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Author: Subject: very high voltage
thefips
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[*] posted on 5-8-2004 at 11:16


It microwave oven transformers are used,you get a problem with a lot of heat.In the microwave oven,the voltage only accelerates the electrons,produced by the hot wire in the magnetron.But if you want to built a spark gap,a very high current goes through the transformer.Here in Germany we have 220V~ and a 10A fuse was destroyed by the current.So you have to limit the current,or I remember,that someone built a metal box mit oil for cooling and isolating the transformers.To get a higher voltage you have to built the transformers in series.If you want to built an energy-bank the capacitors must be built parallel.And the capacitors need DC,so you will need a lot of diodes and resistors.And don´t forget a switch to discharge the capacitors,because it ist very dangerous and days after the last charge it can still kill you!
An other method to produce high coltages is a flyback with a cascade or a tesla coil.
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[*] posted on 5-8-2004 at 13:48


Marvin,
Nicely put; the second transformer doesn't know it's the second it just knows it has a larger voltage so the magnetising current must be larger. that's enogh to show that, in the limit, you cannot stack identical transformers in cascade and expect them to work.

On the other hand, I have a neon sign transformer that runs from a 12 V battery through an inverter so I do run transforemers this way (12 V to oscilator to low voltage AC through a step up transformer to 240V then stepped up again by another transformer (the NST) to 10KV)

Of course the number of steps up and down of the supply voltage between the generator and my house is even bigger.
If the transformers are built for the purpose you can stack them up like this

To get back to the original question, I don't know how hard you can drive 2 MOTs in cascade; but it might be fun finding out:)
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[*] posted on 10-8-2004 at 08:21


my tesla is operative at the moment, we experienced a 1mt arc lenght, and esteemed around 1-1.2MV unfortunately some insulation burned off and at the moment it doesn't work anymore..i think i will fix it this weekend..
if you need VERY high voltage i advice you to use a tesla coil or a VTTC...
the smallest VTTC i've seen ran around 300kV.




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[*] posted on 10-8-2004 at 10:39
Voltage across air


When I was in junior high school(mid 1970's), my teacher said it took
about 25000 volts to arc one inch in the air. I have no I idea if it's true.
Of course in the 70's, science was more primitive than it is now.

BTW, I loved the old Van De Graffe generators. They were fun seeing
someones hair stand on end - at least I had hair then !

Depending on the voltage, today I would go with a transformer for a
neon sign. They're good up to 15kV. Gwalters, out of curiosity, are you
trying to run a CW CO2 laser ?

[Edited on 10-8-2004 by MadHatter]




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[*] posted on 14-8-2004 at 02:37
Re: Voltage required to arc in air


I have a copy of the book "Electrostatics and its applications" by A.D. Moore (Wiley-1973). Section 17.4, headed "Dielectric Breakdown Of The Atmosphere", p.397, says that long sparks can be initiate in a potential field gradient of 4.5 x 10^5 V/m, and that natural lightning can propagate long distances in electric field gradients of at least 6.5 x 10^5 V/m. In the case of very small regions, the electric field gradient can exceed 5 x 10^6 V/m before breakdown occurs.

For the distance between electrodes mentioned of 1 inch or 0.0254 m, the lowest of the above figures requires a potential difference of 11,250 volts, the next 16,250 volts, and the highest 125,000 volts. The 25,000 volts mentioned is near the lower end of this range.

John W.
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[*] posted on 16-8-2004 at 22:31
High voltage is not really that expensive to produce


Or that complicated, when I was in grade ten I wanted an easy mark in my science course, so I constructed an induction coil out of pretty basic materials. (wood 2x4, large nail, strip of metal, spring, some wire coils wound by hand.

It was too primitive to work for very long periods of time, after a few minutes of slapping (you'll understand after you see a picture) the metal strip stuck to the nail because it would become magnetized, but it produced an electric shock that induced tetanus through black rubber lab gloves that were worn under leather mittens that had the cheapo poly fleece about an inch thick inside.

It was when I tried to turn it off that I got froze on the circuit, even my teacher was laughing so hard he didn't bother to try and help.

I was badly frightened, but the metal strip got stuck again and I got released.

Try looking it up, if I could do it........

EDIT: and they could do it..... http://physics.kenyon.edu/EarlyApparatus/Electricity/Inducti...

[Edited on 17-8-2004 by Democritus of Abdera]




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basicelectromechanic
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[*] posted on 7-12-2004 at 20:00


Quote:
Originally posted by thefips
It microwave oven transformers are used,you get a problem with a lot of heat.In the microwave oven,the voltage only accelerates the electrons,produced by the hot wire in the magnetron.But if you want to built a spark gap,a very high current goes through the transformer.Here in Germany we have 220V~ and a 10A fuse was destroyed by the current.So you have to limit the current,or I remember,that someone built a metal box mit oil for cooling and isolating the transformers.To get a higher voltage you have to built the transformers in series.If you want to built an energy-bank the capacitors must be built parallel.And the capacitors need DC,so you will need a lot of diodes and resistors.And don´t forget a switch to discharge the capacitors,because it ist very dangerous and days after the last charge it can still kill you!
An other method to produce high coltages is a flyback with a cascade or a tesla coil.


