Very durable electrolytics or similar?

There are a lot more stresses than just temperature lifetime rating. "Reliable Application of Capacitors", 1996 by RAC is referenced all the tim e, and you're just going to have to borrow it, possibly electronically. I d oubt you can make 90,000 hours MTBF, which may not even be the appropriate statistic for your application, like maybe you want 90% and not 50% chance of lasting 10 years. Then ask yourself if you can't design this crazy requi rement out.

Elsewhere you estimate 500mA ripple, right? Even if ESR=1/2 ohm and cap-bank-to-ambient resistance is 20C/W, that's only an extra +2.5C.

Your worst problem is T(ambient).

You won't like hearing this, but I measured some roadway temperatures some years back, and >100oC is quite common. I'm not sure which notebook my notes are in, but ~110-115oC range, SoCal summer, IIRC.

Cheers, James Arthur

If you can stand the cost, get a few and put them on life test at

150C; if they retained the quality i saw for the Sprague TE series, you may be rather pleased that the caps easily exceed datasheet specs.

** The OP is full of bullshit.

His app does not need special electros - cos the temp is no more than electros in valve amps regularly get to and it is nothing like 100% of the time - more like 10% or 20% at most.

.... Phil

That rule of thumb should apply to semiconductors, but does it also apply to electrolytes ?

A decade is about 100,000 hours and as Phil Allison has several times pointed out, a device would be in heavy sunlight only for about 10 to

20 % of the time each year, then you should search for a component with 10,000 to 20,000 hour life time _at_your_ maximum temperature. This of course assumes that the derate curve is as steep as for semiconductors.

Assuming that the internally generated heat in the device is kept low the 12,000h@105C would not that far away.

I have seen product announcements for miniature fans with mounting hardware for different size electrolytes. Just press fit the fan assembly on top the capacitor :-).

With such high day time temperatures, the air is typically quite dry, so the temperature drops quite rapidly after sunset. In the early hours of the morning, the temperature can be quite pleasant at 25 C or even lower, so this is the best time for any outdoor activities at 6 am :-). The temperature of your capacitors would drop even more from the afternoon sunshine to the cool morning, thus the number of hours pent at high temperatures is not that high.

Is the box mounted on top of something and being exposed to hot sunlight for 8-10 hours a day, or is it mounted on a side of something bigger, then the hot sunlight exposure could only be for 4-5 hours.

Assuming that heat is the only mechanism drying out capacitors, just count how many hours a year the device will be exposed to those top temperatures.

Assuming you are talking about ripple frequencies well above mains frequencies, have you considered using some inductance for energy storage. Of course, ferrites have their own temperature limits, but I have never heard about gradual inductance drop due to temperature (aging).

Peltiers do not like strong vibrations, but they might be usable in non-vibrating environments.

You also need to select sufficiently big one to get even a half decent efficiency. You need to feed about equal amount power into the Peltier as you are going to remove from the cold side in the best case.

Of course you must then remove twice the electronics power (electronics+Peltier) to the ambient on the hot side. There are limits of the maximum hot side temperature, complicating the situation further.

One must be quite desperate to use Peltiers for heat extraction, but of course these devices are good for cooling things well below ambient e.g. for low noise operations in e.g. CCDs cameras.

500mA is a lot for a higher voltage cap unless you parallel a lot. Which I'll do, but that has practical and financial limits.

Yes, definitely.

Well, in my case there will be some natural convection which the roadway won't have on a windless day with scorching sun. The temps have been verified in the field.

--
Regards, Joerg 

http://www.analogconsultants.com/

At least it's often used in that industry.

Make that 30% in places like New Mexiko or Arizona.

IMHO 105C/12000h is a close call, I need something at least a little better.

--
Regards, Joerg 

http://www.analogconsultants.com/

Then one of those haboobs roll through and afterwards the whole works is all clogged up :-)

--
Regards, Joerg 

http://www.analogconsultants.com/

Unfortunately I don't have the >2000h required for this.

--
Regards, Joerg 

http://www.analogconsultants.com/

Sure, bus this machine works 24/7 all year.

Doesn't win you much because the thermal delay in the box is compensated by trailing high temops after dusk when it has to "unbake".

Sitting way out there in the middle, worst case a desert. No shade within the next 20 miles or so and it cannot be covered.

Probably 10-30% in some areas closer to the equator.

--
Regards, Joerg 

http://www.analogconsultants.com/

That would be a challenge because there is nowhere to sink the heat out of the peltier, plus it is prohibitively costly.

From the big space heater up there in the sky :-)

--
Regards, Joerg 

http://www.analogconsultants.com/

Inductors get too big and expensive for this :-(

--
Regards, Joerg 

http://www.analogconsultants.com/

I have a switcher in there already and not a problem to make that bidirectional. The dry polymer caps could be a good idea, didn't know there were so much better than electrolytics.

--
Regards, Joerg 

http://www.analogconsultants.com/

Oil caps fail a lot.

Size should just not be outrageous, meaning not a whole lot more than it would be with electrolytics. Weight is not important.

--
Regards, Joerg 

http://www.analogconsultants.com/

It can be done. Many design from 50+ years ago have proven it. Our Hammond organ from 1961 sometimes was left on for weeks at the first owner (who had it for over 40 years), mainly because you barely notice it's still on, after playing it at a party and then moving on to the martinis. Cap directly next to the rectifier tube, I have no clue what has ridden the engineer back then to place it there. Never failed.

Same for all the tube radios that I recently donated to a museum. At least the one from my grandpa ran pretty much all day. It's still on the first electrolytic.

I repaired a lot of TVs and radios as a kid. Failing electrolytics back then usually were the ones of the more dubious brands. Same with tubes where one had to make sure not to buy any of the "bargain" ones from behind the iron curtain (but that's different now).

--
Regards, Joerg 

http://www.analogconsultants.com/