capacitor question- i am noob at electronics

[abh33]

i want to know that how do i fully charge a 440uf 16v rated capacitor. i need a charger that can do this

can someone direct me towards somethng. it would be better if i can build the charger

basically i want to charge the capacitor then have it dump all its energy in 1 go

[madsci1016]

Just apply 16V to it. No special circuitry required.

[Soeren]

Hi,

Going to 16V is straining things a bit, I'd say 12V max, unless he's in it for the bang 

A 440µF at 12V (or 16V) can't hold that much energy, so don't expect railgun performance.

[abh33]

u hit the right area
basically i need a coilgun or railgun
but the thng is i want to charge up my capacitor using a battery eliminator
it doesnt matter what capacitance i use....that can be modified according to my needs

but how do i charge the capacitor using dc eliminator and how do i know when it is fully charged if i am using a multimeter??? plz answer for a 440uf cap

[knossos]

Here is a decent tutorial on capacitors.  If you just want to know how to measure with a multimeter, take a look at this, although there are better ways.

[Soeren]

Hi,

but how do i charge the capacitor using dc eliminator and how do i know when it is fully charged if i am using a multimeter??? plz answer for a 440uf cap

Regardless of size, it's charged when it reach a voltage close to the DC that you charge it from.

If you build a large bank, you may have to use a resistor to keep the wall wart alive, as a large capacity will look like a short circuit initially.

[abh33]

If you build a large bank, you may have to use a resistor to keep the wall wart alive, as a large capacity will look like a short circuit initially

i dint understand. does that mean that i need to keep the amperage low using a resistor??
and what do i do to dump all the charge in one go. i need quite a high amperage. do i need to use a high voltage rated cap..somewhat like 200v?

[photomark]

The theory your looking for is RC circuits , they are first order circuits and are best solved with first order differential equation ,, complicated ? yes and I am not going to try to explain it here .

The voltage rating of a capacitor has nothing to do with how fast or how much it charges to , that value it not even in the equation , all there is is a resistor and a capacitor in series with a voltage source . for most lab practical uses that voltage source will be from a function generator and it will apply a square wave step to the circuit . and view the response on the capacitor with an oscilloscope.

The time for a full charge is 5RC with RC being (R x C ) Resistance in ohms x capacitance in farads , for example if you have a circuit with a 10k resistor and a 10pf capacitor the RC is 10000 x (10x10^-9) = 100uS , that x 5  gives charge time of 500uS and  a step frequency (charge discharge cycle) of 1mS = 1khz .

you will need a multimeter with a very fast peak hold function to measure the charge on a capacitor as the second you connect it it will discharge  .


BTW if your are going to be using large high voltage capacitors be VERY careful and DO NOT hook your multimeter to them , even a small electrolytic can give you a nasty shock and a burn        

[Soeren]

Hi,

i dint understand. does that mean that i need to keep the amperage low using a resistor??

Yes, at least until it has gathered some charge.
If your wall wart can supply say 1A at 15V, you need a resistor of around 15V/1A=15 Ohm.
Since it takes a charge pretty quickly, a 10 Ohm should fly as well.
That's just for the charging phase, so forget all the stuff about RC filters and the rest of the stuff that's probably a bit too involved for you anyway and a bit off target.

and what do i do to dump all the charge in one go. i need quite a high amperage.

The caps should have the lowest possible ESR (Equivalent Series Resistance) to be able to dump its charge as fast as possible. Spreading the needed capacity on several (lower value) capacitors gives you a lower ESR in most cases.

To dump the charge (without sparks flying), you'd use either an SCR (Thyristor) or an IGBT with a suitable current rating - for serious rail guns, hockey puck SCR's can be found at eBay.

do i need to use a high voltage rated cap..somewhat like 200v?

That would be a bit dangerous if you're not absolutely knowing what you do (and you don't quite sound like you do), so stick to 24V max.  or 50V max if you like to take a good beating
Going lower in voltage will make it easier to find good caps for the purpose.

Let me end this by a warning!
A couple of airheads (well relative at least) in my local electronics club made a rail gun out of bits and pieces and for the smoke test, they fired a metal bolt  (less head) into a concrete wall at about 25..30m. The bolt was buried in the wall!
Imagine if a person had just stepped into the line of sight at that moment.

[abh33]

[Soeren]

Hi,

[img*]http://C:\Documents and Settings\abcc\My Documents\My Pictures[/img]

Linking to a file on a local PC isn't gonna show up anywhere but at that PC 

[Spoil9]

Tag to see how this turns out.

[madsci1016]

Tag to see how this turns out.

You can just hit the "notify" button at the bottom of the page to get email updates, instead of creating useless posts.

[photomark]

What are you trying to do with a capacitor? , what are you trying to achieve? , when you say you want a dump at a few amps what do you want to dump to ?

you must realize that if you happen to get a jolt from that capacitor it will really hurt you .

I remember in my  high school electronics lab we had an old 440uF 400V electrolytic that we would charge up with a mega and just leave sit on the bench , sooner or later some one would touch it and you would here a great yelp .

This was soon stopped once one student touched it with wet hands and got a burn that needed a skin graft on his palm .

This capacitor was very old and did not hold a very good charge for very long .

Another stupid thing we would do is charge up a small 25 volt 1000uF cap and throw it some one to catch , even better if you charge them with a mega

[abh33]

to be exact i am maikng a coilgun.
basiclly i want to dump all tht energy through a solenoid so that i can get a strong magnetic field for a split second from which i can accelerate a ball bearing to a high speed(from high i dont mean to hit someone....possibly harmless but still it has to be presented for science fair)

most of the websites whr i read it says tht u need a high voltage rated cap for tht kind of thng........but i thnk magnetic force depends upon current.

[Soeren]

Hi,

most of the websites whr i read it says tht u need a high voltage rated cap for tht kind of thng........but i thnk magnetic force depends upon current.

Magnetic force depends on energy. It takes voltage to drive the current through the solenoid.

You need to find the inductance (and the DC resistance) of the solenoid, then you can calculate how fast you can dump a certain amount of energy into it, but as a rule of thumb: The more windings, the higher the voltage needed.
Winding the solenoid in a bifilar (w. 2 parallel wires) or trifilar (3 wires) fashion will help towards lowering the needed voltage, at the cost of a higher current.

Since this is for a science fair, better keep it at a low voltage, so that nobody gets hurt and perhaps mount it in a way that it is impossible to shoot anywhere but towards eg. a clay filled box or similar.

[waltr]

I also don't recommend firing a high power rail gun at a science fair. These require a carefully controlled test range as they can easily be lethal, would you fire a .22 caliber rifle at a science fair? 

You may be better off with a very low power demonstration unit like this one:
http://scitoys.com/scitoys/scitoys/electro/railgun/railgun.html

 

[photomark]

Now that your also going to be using solenoid it is no longer a simple RC circuit and it is now RCL .

I suggest you start studying up on calculus and first order differentials  as this is the only way you can really solve and analyze this sort of circuit.


Personally I think there would be far better projects to do for a science fair as this has the potential to be very dangerous     

[abh33]

basically i jst need a velocity of 0.5 m/s
velocity of that sort cant hurt someone

[Soeren]

Hi,

basically i jst need a velocity of 0.5 m/s

My grandson can crawl faster than that... Backwards 

velocity of that sort cant hurt someone

But some may fall asleep during the big moment (with "moment" used in a very loose sense here) 

Seriously, you can't get it to move that slow and get more than a few inches.