# COMMUNITY : HELP : FAQS

## Help with charging capacitors

How do I charge my 450v capacitors with a 120v outlet? I am building a rail gun and need to get these up to a high enough voltage.

iceng2 months ago

A voltmeter, a 600V DC bridge,a variac, a transformer, some clip cords and knowledge about charging EL caps after months of storage...

Jack A Lopez2 months ago

Well, the cheap and dirty way to do this is by using a voltage multiplier,

https://en.wikipedia.org/wiki/Voltage_multiplier

like a doubler, or a tripler.

By the way, the Wiki article for "Rectifier" is, I think, kind of a prerequisite.

https://en.wikipedia.org/wiki/Rectifier

I mean, understanding simple rectifier circuits, like half wave and full wave, this is maybe a prerequisite for understanding the more complicated voltage multiplier circuit.

Keep in mind, the peak voltage of the AC waveform is greater than its RMS value by a factor of 1.4142 (=square-root of 2). For example a sinusoidal AC voltage with 120 volts RMS, has a peak value of around 170 volts, and it is the peak value that gets rectified, and charges the capacitors in these various rectifier circuits. Thus a tripler fed with 120 VAC could produce a DC voltage of around 3*170 = 510 VDC

http://www.kronjaeger.com/hv/hv/src/mul/

is all about voltage multipliers, and the reason I want to link to it is because it has a formula for calculating the impedance offered by a Villard cascade voltage multiplier, and that number will be meaningful, if you are wondering about how big or small the capacitors in your voltage multiplier should be, based on how fast (how much time it will take) for to charge your capacitor bank.

Anyway, that is the cheap and dirty way. Cheap because the component count is low. Dirty because it does not offer much in the way of isolation,

https://en.wikipedia.org/wiki/Galvanic_isolation

from the AC mains.

Perhaps a better way might be to build an inverter, to make your 100s of volts AC, from low voltage DC. Then rectify that AC into DC to give you your 450 volts DC. There are maybe a few advantages to doing things this way. The first is because you can make the frequency faster, the capacitors in a subsequent multiplier stage, or filter stage, can be much smaller.

Also if your capacitor charger is DC powered, then maybe you can run it on batteries, and make your machine portable. You know, that way your ultimate terror weapon, or whatever this thing is, does not need an extension cord.

;-P

Final note: I don't know if you knew this or not, but many recipes out there for homemade capacitor chargers, use a camera flash charger, because these are cheap, easy to find. This page,

http://www.talkingelectronics.com/projects/XenonFl...

does a good job of explaining how these camera flash charger circuits work, it could maybe serve as inspiration, if you want to build a battery powered capacitor charger.