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Electrolytics in a vacuum

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Turkey9

Mon May 18 2009, 04:49AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Does anyone know if electrolytic caps will survive in a high vacuum environment, as in space? I know that when they blow pressure builds up inside so that suggests that there's some kind of fluid inside and won't a vacuum make them burst? Could I fill one with epoxy or something or will that screw with the electrolyte?

...

Mon May 18 2009, 05:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Just plain ole vacuum probably wont hurt them much (my only concern would be if there are air bubbles in the electrolyte they will expand and could cause issues), but if you plan on using them in space you should be more worried about the huge temperature differentials that it would have to handle. I would also expect them to outgass like crazy, which may cause them to burst, it depends on the vapor pressure of the electrolyte. I would recommend using tantalum caps, unless you need kilojoules of energy storage tantalum caps can be quite competitive with electrolytics in pricer performance and size.

Sulaiman

Mon May 18 2009, 06:16PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I repair industrial (not Space) electronics and the MAIN cause of all failures I've come across is misuse/abuse, a close second is electrolytic capacitor leakage.
If the loss of capacitance or increase in esr doesn't cause a problem,
the electrolyte eating away the copper tracks will! (Think 'Alien' blood)

Look at the specifications for electrolytics, especially
- temperature range effects
- expected life ! 1,000 hours is common to 10,000 hours is good. Typically 1 year !!!
(that's here on earth)
I like Multi Layer Ceramic Chip (MLCC) capacitors for reliability and low esr,
though I've not considered an off-earth environment.

There must be a plethora of NASA papers on component choices etc.

I certainly would NOT consider electrolytics for a vacuum environment
even worse given the temperature extremes expected.

P.S. Tantalums are good except that when they fail they go really short-circuit
you'd have to explode them to clear the fault. Not good in space.

Backyard Skunkworks

Mon May 18 2009, 06:41PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

If your dealing with very small electrolytic capacitors, I'd figure you could somehow seal them in in an air-tight fashion to prevent them from ever seeing high vacuum. That in combination with very good thermal insulation might make it workable. Obviously this won't work with larger capacitors, due to the need for a safety vent.

Maybe with some brainstorming and careful engineering it could be possible to design larger electrolytic caps that are both able to contain 1 atm of air, while bursting upon serious overpressure. The thermal issues are still there, but careful design of the whole device along with custom made caps might make it possible for lytics to work in space.

Mattski

Mon May 18 2009, 09:43PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Even if they don't burst, unless the capacitor housing is completely airtight, and I suspect that it isn't, the dielectric will evaporate at a much faster rate. This means it will fail spec faster than expected.

The best solution is typically to go with a solid capacitor, either ceramic, mica, polymer, etc. The new-ish polymer capacitors are supposed to have nice energy densities.

If this is actually for a space application, then you probably want to use space-qualified parts, so you should talk to the manufacturer. Otherwise you'll need to qualify them yourself with a vacuum chamber with temperature control.

hotcrazyfruit

Tue May 19 2009, 12:28AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147

liquids + vacuum = boiling (depending on the case of the capacitor). so my guess is that the electrolytic fluid would simply boil and they would be ruined. and you would have acid everywhere >_>

Turkey9

Tue May 19 2009, 04:30AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I'm going to use the caps as an energy bank with pulsed application into an inductor, like a induction launcher. I think I'll end up using around 250j... so about 400 uf and at 1100 V. Could I series tantalums or MLCC like sulainman said to get to that kind of voltage? How would both those perform in a cockroft-walton configuration? Would building a huge ceramic cap bank be very feasible? I had forgot about the temperature issue... I would like the device to be designed to operate in a space environment but really it just needs to be able to work in a vacuum chamber.

Proud Mary

Tue May 19 2009, 01:20PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

No no no no!

Water boils in a vacuum, doesn't it?

With the larger capacitors, once the differential pressure across the pressure relief valve ( pvent in UK nosology) valve across , the capacitor will start to vent.

Clearly, you must look to the data sheets for these parts to see what pvent is at your particular temperature and pressure.

If you have 'brand x" capacitors without data sheets, don't even waste your money.

GeordieBoy

Tue May 19 2009, 02:09PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Rolled polypropylene/foil capacitors are probably your best bet. You can get these in large values without the associated problems of electrolytics. They are often used in aerospace applications that prohibit electrolytics, but be prepared to pay $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

-Richie,

Bored Chemist

Tue May 19 2009, 05:08PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

It's true that water boils in a vacuum. It's also true that it evaporates in ordinary air unless it's in a sealed container so the caps' cans must be sealed. I'd not like to have to try it but I suspect that you could draw a vacuum on an ordinary 'lytic for a while and let the can handle the excess pressure differential. You can always seal up the vent with epoxy glue- after all, if it fails it doesn't matter what else it takes out- the thing will be dead anyway.

Remember; In space nobody can hear you call the engineer.

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