What to do with big capacitors?

[Doug G]

If charged up at the max rating of 40 VDC - I don't think they will cause anyone that much harm. Resistance of people is on the order of 50,000 ohms - 500,000 ohms. Ohms Law says Current = Volts/Resistance, therefore... at 40 Volts and 50,000 Ohms, Current should be around 0.0008 Amps, which won't do much. It won't get your attention until you get up to 10 mA or higher. Maybe on your tongue will the resistance be low enough to give a good tingle. I use .1 Farad capacitors in my physics class and we charge them up with 9 V and the kids handle them without any shocks; they like putting tin foil on the terminals and watching sparks.

Tesla Coil = NO (Voltage is way to high on a Tesla Coil)

Many of the other ideas mentioned just won't work with this voltage rating of 40 VDC. You can charge it up at 6 V and run a DC item for a while. I bet it could run an iPod Shuffle for a little while??

Power stored in a Capacitor is P = 1/2 * C * V^2, so 1/2 * .025 * 40^2 = 20 W in each Cap if its fully charged up to 40 V.

One word: Boom. But if you do anything destructive, please... remember the rule of boom: If you can see it, it can kill you. So be safe in your haphazardly destructive rampage (if you're going to do that, that is).

[Kyle Love]

We use come big capacitors to make one kick'n ball.

[Richard Wallace]

Originally Posted by Doug G

If charged up at the max rating of 40 VDC - I don't think they will cause anyone that much harm. Resistance of people is on the order of 50,000 ohms - 500,000 ohms. Ohms Law says Current = Volts/Resistance, therefore... at 40 Volts and 50,000 Ohms, Current should be around 0.0008 Amps, which won't do much. It won't get your attention until you get up to 10 mA or higher. Maybe on your tongue will the resistance be low enough to give a good tingle. I use .1 Farad capacitors in my physics class and we charge them up with 9 V and the kids handle them without any shocks; they like putting tin foil on the terminals and watching sparks.

Human body resistance can be as high as you say, but can also be much lower depending on conditions. Look here or here for some interesting discussion and examples.

Originally Posted by Doug G

Power stored in a Capacitor is P = 1/2 * C * V^2, so 1/2 * .025 * 40^2 = 20 W in each Cap if its fully charged up to 40 V.

You mean energy, not power. The formula above gives stored energy in Joules. And 20 Joules is more than enough to stop your heart under the right (er, wrong ) conditions.

[Doug G]

Richard,

True. Body resistance is much more complicated than that of a wire, which is why I used a range from 50k thru 500k Ohm. Measuring the resistance of a tongue and you get as low as 1-5k Ohm, which explains the tingly sensation of licking a 9 Volt battery. Also helps explain why it is common to see medical folks use a "gel" when connecting the terminals of a defibrilator to a patient. No doubt that gel lowers the resistance to help increase the current through the chest. I still think if someone charged up these caps and held them in your hand - the result is most likely not dangerous, probably more dangerous is the burn you may get from accidentally shorting out the cap's terminals. Also notice in your first linked website, that DC is somewhat safer than low freq AC, but then as the freq of AC increases the skin effect takes place and becomes a bit safer. Good sites - I will use them with my students!!

For the Power equation, I stand corrected, PE = 1/2 C V^2 and yes that ends up as 20 J not Watts.

[Doug G]

I've had this Cap for a while now and haven't decided what to do with it either - notice it's only 15 microFarad, but its rated at 3.5 kV. Possible PE = 1/2 * .000015 F * 3500^2 V^2 = 92 Joules. But touching this when charged when your body resistance is 50 kOhm results in up to 70 mA which is on the borderline of lethal. My only idea is to use a HeNe Laser power supply to build a charging circuit (need to research the feasibility of this) and then discharge the cap through some coiled up copper tubing wound around an Aluminum can (Can crusher). The brief max current through the tubing should be in excess of 10,000 A and will create one heck of a magnetic field. Hmmmm???

[Al Skierkiewicz]

Doug,
The gell used with defib actually decreases the localized current peaks to prevent burning by insuring that the current is spread out over the entire area of the paddle for much the same reason we are talking about. The skin resistance varies drastically over different areas and under different conditions. I am not sure what you are referring to with the "skin effect" vs. frequency. High frequency currents tend to move on the "skin" of a conductor but this has little to do with the effect on a body coming in contact with a high frequency voltage. RF burns are real and occur at relatively low voltages. I have some scars to prove that.
The big cap you have likely came from a high voltage power supply. I have seen these in old industrial strength microwave ovens, TWT amplifiers, transmitter power supplies and old style TV projection devices. Handle this one with care as it is oil filled and likely contains PCB if it was manufactured in the 60's or 70's.

[esquared]

Electrical socket + long handled needle nose pliers + safety goggles + a clear path + big capacitor under-rated for said electrical socket = Bottle Rocket++
-Eric

[yongkimleng]

haha please wear goggles and some thick fireproof clothes when exploding them. I exploded a tiny 100uF cap and it was messy.
Save them for some electronics projects lor.. 25mF @ 40V can power a circuit for quite a while..

[Richard Wallace]

0.025 Farad x 40 Volt = 1 Coulomb electrical charge.

That's one Ampere flowing for one second, or 10 mA for 100 seconds, assuming a load circuit that will draw a constant current. So yeah, that capacitor can power a small logic circuit, through an appropriately designed regulator, for at least a minute or so.

More realistically, a load circuit equivalent to a constant 2000 Ohm resistance (the path from your left index finger to your right index finger, through your body, under some conditions) would initially draw 20 milliAmperes, and this would decay exponentially with a 50 second time constant [ i(t) = 0.02 x exp(-t/50) where t = time in seconds and i(t) is current in Amperes ] so that after 2.5 minutes the current will be less than 1 mA and the voltage less that 0.5 Volt.

20 mA is not enough current to send your heart into ventricular fibrillation, but enough to get your attention. Look here for more on electric shock.

[Manoel]

Staying alive near them would be pretty cool.