A shower of sparks

Munch (an ex-cow-orker) sends this link describing in detail what happens with guys and electricity and the square root of two and girls.  Here.

My only brushes with the Irritable Fire God Who Lives In The Walls have been fleeting love-taps, where one moment I’m poking a screwdriver into a circuit board, and the next my hand has teleported to vertical, like I’m trying to get the teacher’s attention in class, and the screwdriver is embedded in the ceiling.

That, and the time I held a whole farad capacitor in my hands.  A whole farad.  Wow…

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8 Responses to A shower of sparks

  1. $mike cremer says:

    Munch’s story was excellent, and while I can’t top it I can relate. In EE181 we built a drive and control system for a robot arm. The main arm was driven by a 90VDC motor. We ended up jury-rigging the power supply from an autotransformer and two hellaciously large caps (but still probably only 250mF or so) to smooth the voltage. After proving it could drive the motor, the prof warned us not to touch the caps since they “packed a helluva jolt; about as much energy as a hand grenade” and pointed out we needed some kind of bleed circuit. I started scribbling some quick calculations while Steve rooted around in the spares box to see what we had on hand. John, meanwhile, decided to drain the caps so they wouldn’t be dangerous using the TV repair method: he shorted the terminals with a big screwdriver. “John! No wait…!” *BANG* Lucky he wasn’t touching the blade since the screwdriver was arc-welded to the cap.

  2. Gridlock says:

    PCB in one hand, wire cutters in the other, old family TV completely dismantled except for this one fairly thick black lead that doesn’t seem to unplug.. Snip..

    Wire cutters exit through window to my left, arm stops twitching within the week.

    Valuable lessons learned.

  3. James says:

    That damn irritable zappo god lives in my car seat too. It’s taught me to fear getting out the car and closing the door. Every single time I slide off the car seat and touch the door *kazap!* *arrgh!*.

  4. harborpirate says:

    I dismantled a Sega Game Gear in high school, eventually frankensteining a nintendo pad onto it because the thickness of the unit made the controls uncomfortable. I made a thick gauge wire stand for it too, so you could take it somewhere and set it on a table and play with the pad.

    Anyway, in this project I got a little careless and left it plugged in while I was working on the circuit board several times during a testing cycle. Everything was fine until I accidentally bridged 4 or 5 caps with my right hand one time. Good thing they weren’t very big, I just got some initial pain and brief numbness out of it.

  5. George says:

    How big, exactly, is a whole farad cap ? In the electronics lab, they had lunch box sized 250 mf caps, so I’m curious how big a 1f cap is (and one that can be held in hand ?!)

  6. Jon says:

    When I was in high school I had built a rather powerful strobe light from a kit. It was unplugged, but there were some whopping caps on that thing that I must have brushed with my hand. I was standing barefoot on the concrete floor in my parents’ basement, and I remember the jolt, running around yelling afterwards and that the bottoms of my feet really hurt. Yowza.

  7. One-farad and even thousand-farad capacitors can be quite small these days; electrolytic capacitors have long allowed your plate separation to be a few atoms in thickness, but getting a large plate area in a small volume was still a real challenge when the plates were made of rolled-up aluminum foil (and the electrolyte).

    It turns out that foams have a lot of surface area; the pores in a foam can be a lot closer together than the thickness of a practical sheet of aluminum foil. An foams with smaller pores have more surface areas. If I understand correctly, the “supercapacitors” of the last fifteen years or so are basically two electrolytic capacitors back to back which use a carbon foam for one electrode; the highest-capacity ones use a carbon gel, which is a liquid-filled foam with extremely small pores.

    The downside of these capacitors is that they can’t withstand more than two or three volts. So people who are building quarter-shrinkers or linear particle accelerators or high-voltage power supplies or other such things have to use capacitors with a thicker, more stable dielectric, and those capacitors are still big, heavy, expensive, and low-capacitance.

  8. Ashleigh says:

    A belt from 300 V dc as used in old valve TV’s and radios aint much fun I can tell ya. Tends to get them little fingers out of the circuit in a damn hurry. Did that when I was about 14 or 15. Learned a big lesson from it. (Like…. writing software is safer!)

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