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Old 22nd September 2004, 03:47 PM   #1
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Question 2MHz, High voltage source?

Hi, I have recently been trying to buid a circuit that will allow me to swicth a signal of around 1700V peak to peak into a mainly capactive load as part of a university project.

The only problem so far is that as soon as my output gets much beyond about 700v peak to peak (using a transistor switch to drive a step up transformer; about 1:10 turns ratio) the transistor goes pop (probably because of the back EMf formed by the transformer when the MOSFET switches off).

I am considering two aproaches to solving the problem, firstly I will simply try and build something which will keep the voltage accross the MOSFEt to less than 200V (using a zenner diode, or VDR e.t.c.).

The second thing that I was considering was to use a valve circuit in place of the MOSFET one I am currently using. However, I don't know a lot about valves, so I was hoping some body here would be able to point me in some sort of direction towards a possible circuit design for this type of device, or at least tell me if I am being a little hopefull in getting this sort of performance in a circuit of this type.

Anyway, if anyone has anyinformation that may be useful, I would be most greatful to hear about it.

Andrew.
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Old 23rd September 2004, 05:48 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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Don't know a lot about it, but many tubes are easily capable of this. An 807 tube will survive up to 30MHz at idle voltage of 600~700VDC.

You might want to look for an oscillator circuit, or try some ham radio guys?
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Old 23rd September 2004, 01:17 PM   #3
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Any horizontal sweep tube will work.
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Old 23rd September 2004, 07:02 PM   #4
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Quote:
Originally posted by Geek
Any horizontal sweep tube will work.

Ditto.
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Old 23rd September 2004, 07:18 PM   #5
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What's a horisontal sweep tube? It sounds like it should be part of a TV set, but that's only a guess at best, I guess I'll have to go and do some googleing.
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Old 23rd September 2004, 08:15 PM   #6
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Sounds like what you want is a vacuum tube tesla coil. Is the capacitive load steady or does the capacitance change during the experiment? If steady you can make an air core HV resonator that will feed this capacitive load, the two forming a parallel LC circuit (or series depending on your point of view). You will need to build an RF power oscillator with a vaccum tube (triode is best). A tank coil from the vacuum tube oscillator can magnetically couple to the HV resonator. You also wind a grid 'tickler' coil in next to the tank coil to supply positive feedback to make the oscillator work. If you can tolerate RF that comes on and off with a 60 Hz sinewave pulse train with 50% dead air between pulses you can use unfiltered AC on the oscillator plate supply. If you need ripple free RF CW you make a filtered HVDC power supply. There are many suitable triodes available. How much power do you need, do you know? How critical is the absolute frequency and the stability of the frequency? Does it have to be phase locked to another device?

Here is a picture of what I am speaking about. This coil system uses four 811A triodes operating in a self feedback RF power oscillator at a much lower frequency of several hundred KHz. You can see the white plate tank coil, the green grid tickler coil underneath both wound on a PVC pipe former, and a HV resonator inserted inside the tank to develop the high voltage RF. This unit is producing about 50-75 kV into the corona load with a little over one horsepower of RF.
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Old 23rd September 2004, 08:23 PM   #7
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Cannot apparently add a picture with edit post mode. Here is the picture.
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File Type: jpg 4x811a01.jpg (21.1 KB, 135 views)
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Old 23rd September 2004, 11:36 PM   #8
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NICE
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Old 24th September 2004, 12:21 AM   #9
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Default Re: 2MHz, High voltage source?

Quote:
Originally posted by bigparsnip
Hi, I have recently been trying to buid a circuit that will allow me to swicth a signal of around 1700V peak to peak into a mainly capactive load as part of a university project.

you know, for a country which invented RADAR...

a horizontal sweep tube won't handle 1700 V -- there were only one or two which were rated for Plate voltage >450V (I know this flavor well, having built several ham transmitters in the 1960's)., nor will the 807 and the the 811 mentioned has a max plate voltage of 1500V

You could use a 8068 if you can find them -- octal base, anode cap, these were used in high voltage tube regulated power supplies for the US Dept of Defense. If you find them, let me know !

the triodes 3-100, 3-150, 572, 810, V70 and tetrodes PL-177, 813, 4-125, 4X150, 4CX250

if using any of the aforementioned you need a really robust filament transformer...

you are probably better of stringing together some beefy HEXFET's together in series -- grab urself a copy of Horowitz and Hill in the University Library and you will see an example.
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Old 24th September 2004, 12:26 AM   #10
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Thanx! Here is a neat parlour trick one can do with such a device under the guidance of a trained professional. There is no hidden wire up the wrist. The man's hand and body are passing all the current used to illuminate the standard household 40 watt incandescent light bulb. Inexperienced amateurs, do NOT try this yourself. Such energy is lethal under normal conditions.

Bigparsnip,

I can email you two jpg. images of a construction article which can be used as a tutorial. PM me a request if you desire with a real email address that can receive it.
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