Not The intended Use by Lizard Boy
February 24th, 2008 2:35 PM / Location: 34.106234,-117.7090Fortunately they're still very good at it. One of my robotics team members recently took apart a cheap disposable camera so that we could get the lens out of it, and as an added bonus we got the flash circuit, including a fairly large capacitor.
120 uF at 330V. That's over 6.5 joules at full charge (which the transformer on the circuit allows). That may not seem like much, but here are some equivalents:
The kinetic energy in a baseball traveling 21 mph.
The potential energy in a 12 lb bowling ball being held 5 inches above your head, or a 1 lb weight 5 feet above your head. (Using your head as the zero height point.)
Now, typically mechanically stored energy is a lot more impressive than electrically stored energy. This is because you can't see electrically stored energy, and transforming it into mechanical energy so you can see it is rather inefficient. The nice thing about capacitors is that there's an easy way to make the energy visible: a wire. When you connect the ends of a capacitor with a wire, they discharge FAST. I'd do the math to give an indication of just how fast, but I'm lazy and it would come out as a differential equation anyway. Nobody likes differential equations.
Without further ado: the video of discharging said capacitor:
(If you want to, you can skip this. I think the pictures are more impressive anyway.)
You'll notice that after I charge the capacitor I don't touch the board with anything that doesn't put insulation between me and it. I do not want to serve the role of the wire in that video (which came away with distinctly blackened ends). Unfortunately, the quality of the video makes you miss some things:
1) When you charge the capacitor it makes the distinctive whining sound you get when you charge the flash on a disposable camera. Not surprising, but much more intimidating when the circuitry is exposed.
2) The spark is LOUD! It's the sort of thing that made my suitemates come out of their rooms asking what happened. Well, until they got used to it, that is.
I've included before and after pictures with commentary below. I've also included the 4 (FOUR!) frames of video that include the shock as still photos. I encourage viewing some in a separate window and zooming in (especially the after shots and the third frame), I didn't crop in order to maintain context.
before1.JPG
The solder joint furthest from the camera just to the right of the flash will be one target.
before2.JPG
The solder joint just beneath the flash will be the other target.
after1.JPG
Nice new scorch marks!
after2.JPG
See the bright spot on the top of this solder joint? That's a new concavity in the solder. I.e. the spark melted and splashed the solder. Now, an interesting note about this: you don't get that splash on the side where the wire first came into contact with the board, only the second one to touch. This happens repeatably. This is notable because it means that the air between the wire and the second contact point is ionizing. I.e. this literally creates a miniature lightning bolt.
spark1.jpg
Just before the spark goes off. You can see a little bit of brightness at the bottom of the picture.
spark2.jpg
SPARK! I find it particularly impressive how much lens flare there is, and how bright the reflections off my hands are.
spark3.jpg
Next frame. This is, I think, the most impressive picture of the bunch. It doesn't look like much, until you notice the small blue spark on the UNDERSIDE of the board (just beneath the near contact).
spark4.jpg
Last frame that includes any spark evidence. There are a few lines of light coming out from underneath the board. I don't really know what caused that effect.
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Capacitors! Which are electric ballast! And fireworks! Oh, yeah!
Yay for electronics! I wish they'd switch to charging circuits that didn't make that whine...
I think that typical flash caps are limited by their series resistance, which is ~ 0.5-1 ohm. So 500 amps would be a reasonable guess, and also seems plausible looking at that picture. 500 amps would discharge that cap in 60 microseconds, and you should have well under 1 microhenry inductance all told, which says it could reach 500 amps in a few microseconds. I would say with some confidence that you probably had a 100-500 amp spark :) And no diffeq's!
Oh, and tantalum capacitors are fun too. You can plug them into your breadboard backward, and they'll explode. Except they'll wait ten seconds or so to do it. You know, long enough for you to turn power on, verify that there wasn't any smoke, and be staring closely at the circuit wondering what isn't working...
See, the advantage you have over me with those calculations is you remember the rules of thumb for how capacitance, inductance, and resistance interact. It's been over a year since I've worked with that in any seriousness (okay, so I made some op-amp circuits last semester), and I never really meshed well with analog electronics in the first place.
Yeah, nobody likes a differential equation :)
You can actually get by with some really really simple rules. Capacitors oppose change in voltage, inductors oppose change in current, and resistors convert between current and voltage. Then all you have to remember is that the units are metric and therefore actually friendly, and you're done. So If I apply 1 volt to a 1 henry inductor, the current through it will change, but not instantly (because the inductor opposes change). How quickly? Well, more voltage should make it change faster, and larger inductance should make it change slower, so we need V in the numerator and L in the denominator. So 1V / 1 H = 1 A/s.
For your circuit I guessed at the inductance (somewhere in the 100nH-1uH range seems likely), so 300V / 1 uH = 300A/uS = 3E8 A/s.
You know, it's almost a shame capacitors have gotten so much smaller... they look cooler when they're huge.
To be fair it looks like that has much higher capacitance (albeit at lower voltage).
But really, would you want to carry around something that size in your camera?
It's nominally 15V at 65000uF, or 7.3J -- only slightly higher than the one in your praxis. I assure you it's equally capable of arc-welding... Physical size of capacitors tends to scale both with C*V and with C*V^2, so the same energy at lower voltage will be larger. And no, I certainly don't want to carry it around in my camera :) Any time I'm actually designing with capacitors, I'm grateful for the tiny, long life, low ESR ones... But I still think ones the size of a soda can look cool. Unfortunately, it's also ancient, and probably not anywhere near its original value any more. If I ever want full output from that power supply, I'll likely have to replace it.
(The power supply it belongs to is fully functional; it's nominally 5V, 22A output, and has the nice property of not producing the awful high-frequency mess of an output that modern switching supplies create.)
Oh, and the little ones also make for a much better game of catch.
> Oh, and the little ones also make for a much better game of catch.
...You are a terrible human being.
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