Capacitor Charging.

[Callidus]

Hi Guys,

Im look for a simple way to charge a capacitor effectively. Any help appretiated. The cap is a 400v 47uF Electrolytic.
I have a PSU that supplies 12v 5Amps. Any circuit diagrams?

Cheers

 

[Phasor]

What voltage do you want to charge it to? And for what purpose?

 

[Callidus]

Yeah, Its to fire a Flash tube. The flash tube is rated at 400V.

Ive read about a circuit that charges by switching a small transformer very quickly but I dont yet understand how it is working. I was looking for something or an explanation, so I could get an understanding of it.

 

[bogdanfirst]

well... if you want to charge the cap at 400V then it shold ve rated more than 400V......like 500 or 450 of 600.....depends......
you might destroy it, it might blow and overheat if you charge it at its full rated voltage.
you can use a simle oscilator and a transformer......and a rectifier after the transformer and maybe a voltage stabilizer with some zenners.
beczuse the cap has a small value......it will charge quite fast at a not so high current.
you can make a 555 oscilaor and use transformer from an adaptor. connect the output of the 555, of course you should use a transistor to drive the transformer, to the 6V output at the transformer. then at the 220V part of the trnaformer you can get around 350-450V....i used a trnasformer once and got around 385V....wich is quite colse to 400.

 

[Callidus]

I know this question may have a long answer but how does an oscilator affect the outcome of a transformer.

I was under the impression that a transformer on its own through eddy currents created the potential difference on the other side. I understnd I am probably completely wrong but any input would be good.

Cheers

 

[Phasor]

how does an oscilator affect the outcome of a transformer

What the oscillator does, is supply the transformer with a quasi-AC waveform. A transformer does not work with DC - it relies on the fact that an AC current is constantly changing in value. When the current changes, so does the magnetic flux, and in doing so, cuts the secondary winding. This cutting action produces the potential difference.

If we were to put DC into a transformer, the amount and direction of flux remains constant, so there is no cutting of the secondary.

 

[Callidus]

Do they then use a bridge rectifier between the transformer and the cap? Im assuming you get a quasi-AC waveform out of the transformer.

 

[Phasor]

Im assuming you get a quasi-AC waveform out of the transformer.

You do get an alternating waveform, but not necessarily the same wave which you put in. A sine wave is the only waveform which retains its shape when transformed. All other waveforms will have some sort of distortion on the secondary.

Do they then use a bridge rectifier

You could use a bridge, but for your purposes, I think a single diode (half-wave rectifier) will do.

 

[lunatic2]

If you're to the point of quasi AC you could look into building a cockroft walton multiplier. The are very easy to construct.

Use an input resistor to limit current into multiplier, and use and output resistor to limit current draw from the multiplier, otherwise you smoke the diodes. They can be powered from 120Volts, but it is too dangerous if you are not experienced with it.

I've built this type of circuit to charge capacitors for laser flashlamp power supplies and they work great.

Check Jochen's High Voltage Page for details on this kind of circuit(look under the high voltage section under mutilpliers) to get the voltage you need you'd only need a 2 or 3 stage multiplier as opposed to the 100,000 volt units he shows. You could even power it off 12 volts with a low frequency DC inverter driving the multiplier.

The transformer method would work as well although they will not be tuned well for your circuit, CW multipliers are a solid state solution to the same problem.

And when in doubt, just modify the flash circuit out of a disposable camera to do what you need. There is information on these.. Schematics of a Kodak disposable camera **broken link removed**

 

[jjjasesino]

well... if you want to charge the cap at 400V then it shold ve rated more than 400V......like 500 or 450 of 600.....depends......
you might destroy it, it might blow and overheat if you charge it at its full rated voltage.
you can use a simle oscilator and a transformer......and a rectifier after the transformer and maybe a voltage stabilizer with some zenners.
beczuse the cap has a small value......it will charge quite fast at a not so high current.
you can make a 555 oscilaor and use transformer from an adaptor. connect the output of the 555, of course you should use a transistor to drive the transformer, to the 6V output at the transformer. then at the 220V part of the trnaformer you can get around 350-450V....i used a trnasformer once and got around 385V....wich is quite colse to 400.

This Seems to be a ver y simple way to do it but My question is how to connect all, i mean, im New on this , My tought is that You connect the ac ouput of The oscilator to The Gate of The transistor And Then aire The better y in series with The transformer and transistor, im right¿, can You post a pic of how is it connected¿