# Charging 250 volt capacitors without flash camera circuit

I'm building a coil-gun in my high school engineering class and am having a little trouble charging the 5 250-volt, 1000 microfarad capacitors. Would it be possible to connect the wall to a voltage doubler, and then just hook that up to the capacitors(obviously with a fuse along the way for safety)? I'll only be using the circuit under careful supervision, so reasonable danger is OK.

• I will say one thing. Nothing is more dangerous than touching a charged capacitor. It can burn your skin or blow a finger off your hand. A direct connection to the wall outlet through a bridge rectifier will get you 120vac x 1.414, or about 170vdc. Add a transformer too boost the voltage if you wish, but stay well below the 250 volt limit of the capacitors, about 220vdc at most. NEVER run capacitors at their maximum voltage.
– user105652
Commented May 5, 2016 at 1:19
• The safe response to this is "if you have to ask, you shouldn't be doing it". You are building a device that will easily put a hole in your hand or vaporize a chunk of metal, and your question demonstrates your lack of electrical knowledge. This is concerning. Commented May 5, 2016 at 2:01
• @unit128_t. If this forum has a safety threshold where we do not help create what could be fatal circuitry for the amateur OP then I would like to know what that limit is. I know how to build 5 GigaWatt lightning generators, but I am not passing that knowledge to others.
– user105652
Commented May 6, 2016 at 1:39

What voltage do you have at the outlet? If is 110-120V, the peak voltage which the capacitors will take will be approx 150-160V which is too less. If outlet is 230V, the voltage will be 325V which is too much. If you want 250V dc, the transformer you'll need is to be one who deliver in secondary 250V / sqrt(2) = 175V ac (which will not be easy to find).

A "dirty" approach will be to just use from 230V (outlet or transformer) a simple diode as a rectifier (yeah, is just as simple) series with a large resistor (something alike 22-33 kiloohmi/1W) and you'll observe carefully the capacitors voltage with a multimeter and stop charging when its reaching 240-250V. It shall take about two minutes to charge to that voltage (will approach time constant T=RC, thats 5000uF x 22k = 110 seconds if i don't miss). Edit: i missed, its very late now in Romania; rectification is single way, time will be more than double, also considering the voltage should arrive close to top of the RC courve; use a 5-10 kilo resistor, at 5W.

• Please don't suggest lethal solutions to inexperienced students.
– pipe
Commented May 5, 2016 at 1:33
• Students will learn, no risk no life. Commented May 5, 2016 at 9:05
• I DO NOT recommend charging a capacitor with a 250 vdc rating up to its 250 volt limit. That is asking for trouble. Granted that manufacturers of quality caps will under rate the max voltage, but even they would ask you for a 10% safety margin if possible. When large value electrolytic caps short...kaboom!
– user105652
Commented May 6, 2016 at 1:32
• @Avraham. Glad this is under supervision of a teacher,etc. Please add 1 220K ohm 2 watt resistor across each capacitor to bleed off the charge if the setup is in an idle state. Only charge the caps just before firing the object. Cover the caps in plastic once construction is complete. Play it safe please.
– user105652
Commented May 6, 2016 at 2:37
• Also when you use a voltage doubler as you suggested. The voltage will not rise to 220V but to 325V again, because that (325) is the peak voltage of the waveform which have the RMS value of 230V or 115V in your case, and capacitors are charged by the peaks of the waveform. Commented May 7, 2016 at 1:42