A " choke " in series with the transformer primary would limit current, the impedance would have to be matched to the transformers capacity, the formulas can be found in any good electronics text.
Of course the open circuit secondary voltage could not be maintained once arc established. Similar to the way a ballast limits current flow in a fluorescent lamp after the mercury vapor becomes conductive.
I seem to recall that the voltage tripler used in CRT used a combination of capacitors and rectifiers ( with an AC source?)
Discharge of a capacitor can be accomplished with a switch in series with a fairly large value resistor.
Throwing a high frequency power source into the experiment instead of 50 or 60 Hz could make it even more interesting, not just in the way it changes a factor but also by skin effect.
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[*] posted on 20-12-2004 at 01:45


I have used Microwave transformer extensively in research purposes for high current applications. I can say wothout a doubt most microwave trannies run between 1200 volts to 2600 volts AC ---with diode/cap you looking at 3 kVDC to 5 kVDC with currents usually 300mA-400mA.
------
You can easily series 2 transformers center tap grounded with 2 more floating on either side with cores isolated for 4.5-5kVAC per side---or a total short of 9-10 Kv(AC). Power output is DEADLY!
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[*] posted on 22-12-2004 at 11:02


Quote:
Originally posted by Marvin
...Voltage rises with the turns ratio of the transformer, impedence rises with the square of the turns ratio (required by conservation of energy). The output of the first transformer will be very high impedence, the input to the second one will be low impedence. For a working circuit the impedence of the primary coil of the second transformer must be at least as high, and preferably a lot higher than the secondary from the first. Simply wiring 2 MOTs together, I think youd be lucky to get your 4kv back out the other side, not to mention burning out the first transformer from power issues after a short time.


MOTs can easily pass 2X plate voltage rating. I know Greg hunter from http://www.hot-streamer.com/greg who has built a tesla coil with 4 MOTs in series an its a nasty little coil. I know another guy whos gone 6 pack for 14 KV @ 7kW. MOTs can provide a lot of power---yes, they aren't ideal but they do work.




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[*] posted on 8-2-2005 at 00:45


I have a lot of experience with MOTs, so I should comment here. There are several issues to address when using MOTs outside their intended circuit.

Most MOTs have one end of the secondary grounded to the core, since that allows minimum insulation between that part of the winding and the core (usually the inner part). You may not see this connection when the MOT is out of the microwave, but it's usually there. If you break that connection, there is a serious risk that the insulation between the winding and core can fail.

The second issue is that MOTs are too lean on core material, to save money. That means they leak a lot and distort the waveform. One of the buzzing noises in the microwave is the metal enclosure vibrating due to leakage from the MOT.

The core has substantial losses because the laminations are usually welded together in several spots. This causes more heating and buzz.

Here is how I've dealt with these issues.

Using a grinding wheel (the one that is thin and fits in a circular saw) I carefully grinded out the weldings. The E and I sections of the core can now be taken apart (you may need to split off I laminations several at a time with a sharp edge if the whole thing doesn't come off). Do it carefully as not to bend the laminations.

Now, using a screwdriver very carefully loosen the windings (together with their insulation) from the E section. This may take some time, as the thing is held together by epoxy or (in my case, shellac). You must take care not to scratch the enamel of the wires -- take your time! Eventually, you should be able to pull out both windings without damaging them. There will be a third winding, just a few turns of heavy gauge wire, sometimes on top of the high voltage one; this is the heater winding and should be removed. The magnetic shunts -- the blocks between the primary and secondary -- should be thrown out. This will increase leakage, but that will be fixed the proper way as explained below.

Separate all the laminations from one another and soak in some lacquer thinner. If you let them sit long enough, you won't need to scrub. All the epoxy/shellac should come off.

Examine the secondary winding. One side, usually the inside, will have very little insulation. This was explained above. Use mica insulators (from an electronics shop) to get about 1mm thickness. That will probably use two mica layers, so when you layer them, use an offset on the second layer so that the gaps between sheets do not overlap. Use clear nail polish between the layers to hold them together, and put some paper on top, and again brush some nail polish. That takes care of the insulation.

When you put together the laminations, going through the windings, it will be difficult due to the extra insulation. Do it carefully as not to break the mica. The laminations should not be in electrical contact with one another; use clear nail polish between them for insulation, as well as holding them together. Press together tightly. Now, it is important to interleave the E and I sections. This means EI, then I mirror-E, EI, I mirror-E, etc. Clearly, putting them together this way will have to be done with the windings in place, as there's no way to add them later. You should manage to fit in at least 95% of the laminations.

