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Registered Member #1378
Joined: Thu Mar 06 2008, 01:08AM
Location:
Posts: 4
Hey all. I've played around a bit with electronics in the past, but I've never really done any high voltage or high frequency AC. Anyway, in order to get into it I was hoping to make a marx generator powered off a CW multiplier running at 240 kHz. I don't know if this is feasible, but I'm hoping that it is. The inverter I was thinking of using is here: http://ocw.mit.edu/NR/rdonlyres/ED571AB5-1285-4B77-B4C4-CB1B0A986518/0/building_hvps.pdf, which outputs at 240 kHz. I was going to drop the voltage doubler included in it, though, and replace it with a multi-stage one. Also, I would remove the current-limiting resistors on the output, and use a 4.5 V input to get an output of 225 VAC(peak). This would then be used as the input for a 10 stage CW, which would then power a 5 capacitor marx generator. The CW would use 10 nF, 500V ceramic capacitors, and the Marx 60 kV, (about)3.5 nF homemade capactitors described here: http://wiki.4hv.org/index.php/Rolled_foil_capacitor_-_60_kV%2C_3.5_nF
Anyway, I have a few questions about this. The first, naturally, is if it would work. Like I said, I'm not experienced with HV, and there could easily be something I left out/got wrong etc. Anyway, if it would work, what kind of diodes would I need? I'm a bit confused about reverse recovery times. The diodes used for the voltage doubler are 1N5062's, which have a reverse recovery time of 4 microseconds. It says here: http://www.maxim-ic.com/glossary/index.cfm/Ac/V/ID/1015/Tm/Reverse-Recovery-Time
"When switching from the conducting to the blocking state, a diode or rectifier has stored charge that must first be discharged before the diode blocks reverse current. This discharge takes a finite amount of time known as the Reverse Recovery Time, or trr. During this time, diode current may flow in the reverse direction. "
Does this mean that the diode conducts in reverse for 4 us after the voltage switches polarity? A 240 kHz signal has a period of 4.2 us, and switches every 2.1 us, so wouldn't this mean that the diode will always be conducting? Anyway, I'm not sure what it is that I got wrong here, so any help would be greatly appreciated.
Third, why do marx generators have their resistors placed so each capacitor has one more resistor limiting the current to it than the one before it, for example, here: http://en.wikipedia.org/wiki/Image:Marx_generator.png. Wouldn't this make the last capacitors in the series charge slower, so when the first fired, the rest wouldn't be ready (Because the first one has 1 Mohm, the second 3 Mohm, third 5 Mohm etc. in front of it)? I understand that you need to prevent the capacitors from discharging in parallel, but couldnt' you put one in front of each individual capacitor, but still before the spark gap, so it doesn't interfere with the others or the discharge?
Finally, would this (hopefully 20 kV discharge), be enought to light thermite?
Anyway, answers, suggestions or comments are greatly appreciated, and hopefully someday I'll have enought knowledge to repay the community and answer someone else's questions.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Welcome to the forum!
240kHz is a pretty high frequency for a CW multiplier, and would require some pretty fast diodes, thus increasing price. You could simplify your design a lot if you just forgot about the inverter and CW system and used a common TV flyback driver. These will give you a very high DC voltage that you can use to charge you Marx directly. 50kV is not a tough achievement with the right flyback. Or you could have an AC flyback (~15-20kHz) powering a high voltage CW multiplier.
But whatever you do, turn the frequency down by a factor of around 10. Then you don't need to worry about the whole diode mess.
However, as far as igniting thermite goes, I don't think pulses from a Marx Generator would work. A continuous arc would be much more reliable for such a task, such as those from a current-limited CW multiplier. However, arcs from a half-decent flyback driver by itself can ignite thermite.
If this is a proof-of-concept thing, like to just demonstrate that electrical arcs can do the job, then its all good. But if this is meant to be a practical thermite ignition device to be used often, I would have to say that there are MUCH easier ways to light the stuff (sparklers come to mind).
Oh and by the way I can't open any of those links on Firefox or Explorer. Anyone else have this problem? I get 4HV.org: Error 404
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Exactly my point; a thermite ignition system need not be complex. But the system he describes may not only be for thermite use.
It's actually one hell of a build; combining three things that most here would consider a project by itself: First a high-frequency inverter, then a CW multiplier, then a Marx gen.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
you cannot trigger a Marx with 235V input. That's too low for spark creation. And you do not need 60KV rated caps in case you are not going to load to 60KV every stage in your Marx. You can change the amount of layers in the cap and increase its cpacacitance. Two layer of transparencies between the alu foils inside the cap gives around 8,5nf and still can handle around 25KV. My first Marx used only two layer caps and was loaded by 20KV DC flyback. It was giving around 80J at 400KV in a pulse rated in nanoseconds!
