How To Wire A Capacitor To Charge And Discharge


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well, as some of you know, ive made a (very) simple 2 stage microswitch triggered coilgun

at the moment on a standard 5A 12V powersupply, the gun fires about 1 - 2 meters

however, because my school is sponsoring me for this project (i am donating it to them when it can reach a distance of about 10 meters) i am looking for a more powerful supply of power.

many sites suggest using a capacitor bank to charge and discharge into the firing circuit

i have read alot of sites, all which have overly (imo) complex circuit diagrams or show no circuit diagrams of the capacitor bank

at the moment i only have two 3300 uF capacitors

how would i go about wiring them to my coil so that they charge and discharge and hence, fire the gun?

thanks in advance

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Imagine the capacitors as a simple power source. Wire a simple switch that switches between 2 circuits, one that just has the battery and capacitors, and the other that just has the capacitors and firing mechinism.

The caps will be charged to the voltage of the bat on the first circuit, and will discharge on the second.

You want the most energy possible, and since energy is 1/2 * C*V, you want each capacitor to have the max voltage possible, which is what happens in a parallel circuit. (since each cap will have the voltage of the bat). When you do this, you'll get a higher net capacitance, since capacitance adds when in parallel. (Which is an alternative explaination for why you get more energy)

However, when discharging, you want the single highest voltage you can get, which occurs when the caps are in parallel. (This gives you a lower net capacitence, and thus a higher net voltage, since the energy stored has to be the same). Since each was at v_batt and you have 2 of them, you'll get v_bat + v_bat = 2 v_bat, which is better than just v_bat. Higher voltage gives you a faster discharge and a higher current.

So, that's why it's complicated. You want a parallel circuit when charging, and a series circuit when discharging.

Note: this was done on the fly, from the top of my head. I'm a year removed from taking a circuits course. Take with a grain of salt accordingly.

edit: I've assumed all along you are using a DC power source (which I called the battery, but it doesn't have to be). You can't use AC power, because it will charge and discharge the capcaitors itself, at the frequency of the power. You can use an inductor to store energy from AC, but that's a lot more complicated.

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ohh, that is what the cockroft walton voltage multiplier is doing

thanks, i sorta get it :) the direction of the current changes when discharging, right?

what i mean is when charging the current flows from the negative cell through negative capacitor, out of positive capacitor and back to positive cell

but when discharging current flows from the negative capacitor, through circuit back to the positvite capacitor?

thanks :)

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ohh, that is what the cockroft walton voltage multiplier is doing

thanks, i sorta get it :) the direction of the current changes when discharging, right?

what i mean is when charging the current flows from the negative cell through negative capacitor, out of positive capacitor and back to positive cell

but when discharging current flows from the negative capacitor, through circuit back to the positvite capacitor?

thanks :)

Yeah, that's right. Current flows into the terminal connected to the positive terminal of the powersource to charge, and then flows out of it when discharging.

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Do you have any images of this contraption? :D

they would break a mirror :p

just lots of wires and coil and stuff :p

quite fugly - but when i get it finished i will post some pics :p

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Imagine the capacitors as a simple power source. Wire a simple switch that switches between 2 circuits, one that just has the battery and capacitors, and the other that just has the capacitors and firing mechinism.

The caps will be charged to the voltage of the bat on the first circuit, and will discharge on the second.

You want the most energy possible, and since energy is 1/2 * C*V, you want each capacitor to have the max voltage possible, which is what happens in a parallel circuit. (since each cap will have the voltage of the bat). When you do this, you'll get a higher net capacitance, since capacitance adds when in parallel. (Which is an alternative explaination for why you get more energy)

However, when discharging, you want the single highest voltage you can get, which occurs when the caps are in parallel. (This gives you a lower net capacitence, and thus a higher net voltage, since the energy stored has to be the same). Since each was at v_batt and you have 2 of them, you'll get v_bat + v_bat = 2 v_bat, which is better than just v_bat. Higher voltage gives you a faster discharge and a higher current.

So, that's why it's complicated. You want a parallel circuit when charging, and a series circuit when discharging.

Note: this was done on the fly, from the top of my head. I'm a year removed from taking a circuits course. Take with a grain of salt accordingly.

edit: I've assumed all along you are using a DC power source (which I called the battery, but it doesn't have to be). You can't use AC power, because it will charge and discharge the capcaitors itself, at the frequency of the power. You can use an inductor to store energy from AC, but that's a lot more complicated.

I am stunned by your reply. If he could make a coilgun, why could not he know the common sense about the series-parallel connection of the capacitor bank? All you had to introduce is how to make a circuit that change the state of parallel to the state of series. :)

Also, what you said about using AC power to charging caps... is wordy.

@Powelly: that is not about hi-tech weapon, just a weapon toy.

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lol, high tech weaponary :blink: yep sure :D fires 2 meters when at an angle of 30 degrees and 12V 5A is passed through the coil

PhW0AR :pinch: KiLLAH :p

@Johnson

is the circuit to which you are refering the cockroft walton voltage multiplier/amplifier? uses diodes to charge capcitors in parallel and discharge in series?

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@Johnson

is the circuit to which you are refering the cockroft walton voltage multiplier/amplifier? uses diodes to charge capcitors in parallel and discharge in series?

Cockroft Walton Voltage Multiplier uses AC power as its energy source, and can you put an AC power into a coil gun?

It is impossible.

theoretically speaking, for swithing series state to parallel state, all you need is a simple switch consisting of a group of SPDT switchs. But the switchs should endure strong current! :p

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update:

begged camera shop for free used disposable camera so i could 'study' the internal components :p (eg FLASH UNIT :D )

got one, tried to take it apart on the way home from school - got zapped 3 times in doing so :pinch: (the female dog still had charge left - even after after zapping me :s )

finishing the report on it, before testing :D

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