after the warranty expires. (Hint: I always recap with
better or higher voltage caps).

In theory if the caps are not actually operating at even 85 degrees C
then why wouldn't you be able to use a lower rated temperature cap
for that application?

Look at Fig 13 in the URL I cited. At 85C, the maximum working
voltage is zero.

It would seem to me that if a piece of equipment were designed to
run a capacitor that hot or even at 85 degrees C for whatever reason
then in my mind that would certainly constitute a very poor design.

Well, the math is easy enough. 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.

I have been repairing TV's for many years and the only capacitors
I've ever seen get too hot to touch were bad ones.

Generally true. You can take an IR thermometer, thermistor probe,
thermocouple, or if you have money, an FLIR thermal imager, and
measure the case temp. Note that the inside temperature is much
hotter. That works, until you try it with CPU filter caps, which are
heated via radiation by the CPU and by the fan via convection.

Could someone please explain this rating to me? Thanks, Lenny

See Fig 13 again.

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