Stacking Capacitors

[Yoa01]

Hi all,

Ok, so I know it's super dangerous and stuff, but I was wondering exactly how one would go about stacking capacitors.

You see, for a filter design I'm testing, I need a 22pF capacitor. I seem to have every size but that, so I Google'd stacking capacitors and found that it is possible. I read on another forum that they add like resistors.

Regrettably, I don't understand exactly how capacitors work in various situations, but it appears that, though it may exponentially increase voltage, I could stack 2 10pF and 2 2pF capacitors in parallel, and I would get 22pF, like this:

Out of fear of blowing stuff up, I have not wired this up and tested it.


Any help would be greatly appreciated

 

[(*steve*)]

That's not at all dangerous. Yes you can do that.

You will probably be close enough just stacking (placing in parallel) the two 10pF capacitors.

I suspect you mean 2 x 10pF and 2 x 1pF as that works out and is what you have on you schematic.

 

[Raven Luni]

They dont add like resistors. Resistors and capacitors have the opposite rules for series and parallel configurations. So in the case of capacitors all the capacitance values add if they are in parallel and for series it is 1 / (1/C1 + 1/C2 + ..... 1/Cn).

The dangers of capacitors are nothing to do with stacking them - its more to do with the fact that they hold their charge for very long periods and can supply very high instantaneous currents (meaning they can dump all their charge in a very short space of time if the conditions are right). Its more a problem of larger electrolytic types but all capacitors should be respected - make sure they are discharged when connecting or disconnecting them. I've blown a few components by forgetting this - the most spectacular being an LED which exploded and sent bits flying everywhere (that was 4700uF capacitor which was part of my power supply).

 

[Yoa01]

Sweet! I actually tried it wwith a 68pF cap, and that had some strange results. I'll see if I have enough room on my breadboard to try this setup (it's a little full now...)

Right, sorry I was typing two a lot, I guess I misread it when I proofread.

And Raven, sorry for not specifying, but thanks for the clarification. And how much can be stored in 22pF in this case? It doesn't seem like much since it's in a passive circuit (unless the OpAmp's output creates feedback...)

 

[Raven Luni]

That wont do much

(You know - I'm tempted to try the LED thing with one of my 3kF beasts - might have to leave the room and film it )

 

[(*steve*)]

And how much can be stored in 22pF in this case?

If it had 100V across it, you *might* be able to feel it (if you discharged it across your tongue)

The energy in Joules is given by 1/2CV^2. A 22pF capacitor with 100V across is stores 0.00000011 Joules.

In contrast, a flash capacitor in a disposable camera has about 300V on a 300uF capacitor stores about 13 J of energy. Discharge that through your fingers and you'll never forget it. Discharge it across a screwdriver and you can vaporise small pieces of metal.

 

[Yoa01]

I always knew capacitors were dangerous the larger they got...

 

[CocaCola]

I always knew capacitors were dangerous the larger they got...

FYI, larger value, not necessarily larger size... The caps in small xenon flash applications can be quite small and still kick like a mule...