High frequency capacitor discharges

[johnb003]

Hello.

I've been trying to follow Tesla's descriptions for creating high frequency spark gap discharges from a capacitor, and I'm having some trouble getting really high frequencies.

As I understand it, Tesla was using a magnetically quenched spark gap, such that when the capacitor discharge is formed it would immediately be blown out. However, so far I haven't been able to produce any noticeable effect with a strong magnetic field.

So far I've built a hv power supply using a flyback transformer and a 555 timer:

edit - 2/23/09 - Updated video to show more detail of the power supply:
YouTube - High Voltage Power Supply

edit - 2/25/09 - Added a new video with oscilloscope readings:
YouTube - HV Power Supply Oscilloscope Readings

So I have my hv output connected to a hv capacitor, and I've started with a very small spark gap. I get rapid discharges, but they are pretty distinct and I can tell it's not that high of a frequency. I tried putting a fairly strong magnetic field surrounding the spark gap, but it sounds very much the same. I thought maybe the spark gap has to be larger so the discharge is more intense for the for the Lorentz forces to have a stronger action. So I made the gap wider but still I can't really make out any observable difference with the magnetic field vs without.

In the article from Tesla "On Light and Other High Frequency Phenomenon" he clearly states that with the magnetically quenched spark gap, the sound of the discharge is more like a gun shot which he says is because the discharge is formed and broken several times. I guess he means formed and broken several times for what would have been a single discharge.



Has anyone created a nice magnetically quenched spark gap, and noticed a considerable difference compared to the same without the magnetic field? Tesla's assembly seemed pretty nice, especially since he was using an electromagnet which he could vary the intensity to fit his needs.

Thanks,
`John

[johnb003]

Also where does everybody get the materials for their experiments?

I feel handicapped when it comes to constructing nice rigs. Even for a spark gap, I mean sure I can go to home depot and find metal rods and probably a block of wood, but I feel like I'm trying to make fire with twigs and stones.

What about mounting brackets, or what if I wanted some kind of magnetic metal plate that I could adjust into place to fine tune the poles of an electromagnet or something. Or what about the shape of the rods, perhaps I want them tapered, or maybe even threaded near the neck so I can adjust the gap size.

What do people do these days, if you don't have the equipment to machine or mold everything yourself? Is there a cost effective solution to prototyping?

`John

[Xenomorph]

Hmm, most people improvise and use their imagination. Experiment around, there is no rules.
Check for example Aromaz´ spark gap: YouTube - Aromaz 026 - Final Lab Note on Sparks, Corona and Water

I mean Your gap looks okay.
Home depot is a good start, look for drawer knobs preferrably brass. Other metals will work too of course.
Thats my 2 cents, you will get MUCH more advice here in the course of the day for sure

P.S.: Nice idea with with chaining a couple of shunt resistors to a bridge thing to protect the Mosfet

[sucahyo]

How do you layout the magnet?

[Electrotek]

Spark Gap Magnet

Here's how I made a spark gap with a magnet.

I took an old Hard Drive magnet, covered it with 4 layers of Crystal Clear shipping and storage tape, then stuck two spark plugs onto the magnet. I don't know if the plugs were resistor type or not, but I clipped the ground prongs off, and used the end electrodes for the input.

This is a very powerful magnet and produced an extreme effect, perhaps more so than what Tesla was working with. I charged a 16uF cap with a 7.5kV NST. Then when I hooked the cap to the plugs, it made a loud bang. But there wasn't a spark between the plug electrodes themselves, just a dotted line half way between them, perpendicular to the line of the plugs. Five little thin blue dashes, separated by spaces. And the dashes flinged themselves downwards, etching the surface of the magnet.

I read somewhere that the magnet can be attached to one jaw of a vise, allowing it to be moved closer or further from the electrodes. With a magnet this strong, only one of them is needed.

I also read that the discharge wires can be bent like a tiny Jacob's Ladder, only curved. Place this next to the magnet and you should get a pretty fast blow out. Hopefully upwards, depending on polarity. This might be better than the spark plugs since I was only getting one discharge per charge of the cap. Then again, Tesla was powering his setup with a HV dynamo generator. My cap only stores around twice the Joules I get with each pulse from the transformer.

[johnb003]

Thanks for the replies!

Here's a description of the spark gap I tried:

I took a chopstick, it was pretty square but also pretty thin, and I taped two wires onto it such that there was a small gap between them. The wires were stiff enough to not move too much, but could be bent into place. I bent them out a little so the spark wasn't right against the magnet. Also the wires were non-magnetic. Then I took another chopstick just like the first and placed it on the other side of the gap so the chopsticks were like rails and the gap was suspended between them in the same direction as the chopsticks. Then I had 4 disk magnets two on the bottom two on the top, such that they clamped down on the chopsticks and held it all together.

I can also take the magnets off and just use the one chopstick by itself supporting the spark gap at the same distance as with the magnetic field.

