Pulse power > Capacitor Banks

Cautionary tale of unintional pulse power and hearing loss

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davekni:
I purchased some bulk caps on EBay for DRSSTC use, so was testing them.  While connecting a resistor to discharge a cap after leakage-current testing, I bumped an alligator clip across the capacitor terminals.  The resulting bang hurt my ears, and it now appears I will have permanent hearing impairment in my right ear.  The capacitor is rated 20mF (18mF measured), and probably had between 350 and 400V charge at the time of shorting.  (Was charged to 450V, but it had discharged some before my accident.)

I regularly wear hearing protection when running my DRSSTC or Marx generator.  Going forward I'll wear protection in many more situations.

Mads Barnkob:
I wear safety glasses and ear defenders in all cases of testing and high power/current/voltage. This is after several experiences with exploding silicon, flying scrapnels of plastic casings, copper pieces and arc flashes.

Worst was an underestimation of the remaining charge on my 4kJ maxwell capacitor, so with ear defenders around the neck after a shot, I just wanted to short the remaining out with the earthing crowbar circuit, it was however around 3kJ left on it and it was ear deafening for hours. No permanent damage that I can tell, short pulses are usually not damaging unless you have bleeding ears, compared to long exposure like going to a rock festival for 8 days straight.

Hydron:
Ouch, sorry to hear that, hopefully it's something that recovers somewhat over time.

I have a rather similar capacitor (well a bank of 4 smaller ones) - 24mF @ 450V. When I did the maths on the energy stored it made me nervous, so I've recently build an over-current tripping device (using a massive low-Vce(sat) IGBT and DC current sensor) set to interrupt things in a few micro-seconds if it sees fault currents above what I'd expect to be drawn during use. This way I can use it to power my QCW coil (especially during development) without quite so much fear of loud bangs and flying silicon in the case of a mishap. I think I'll still don the safety glasses and ear defenders when using it at higher voltages though, at least during development.

MRMILSTAR:
Sorry to hear about that Dave. A 2 KJ shot will definitely wake you up! Just one of those caps would make an excellent can crusher if it can withstand the voltage reversal. When you wrote "mf" I assumed that you meant 20 microfarads but then I looked at the link and saw that it was 20 millifarads!

Even though the solid-steel blast box on my coin shrinker greatly reduces the noise and contains the shrapnel, I still wear ear defenders and eye protection when operating it. Its still quite loud when firing at around 7 KJ, about like a small pistol. Before I improved the blast box seals I actually got injured by a piece of copper shrapnel escaping through some hair-width seams in the lid.

klugesmith:
Sorry to hear about the accident.
Could be worse -- thread on laserpointer forum by an enthusiast who got a permanent vision defect from momentary accident, and on chemistry forum by a guy who lost most of his hands while preparing an explosive for the 10th time.
Steve, re. can crushing: I think there's a huge efficiency penalty when using an electrolytic capacitor, no matter how many joules.  Can't make a work coil with less than 1 turn to match the relatively low voltage, and I think the parasitic R and L don't support 10 microsecond discharges.   

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