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Another capacitor question (durability).

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LAN_403

MikeT1982

Mon Jun 04 2007, 04:29AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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It seems to me that a capacitor can take a heck of a "hit" during discharge. By this I mean like my electrolytics will into my flashlamp or when used in coil guns and the like. Can an electrolytic stand up to being charged nearly fully and dead shorted many times or is there a better capacitor for the job? Also can you "slam" charge a capacitor the same way many time (like hook a small electrolytic to a very stiff current supply so it charges as fast as it discharges into a dead short, or will this harm it? Thanks.

...

Mon Jun 04 2007, 04:57AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
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You can charge the caps as fast as you want. We use a resistor because the caps appear as a dead short when you start charging them, so if you do not have a current limited power supply you blow out fuses, switches, transformers, etc. When a NST you can only get ~100ma out, so you do not need any ballast.

MikeT1982

Mon Jun 04 2007, 05:02AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Cool man thanks for reading my first post, I re-typed it more to the point thats why it asks a sorta different question now LOL! My apologies. To come to think of it, caps must be pretty sturdy in those BIG pulsed lasers that rapid fire! I guess thats why the darn power supply towers are as big as me to charge them fast.

Bjørn

Mon Jun 04 2007, 05:13AM

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The datasheet for each model of capacitor will have most of the information you need. Some capacitors will take damage or have greatly reduced lifetime in pulse applications.

Electrolytic capacitors have a limited life time that is very dependent on temperature so charging/discharging often enough that there is a temperature rise will reduce life time.

Hazmatt_(The Underdog)

Mon Jun 04 2007, 06:57AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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You might want to consider how the capacitor is constructed internally. The very thin conductors that make up the plates must handle that di/dt. When they cannot handle that surge they can detach themselves from where ever their weakest point is. That may be the connection to the capacitor roll, or maybe at the terminal stud. In any case, the thin 'leaves' have to conduct that energy to the outside world and they may not be able to do that without heating, distortion from eddy currents, or what-have-you.
Likewise the anodic coating on the capacitor roll and its dielectric must be robust enough to handle the stress you put on them, wether it be charging or discharging.

I have used electrolytics that are not pulse rated for some pulse applications and haven't seen one die yet, but I'm sure they're not happy. I should have counted the number of shots so far to see when they decide to short. My guess is you will get a pretty good amount of use out of the electrolytics, and they're pretty cheap to replace compared to pulse caps.

Dr. Shark

Tue Jun 05 2007, 10:13AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
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I think the situation with electrolytics is a bit different from pulse caps. Since they are not constructed for ultra-low inductance, they cannot discharge fast enough to cause mechanical failure due to elecromagnetic forces. They also have quite a bit of internal resistance because of the poor conductivity of the electrolyte, so this is where most of the energy will be dissipated as heat. The temperature rise from one shot will probably not do much bad, but repeating this at a high rate would be a sure death for the cap. So if you allow for a long "inter pulse interval", electrolytics are probably most forgiving when shorted out, just because they cant be "shorted" as "short" as proper pulse capacitors.
My experience seems to back this up, as long as you don't overvolt or reverse charge them, electrolytics take abuse quite happily.

MikeT1982

Fri Jun 08 2007, 04:24AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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That's good to know man, I used those formulas you gave me in that awesome link to the laser lessons, and came to the conclusion that the resistance within my flashlamp during disharge is approx. 4-5ohms. The reason I say approximate is because I did the equations twice with a low value and high value of my possible error for measurement and both answers fell between that. SO the caps will have a 4-5 ohm load to fire into so that's good then! That plus the fact that my tube and rod will need to cool down for 3-5 minutes between shots and my charging supply can only yeild 90mA max to the 16 3,900 microfarad caps (seriesed for 244uF at 6400 volt capacity)...I think I will gamble and throw out a conclussion (based on little experience but from what I have observed and learned) that I feel they will have it decently easy!

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