Registered Member #1378
Joined: Thu Mar 06 2008, 01:08AM
Location:
Posts: 4
Shaun Welcome to the forum!
240kHz is a pretty high frequency for a CW multiplier, and would require some pretty fast diodes, thus increasing price. You could simplify your design a lot if you just forgot about the inverter and CW system and used a common TV flyback driver. These will give you a very high DC voltage that you can use to charge you Marx directly. 50kV is not a tough achievement with the right flyback. Or you could have an AC flyback (~15-20kHz) powering a high voltage CW multiplier.
But whatever you do, turn the frequency down by a factor of around 10. Then you don't need to worry about the whole diode mess.
However, as far as igniting thermite goes, I don't think pulses from a Marx Generator would work. A continuous arc would be much more reliable for such a task, such as those from a current-limited CW multiplier. However, arcs from a half-decent flyback driver by itself can ignite thermite.
If this is a proof-of-concept thing, like to just demonstrate that electrical arcs can do the job, then its all good. But if this is meant to be a practical thermite ignition device to be used often, I would have to say that there are MUCH easier ways to light the stuff (sparklers come to mind).
Oh and by the way I can't open any of those links on Firefox or Explorer. Anyone else have this problem? I get 4HV.org: Error 404
Thanks for the welcome. The thing about flybacks, is, where would I get them? I know that they're used in CRT TVs and computer monitors, so would I have to buy a broken one on ebay?
About the inverter, I would have preferred to use a lower frequency, but I couldn't find any other schematics that weren't designed for flybacks. Would one of these work with a different type of transformer? Also, the inverter guide says that it operates at the resonant frequency of the LC circuit on the secondary's side. So in case I can't get ahold of a flyback or MOT, could I increase the value of the capacitors to change the resonant, and therefore output frequency? Or would this not work?
On the thermite, no I don't intend to build it only to light thermite. I was just wondering if it could be used this way.
teravolt: if you want ti ignite thermite use a MOT or neon sign tranformer
Again, I'm not sure if I can get a MOT or NST, and the thermite ignition would just be a nice side effect, not the main reason I want to build it.
Shaun:
Exactly my point; a thermite ignition system need not be complex. But the system he describes may not only be for thermite use.
It's actually one hell of a build; combining three things that most here would consider a project by itself: First a high-frequency inverter, then a CW multiplier, then a Marx gen.
Yeah, I figured it would be hard combining the three. I thought I had to because I didn't have any HV transformers, so the only option I knew of to step it up into the kV would be a CW. Using a CW I could only have so many stages without it becoming too inefficient, and it alone wasn't enough to get above 10 kV. So I decided I'd have to use a marx after that, to get it up higher.
Mates: you cannot trigger a Marx with 235V input. That's too low for spark creation. And you do not need 60KV rated caps in case you are not going to load to 60KV every stage in your Marx. You can change the amount of layers in the cap and increase its cpacacitance. Two layer of transparencies between the alu foils inside the cap gives around 8,5nf and still can handle around 25KV. My first Marx used only two layer caps and was loaded by 20KV DC flyback. It was giving around 80J at 400KV in a pulse rated in nanoseconds
I wan't intending on using the 225 V to power the marx gen. I was going to put the 225 VAC (peak) into a 10 stage CW with an output hopefully close to 4 kV and use that for the marx gen. Sorry, I should've been clearer about that. Thanks for telling me about the capacitors, though, I forgot that you could make it to varying thicknesses if you didn't need as high voltage rating.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Buy a broken TV on Ebay?!! You must be joking! Go to local yard sales; they can be had for $5. The older the TV, the better! Plus can you imagine paying the shipping for that?
But seriously, yes any CRT TV or computer monitor will have one. As for drivers, I'd have to recommend the ZVS driver since I've been using it successfully at very high powers for months:
However, the 555-based half bridge driver is known to be very good too.
The inverter you linked to looks simple enough that you could easily change the frequency, should you choose to go that route. Just make sure the components you use can handle a reasonable amount of power throughput.
Registered Member #1378
Joined: Thu Mar 06 2008, 01:08AM
Location:
Posts: 4
Wow.... never really thought of looking for a CRT screen at a yard sale. Don't think I've ever seen any of them at one, but I'm sure I can find something if I look hard enough. They're actually pretty cheap on ebay, but you're right, it's the shipping costs that kill you. Anyway, I'll look for one next opportunity I get.
Thanks for the link to the ZVS, I'll build one when I find a flyback.
Registered Member #1157
Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307
Logan Kennedy, from the forum here, has old disk style flybacks for sale in the sale and trade area. I think he is charging $10 for the flyback, and $5 for shipping. See here
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