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Voltage Tripler - Cap Charger

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Jordan aka Axiom

Fri May 07 2010, 01:00AM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27

I'm looking to construct a cap charger to reach around 500V. I just bought the parts for a voltage tripler, which will be constructed roughly as seen below:

My cap bank is 68,000 uF at 450V, so I'm hoping for a decent amount of output current. I'll be using mains power as the input. The parts that I purchased are listed below:

Capacitors: 1000uF; 450V (x3)

Diodes: 25A; 400V (x3)

To limit the current, I purchased:

Resistors: 225W 50 ohm (x4; to be connected in parallel, in series with the input)

The resistors can dissipate 1000W and should allow no more than 9.6 amperes of current (120V / 12.5 ohms)

So, I'm here to consult you guys on my plan so far, if I hook up the parts I have exactly as seen in the image above, everything should be ok, right? Thanks in advance!

GhostNull

Fri May 07 2010, 09:54AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

What kinda of voltage mutiplier this?
How is it supposed to work?
I've never this voltage multiplier before

Is this meant to be a Villard cascade?

In voltage multipliers the current is limited and determined by the capacitor size, limiting resistor are unnessesary and will reduce mutliplier performance.

see this:

for a detailed guide to the Cockcroft Walton Multiplier
and this:

a calculator for CW multipliers

Proud Mary

Fri May 07 2010, 02:51PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Taking your HT+ output directly from the mains looks very odd and very dangerous.

I would never have confidence in any circuit that was not isolated from the mains by a transformer.

And you don't needs lots of current to charge your capacitors up.

Forget your well-meaning support for your local funeral parlour, and learn a few basics before doing anything that involves mains electricity.

Jordan aka Axiom

Fri May 07 2010, 05:47PM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27

Well, I do need a significant charging current because I want to be able to charge the bank quite quickly, plus, it's 68,000uF.

I was under the impression that the resistors would be necessary for the moment charging is initiated, where the current drawn would be thousands of amperes. All of the capacitors in parallel present 2.5 milliohms of resistance.

Are you reccomending I install a 1:1 isolation transformer between the mains input and the circuit?

Proud Mary

Fri May 07 2010, 05:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Jordan aka Axiom wrote ...

Are you reccomending I install a 1:1 isolation transformer between the mains input and the circuit?

Absolutely. Yes.

As for the switch on surge current, you could deal with that by using a soft starter circuit upstream of the transformer, but such things are often a matter of taste.
Remember that the switch-on surge is only momentary, so your anti-surge resistor need not be rated for continuous dissipation.

Jordan aka Axiom

Fri May 07 2010, 06:32PM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27

I browsed AlliedElec.com and saw several suitable isolation transformers, the only problem, however, is that without paying hundreds of dollars, they all were only rated for 10-15 VA.

Jordan aka Axiom

Fri May 07 2010, 06:46PM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27

I have a General Radio variac, that would suitably isolate the circuit from the mains power, right? It would also allow me to slow down charging when the caps are nearly charged.

Proud Mary

Fri May 07 2010, 07:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Jordan aka Axiom wrote ...

I have a General Radio variac, that would suitably isolate the circuit from the mains power, right? It would also allow me to slow down charging when the caps are nearly charged.

No. Variable voltage transformers are a class of autotransformers where a tap is made on a single winding. They provide no isolation between input and output.

Jordan aka Axiom

Fri May 07 2010, 08:14PM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27

I found a 50VA isolation transformer on allied:

Even with that, 50 VA is equivalent to 50 W, which is equivalent to 50 J per second. 6,885 joules / 50 joules = 137.7 seconds of charging time. That's too long, as I intend to further expand the capacitor bank eventually.

edit:

Also, I guess this would be a more accurate image of the circuit:
Voltrip

As for how it works, it seems identical to practically all "voltage tripler" circuits I have found over the internet; I don't think it's anything new. The one in the OP was from

and the one in this post was from

which seems to be a reputable source of information. There is also this site

which goes into detail about the actual operation of the circuit, which also seems identical to the ones above.

Proud Mary

Fri May 07 2010, 08:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Jordan aka Axiom wrote ...

Even with that, 50 VA is equivalent to 50 W,

Not so. VA is only equal to W in a purely resistive circuit, and their ratio is called the Power Factor. With a capacitor input supply, the power factor can be as low as 55%, so your 50VA transformer might only be good for 27.5W.

Anyway, good luck with it all!

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