Normally, the magnetic shunts send excess flux back to the primary. With them removed, to deal with the leakage, do the following.

Find the turns per volt. You can do that by threading a winding of a few turns in the rebuilt transformer and measuring the voltage with the primary plugged in the outlet (use gator clips and hands away from the setup when doing this). Now, taking the core area (crossection of the part going through the windings, not outside them; also, consider that some percentage of the space is taken up by the nail polish between the laminations), you can calculate the Bmax (Google for the calculations; I'm not giving the formula here as you should read the explanation behind it for better understanding). You'll almost certainly find that it exceeds saturation for your core material (most often grain oriented silicon steel). Again, these transformers are made with saving money in mind, so they do not have enough iron as a proper transformer would -- just to manage for their specific application in a microwave.

Now just figure out how many turns to add to the primary to go well below saturation. In my case, to a 1.5 kW MOT I added 20 turns (it's was about 1 turn/Volt), so I have about 130 primary turns now. I did this after I had rebuilt the core, and to be able to thread the extra turns faster I made a makeshift shuttle that could squeeze through the space left from the trashed magnetic shunts. Make sure you have enough insulation between these turns and the rest of the primary; since the voltage is low, a copule of sheets of paper (and the nail polish) will be fine. The insulation between primary and secondary has to be good, though. Use an enameled solid copper wire, of gauge that is either the same or just a bit thicker. It is very important to wind in the same direction as the original windings! The extra turns are of course connected in series with the original winding. Now, to avoid a drop in secondary voltage, you may add a proportionate number of turns to the secondary. That will be hard to do, but it's possible. I managed about 10% more turns, so I only ended up loosing a bit of output voltage. You must be very careful with insulation between the extra turns of the secondary and the secondary, as well as between these and the primary, and the core as well. Use mica. I found I could get some extra space by removing some of the insulation around the outside of the secondary, as there was too much, and fitting some more turns there.

Brush the whole thing with more nail polish. The laminations should be squeezed together while the nail polish is drying. The best way is to use bolts and nuts in the corner holes. Do not use metal ones as they interfere with the magnetic path; use large nylon bolts from the hardware store and just leave them in place.

Extra -- about getting good DC: rectifying -- microwave oven rectifiers generally have low current ratings. Use chains of 1N4007 instead. Put seven or eight of them in series to make one diode. In old times, this configuration needed paralleled resistors and capacitors for even distribution, but this actually makes it worse with modern components. Now diodes are manufactured with chaining in mind, and when not perfectly matched, the slightly overvoltaged 1N4007 in the chain will start softly leaking, and the chain equalizes. Just make sure to use diodes from the same manufacturer. Also, at these currents cooling is not an issue.

For filter capacitors, you can get film in oil 3 kV capacitors on eBay. I got several 40 uF for $15 each. Otherwise, you can chain electrolytics for higher voltage. Rate for 150% of actual voltage. Don't forget that capacitance decreases when putting capacitors in series, so you'll need a lot. It is very important to have equalizing resistors in parallel with each capacitor in the series chain, as capacitors are usually poorly matched and they'll get overvoltaged; once one blows in the chain, the rest will fry in a chain reaction. At the very least ten times the capacitor's rated leakage current should be bled through the equalizing resistor. Usually it should be much more. This has been discussed at the diyaudio forums, so you can search there for estimating formulas. Watch the wattage ratings on the resistors.

In a filter configuration of CLCRC following a bridge rectifier (and I actually split the R so half was on the ground side), I get a few tens of millivolts ripple for 2.5 kV DC at 400 mA (totally 160 uF capacitance). I wound my own L by rebuilding the core of a transformer I ripped from some busted amplifier; it's about 4 Henries and less than 10 Ohms, so it doesn't drop much voltage. The resistors are four 75 Ohm, 22 Watt each, which I got very cheaply from allelectronics.com. Regular coaxial cable makes excellent high voltage cable, and is safe if the cable shield is grounded (so use the center solid conductor for the high voltage). Connectors are a problem and you should hardwire. Using BNC is an option, but they are only rated to 500-600 V, and it is possible for an arc to form. Make sure you have fuses on the primary side no matter what! SHV and MHV connectors are designed for high voltage, but they are extremely expensive.

You may want to add a fan if you put all this into an enclosure. Film in oil caps usually like <45*C. I recommend a simple thermistor based speed control (you only need one transistor for this) to keep noise down when the temperature is low.

I'm working on a regulator, but I'm driving class A plasma amplifier, so I don't think it's really necessary.

Finally, the capacitor bank stores several times the energy of a defibrillator, which means instant death if they discharge through you!

[Edited on 8-2-2005 by Quince]




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