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GrantX

Fri Jan 18 2013, 03:09AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

The amount of current you can pull continuously from a voltage multiplier is related to the size of capacitors, as well as the output resistance. If you want all 750W from your MOT to make it to the output at 50/60Hz you are going to have to use rather large capacitors. At least several uF. With 2.1kVac input you would need to look for some large oil-filled caps, capable of about 4.5kV DC, and at the very least a few uF (these are just ballpark figures, I'm very tired). You will also want an output resistor of at least several hundred thousand ohms (if not megohms, it depends on the final output V), and a couple of hundred watts (the output resistor must handle the full 750W).

A mains frequency HV multiplier is a large and expensive device. I tried the same thing a couple of years ago, never got anywhere due to prohibitive costs.

Also, once you start stacking up multiple microfarads and multiple kilovolts, you begin dealing with some seriously dangerous stored energy. Hundreds of joules.

StormInABottle

Fri Jan 18 2013, 05:43AM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

Is there no way to just make a simple voltage multiplier that runs off a mot or mains to get me 12 kv at atleast 30 MA

GrantX

Fri Jan 18 2013, 06:46AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Well the circuit is indeed very simple, It's just the parts are expensive, because the capacitors will have to be truly enormous to let 30mA through while maintaining 12kV across the load.

A capacitor is reactive, and impedes the flow of AC current. The impedance will drop as the capacitance rises, or as the frequency rises. Since you want to use 50 or 60Hz, you need more capacitance, so that the multiplier has less impedance. Less impedance = more current and less voltage sag.

EDIT: As Ash Small has said, you're best hope would definitely be microwave oven capacitors and diodes, since that will probably be the cheapest way to build up enough capacitance in each stage.

I recall building a three stage MOT multiplier with MO caps and diodes. It produced a decent amount of voltage at open circuit, but when I added about 4 meg ohms to the output and drew an arc it really wasn't very impressive. I got better results with a basic 555 flyback driver, which was much less expensive to build, and far smaller.

So far as I can see, you want a robust HV power supply capable of producing 12-15kV DC at approximately 30mA? This supply is going to be used with a SGTC?
If you're buying all your parts new, then you might want to look into an NST and some chinese ebay diodes, since the cheapest I've seen microwave oven capacitors on ebay is about $10-15 each, and you'd need at least 12 for a 6 stage multiplier.

StormInABottle

Fri Jan 18 2013, 07:01AM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

So the only way to go is making a large ass high voltage transformer. For 20kv at about as much amps as i can get

What 'd the turns be. and how much iron 'd i need?

StormInABottle

Fri Jan 18 2013, 07:47AM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

XravenorX wrote ...

Well the circuit is indeed very simple, It's just the parts are expensive, because the capacitors will have to be truly enormous to let 30mA through while maintaining 12kV across the load.

A capacitor is reactive, and impedes the flow of AC current. The impedance will drop as the capacitance rises, or as the frequency rises. Since you want to use 50 or 60Hz, you need more capacitance, so that the multiplier has less impedance. Less impedance = more current and less voltage sag.

EDIT: As Ash Small has said, you're best hope would definitely be microwave oven capacitors and diodes, since that will probably be the cheapest way to build up enough capacitance in each stage.

I recall building a three stage MOT multiplier with MO caps and diodes. It produced a decent amount of voltage at open circuit, but when I added about 4 meg ohms to the output and drew an arc it really wasn't very impressive. I got better results with a basic 555 flyback driver, which was much less expensive to build, and far smaller.

So far as I can see, you want a robust HV power supply capable of producing 12-15kV DC at approximately 30mA? This supply is going to be used with a SGTC?
If you're buying all your parts new, then you might want to look into an NST and some chinese ebay diodes, since the cheapest I've seen microwave oven capacitors on ebay is about $10-15 each, and you'd need at least 12 for a 6 stage multiplier.

I can't find a NST anywere.

Also a flyback cannot give a tesla coil enough MA Even on a zvs driver.

GrantX

Sat Jan 19 2013, 04:05AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Arcingnoob wrote ...

Also a flyback cannot give a tesla coil enough MA Even on a zvs driver.

If you wind your own ferrite transformer you can build it to any specification you desire. Also, I've seen ZVS drivers up to 2200W on youtube. Much more than your average NST. I have no idea how it would perform with an SGTC though.

StormInABottle

Sat Jan 19 2013, 07:35AM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

XravenorX wrote ...

Arcingnoob wrote ...

Also a flyback cannot give a tesla coil enough MA Even on a zvs driver.

Hmm. How can i wind a flyback lol? i heard that you cannot wind a flyback like any other transformer
I think you make a layer of wire. then insulate then how to make the next layer and connect it to the previous... Also i saw the 2000 watt video..
Also because it would be AC. and i can make it hit resonant with a nice cap bank.

So lets say i wind it like that.. I wind from left to right. put a layer of insulation and then from right to left with the remaining wire?
Also which AWG should i use of enamel wire

StormInABottle

Sat Jan 19 2013, 09:31AM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

Look at that guy..

with alot of work i can wind 3000 turns

Ash Small

Sat Jan 19 2013, 02:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Arcingnoob wrote ...

So lets say i wind it like that.. I wind from left to right. put a layer of insulation and then from right to left with the remaining wire?
Also which AWG should i use of enamel wire

Always wind from left to right, or vice versa, in the same direction.

Wind one layer, then insulate, then take the wire back to the start, insulate again, then wind the next layer.

Anything below 0.1mm will be difficult to wind. Generally it's best to use the thinnest you can, and you are only limited by breakage/stretching.

Current rating also needs to be considered. It's mostly 'trial and error'.

Others may have other suggestions.

StormInABottle

Sat Jan 19 2013, 03:51PM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

I took apart a old flyback core... it is ferrite i think. it is 2 U shaped and assemble togeather into a O shape. The lenght of the O is 6 cm

And the width is 3.5 cm.
The air gap inside that O
4.3 lenght
1.6 cm wide.

I am planning for 3000 turns. 100 turns per layer for 30 layers

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