curious about Capacitors !

M just curious about capacitors that the voltage they store when they get charged

i have two capacitors laying around for test 
on one is written 400v 22uf
other is 420v 100uf

so how much voltage can they hold when they get charged fully ???

and any simple way to charge them ???

i want make something for shocking pranks.... :D

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The voltage rating printed on the side of a capacitor tells you the
maximum voltage you can place across it, without breaking it. There may
also be some markings indicating polarity. Its sort of the same deal with polarity. If you reverse the polarity there's the possibility of breaking the capacitor, sometimes spectacularly.

So there's your answer. A capacitor, rated at 400 VDC, is fully charged when it has 400 volts DC across it. A capacitor, rated at 420 VDC, is fully charged when it has 420 volts DC across it.

That's why the question, "How much voltage can they hold when they get charged fully?", the question is not all that meaningful.

It is sort of like asking, "How much stretch can a rubber band hold?"

For example, I've got a rubber band here that can hold 10 centimeters of stretch.

So you've got a mental picture of this now: A rubber band. It started out in a roughly circular shape, maybe one centimeter in diameter, but now I have stretched into a taught, thin line, 10 cm in length.

What happens if I try to stretch it some more? For example, lets say I want to stretch it out to 11 cm. So I pull on the ends a little harder, but before I can get it to stretch to 11cm, the rubber band breaks.

Why did it break? Because it was only rated for 10 cm of stretch.

A question that might be more meaningful is to ask how much energy can a capacitor hold. The amount of energy stored in a capacitor, ideally, is

U = (1/2) *C*V^2

where V is the voltage measured across the terminals of the capacitor, C is capacitance of the capacitor in farads, and U is in joules of stored energy.

The amount of energy a capacitor stores is equal to the work required to to move charge into it and increase its voltage. Similarly, the energy stored in a rubber band is equal the work required to stretch it. Actually the formula for energy stored in an ideal rubber band, or metal spring, is similar, something like

U = (1/2) *k*x^2

where k is a "spring constant" with units of force divided by distance, and x is displacement, the length, or distance the rubber band, or spring, gets stretched.

It is sort of the same story with capacitors and rubber bands. Both can store energy, and for both you can break the darned thing if you try to store more energy than the materials can physically withstand.

Anyway, that concludes the physics lesson. Hope that brings you closer to enlightenment. If not, you can always read the rest, here:


Regarding the thing you want to build, the capacitors you've mentioned are roughly the same size, voltage and capacitance, as those found in a flash for a camera, be it disposable film camera, or obsolete film camera. Both of those are pretty cheap and easy to find, at the time of this writing. The circuit that charges the big capacitor, "big" in this case meaning around 300 uF or so, this circuit produces around 300 volts DC, so that's not going to break the capacitors you mentioned (since those are rated for voltage higher than 300 VDC)

So I'm going to recommend looking into camera flash charging circuits. Here's a search for instructables about "camera flash shocker" :


and I'm guessing that some of those are, pretty much the same thing you want to build.

I'm not sure how safe toys like this are, but at the same time I'm not very good at lecturing on safely. Just be careful, because those goddamn monkeys bite, I tell ya!


D3zire (author)  Jack A Lopez3 years ago

thanks for the pretty good explaining ... :) but lets say i cant get a disposable camera circuit... any other method to charge them ??

i have other circuits that has capacitors .. cant i use them ??

Well, the camera flash charger pretty much has everything you want; i.e.

It runs on batteries, is small enough to fit in one hand, and the complete circuit is cheap and easy to find.

If you want to build your own, the part that's hard to find, if you try to build it part by part, is the transformer.

I think that's why the circuit Iceng just uppped is using a 8ohm-to-1000ohm impedance matching transformer, because that's a pretty common part. If you live in the former US, I think RadioShack(r) has that part.

But if you live in the former US, then you could just buy a disposable camera from some brick-and-mortar store. Or buy an obsolete film camera from a thrift store. Or buy the used circuit board from place like this:


Anyway, going back to this idea of building such a thing from discrete components, this page:


has a bunch of circuit diagrams which the author copied from found flash charger circuits.

Also the circuit Iceng upped, here


looks to me like it would work. The part of that circuit on the output side of the transformer is a network of diodes and capacitors, called a voltage multiplier.


and you can sort of increase or decrease the voltage it produces by adding or removing stages. If I remember correctly the trade off in adding more stages, is that current available at the last stage gets smaller and smaller, so the time required to charge a capacitor in the last stage gets longer and longer, until it becomes an inconveniently looooooooooooooong time.

D3zire (author)  Jack A Lopez3 years ago

that is the problem i live in india where even disposable camera are pretty costly and hard to find... i have this mosquito zapper which has capacitor attached to it.. so can i replace it with bigger capacitor and charge it how much volt needs to charge the capacitor.. and i am not ee not even close i cant understand that diagram thats what i was trying to tell iceng too..


Great pointers, especially the



iceng3 years ago

As an older EE ... I can tell you that four hundred volts will provide a good shock and probably kill the subject of your foolish desire for attention.

Here is a circuit that can charge to 400 VDC if you simply take out 3 identical sections.

Be aware if you exceed the capacitor voltage you have, they do have a tendency to steam explode and fling its chemicals at your eyes and face.

D3zire (author)  iceng3 years ago

well that is scary.. anyway to make it safe i have some reduced volt capacitors too.. like 240v .. i don't want to kill anyone :( and i i am not a ee not even close.. so can you simplify that diagram please if possible

iceng D3zire3 years ago

The process of creating the pain of electric shock penetrating the skin without killing is to limit the DC current.

The 10nF/400v capacitors are 0.01uF/400v compared to your 22uF they are 2200 less current storage capacity.

Also the stun gun puts eight of these in series further diminishing the capacitance to .00125uF which is 17,600 less then your 22uF cap.

And ... that circuit is as simple as it comes !

If someone tried to "prank" me with an electric shock, they would find themselves on the deck, nursing broken teeth.

Just sayin'

If someone tried to "prank" me with an electric shock, they would find themselves on the deck, nursing broken teeth.

Like the caps say on there side they can hold 400V and 420V respectively. To charge them give them any voltage between 0 and 400v/420v and your set. If you want to play around with shocking pranks then get a hold of some disposable camera flash cicuits. They only need slight modification to make some fun little toys.

D3zire (author)  mpilchfamily3 years ago

thanx for explaining :)