I ran the power supply connected to the cap and the gap and I get the loud repeated cracking, It sounds like two stones being slapped together really hard. But there's no real audible difference between with magnetic field and without.

I ordered stronger magnets, so I'll give those a try this weekend, but otherwise I'm puzzled why it's not working. As I mentioned I tried this experiment again with a larger gap size, but though the cracking was less frequent and much louder, there was no difference between with and without the magnetic field that I could detect.

Xenomorph:
Yeah, the shunt resistors are rated for 10W of power each, yet they were only 1.5 ohm. And I figured if P = V^2/R, solve for voltage:

V = sqrt(10*1.5) = sqrt(15) = close to, but less than 4.

So, since my supply is 12V, I need to put them in series, which gave me 6 ohm of resistance, so then I was like, well I want more current, so I'll put 2 more of these chains in parallel.

It's probably not needed with the inductance of the coil and all, but it doesn't really hurt the HV, and if something goes wrong in my 555 then once the field stabilizes the inductance goes down and suddenly I'm drawing a lot of current it would fry the mosfet probably burn up the flyback, and blow my fuse :P

Anyway that video looks interesting, but I'll have to wait until tonight to check it out.

sucahyo:
That's precisely the setup I was referring to from Tesla. I wish I could build something study that resembles that layout but it's hard without any machining equipment.

Electrotek:
I would think with bent electrodes, you have to be careful to make sure the magnet blows the flame away from the rails and not along them. Also I'm curious why your spark was drawn into the magnets and not perpendicular to them, I thought with the lorentz force it would be perpendicular to both the wire E field, and the B field.

`John

[Electrotek]

Thanks for the replies!

Electrotek:
I would think with bent electrodes, you have to be careful to make sure the magnet blows the flame away from the rails and not along them. Also I'm curious why your spark was drawn into the magnets and not perpendicular to them, I thought with the lorentz force it would be perpendicular to both the wire E field, and the B field.

`John

The Hard Drive magnets have N on one end and S on the other end, so the direction of the B field is parallel to the surface. With my setup, both the E field and the B field are in the same direction. So the perpendicular direction is towards the magnet's surface. And the lateral spread of the spark dashes is also a perpendicular direction.

[johnb003]

The Hard Drive magnets have N on one end and S on the other end, so the direction of the B field is parallel to the surface. With my setup, both the E field and the B field are in the same direction. So the perpendicular direction is towards the magnet's surface. And the lateral spread of the spark dashes is also a perpendicular direction.

Your hard-drive magnets are like this?...


The magnetic poles are on the flat sides right? So if you picture just putting your gap inside this entire mount, wouldn't the B field be perpendicular to your e field?

The B field would be between the N and S facing edges, and the E field would be in the direction of the spark gap right? + to -

Have I misunderstood something?

Or are your magnets N and S the long way for some reason? (such as top to bottom in the picture)

[Electrotek]

The magnet assembly in the picture actually shows 4 magnets. Mine is similar, but only has two full length magnets, one on each side. I guess it varies from one brand to another. I took the whole assembly apart, and just used one magnet. With only one magnet, it is N and S the long way.

[johnb003]

I see. Interesting that's it's long ways. Too bad though, I think it'd be really cool if the magnetic poles were on the facing sides.

I just got some new magnets in the mail. These ones are very strong, I'm excited to try to use them, but they will seriously hurt my fingers so I need to find a better way of bringing them close together.

`John

[Electrotek]

Well, when you have TWO magnets spaced like your picture, the poles are on the facing sides. Also, with one magnet, one end is N on one side and S on the other side, with the poles reversed at the other end. Tricky.

[johnb003]

I understand how the field would look with two magnets interacting. I guess I was just curious about where the poles were.

Lets say the magnet is 1x2x5 units, and ignoring the the bend. You're saying your single magnet has the poles on the 1x2 side?

But in the picture it's probably on the 2x5 side? Or are you saying they are probably the same, but 2 linked are probably forced together with the same poles so that you have something like this:

N----SS----N
==========
S----NN----S

[Electrotek]

It's actually like this:

N----SN----S
==========
S----NS----N

Or, with a single magnet:

N-----------S
S-----------N

[johnb003]

Anyone have tips for constructing vacuum experiments? I wonder if I could create some kind of plexiglass box or clear pvc tube or something, with pressure fittings that could be connected to a running vacuum.

Anyone know how much a vacuum pump suitable for this kind of thing would cost? I think I recall Stanley Meyer had some sort of vacuum hooked up to his cell in the video.

I'm really just interested in studying electric discharges in a vacuum.

[Inquorate]

Vacuum pump

Both nat and I bought 12v car tyre pumps - some have an inlet hole in the top of the pump's compression cylinder. I cut a piece of tv aerial tube and glued it on over the hole, then glued a gauge from a foot pump on there but backwards: I haven't actually fired it up for use yet but nat has; his gray tube collapsed? from the vacuum.

My compressor cost me $15au, the same one that nat got.

I'm thinking of vacuum tests too, including turning 12v car globes into vacuum tube diodes...

Good luck!