multiplying dc

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Adam Binns
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multiplying dc

Post by Adam Binns »

for the power supply for my fusor, I am going to use an NST and a voltage multiplier and I was wondering if a croftcocker-walton multiplier would multiply a dc supply or would I have to use an ac supply. and if I need to use ac how can I convert dc to ac.
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Dennis P Brown
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Re: multiplying dc

Post by Dennis P Brown »

Adam, that you are unsure how a voltage multiplier works and whether one needs DC or AC driving voltage indicates you haven't read anything at all about these types of devices. That you don't know how to convert DC to AC (edited - sorry, had that backwards) indicates you have never even read anything at all about even basic electronics. Learning just elementary concepts of electronics is essential if you ever intend to be taken serious here with your questions about electronic systems.
Silviu Tamasdan
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Re: multiplying dc

Post by Silviu Tamasdan »

A NST will not give you enough power for fusion (well maybe for a demo fusor it might be OK if you understand that you won't get fusion from it)

A CWM requires AC at input. It's a fancy rectifier - which, apart from rectifying the AC has the added benefit of increasing its voltage. And reducing its intensity proportionally. And dissipating a non-trivial amount of the input power as heat.

If all you have is a source of DC you cannot multiply that directly. You must first turn the DC into some sort of alternating or pulsed current. Usually that's done by switching it on and off quickly with some low-losses devices such as MOSFETs or IGBTs in a bridge configuration at high frequency.

Please read more about the subject before trying to play with high voltages. Head over to the FAQ section on this board and read everything there.
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prestonbarrows
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Re: multiplying dc

Post by prestonbarrows »

Attached is a good quick primer on multipliers in theory and practical application.

To do easily detectable fusion in a fusor, your supply needs to be capable of at least 10-20 kV and 10-20 mA. More is better; this puts you into the 100-400 Watt range. This voltage and power level is a serious supply; duct tape and bubble gum will not get you here.

Basically, CW stacks are limited by the driver power; If you try to just put an arbitrarily large number of stages on top of a driving transformer, the output voltage increases but the output current capability and ripple degrade simultaneously.

If you sit down and do the back-of-the-envelope-calcs, you will find that a typical NST is just on the edge of this power range if you are lucky. Most commercial HV units use a full bridge rectifier to mosfet resonator to small stepup transformer to produce a few kV at 10s of kHz then pipe that into a CW multiplier stack and smoothing cap to the final output voltage.

Additionally, NSTs run at line frequency (60/50 hz) CW multipliers tend to be more effecient at higher frequencies. That is another benefit of the rectifier to mosfet to transformer topology; you can run up at 10-100 kHz easily. Higher frequencies means you can have a much more stable and stiff voltage output for a given set of capacitors in your CW stack and volume of iron in your transformer (which link to the size, cost, and stored energy of your supply).

So again, CW stacks are very viable if you know what you are doing. If you try to use one on an NST, expect to either get an output voltage that immediately sags down when any current is drawn or spend thousands of dollars on big HV caps to prop up the output. High frequency is the way to go if you are serious about rolling your own HV supply.
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Multiplier Design Guideline.pdf
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Dennis P Brown
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Re: multiplying dc

Post by Dennis P Brown »

Wonder if this person (who started the thread) is still active?

Also, for future reference, fusion in fusor's start to be detectable (using std equipment) from about 20 kV (seen one example at 19 kV. Generally mid 20 kV is better for beginners.) So the generally accepted range is 20 - 30 kV.
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Bob Reite
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Re: multiplying dc

Post by Bob Reite »

Unless you are using a bubble detector, which *only* responds to fast neutrons (if you don't overheat it and get big bubbles), you need to be at 25 KV or higher to be well enough above the noise floor of your counter. I usually operate at 40 KV. Needless to say at this voltage, the stainless steel of the chamber will not be enough to block X-rays, additional shielding or operating at a distance will be required.
The more reactive the materials, the more spectacular the failures.
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Richard Hull
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Re: multiplying dc

Post by Richard Hull »

Right you are Bob! My mega mark in just over 40kv, normally. 30kv is a very easy detect with smaller electronic system detectors. 25kv is just a start for bubbles. 20kv detection is terribly difficult without plus ultra-gear. In the end, it is all about the operator, their fusor's build and detection system at what voltage, current and pressure where fusion detection occurs.

As already noted, some amazing results have been had in tiny fusor devices near 20kv at high pressures and currents by very adroit hands. Adroit hands rarely show up here. When they do, we often learn from them. That is how we move onward and upward.

I have been fighting off the urge to do a 6 way cross 2.75" fusor and check out what can be done, but it would mean a redo on my current system and I have a mentality that says' "if it ain't broke don't try to fix it."

I will muse a bit here and propose that such a 2.75" conflat system, in good hands, that is well sealed, might not require a diff or secondary pump. This is provided you have a fabulous mechanical pump that has no issue hitting single micron vacuums. Why?.... Well, these systems seem to function at 20 to 30 microns of D2 and in such a small volume, flowing D2 will quickly displace any residual gases with little wasted D2.

Fusor III never had a diff pump, but did have a micromaze that barely let the mechanical dip fusor III to sub-micron vacuums. I got to almost 200,000n/s at 28kv in that 6" chamber before building fusor IV.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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