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gwalters
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very high voltage
i read that a microwave transformer takes 120 volts up to ~ 4000. 120/4000=.003 so if i connect 2 trans. together in series i get 4000/.003 = 1 333
333. will that work? will the wires in the trans. arc at that voltage? what about if i run it in oil? any other ideas?
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chemoleo
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well of course, the insulation of the transformer wiring will not withstand such high voltages. Look at power lines, and see how far they are
separated, or how long the insulator are (connecting the mast to the powerline). It's not for no reason!
The only way to easily generate high voltages like that is to either build an appropriate transformer with massive insulation, or better, to work via
capacitors in series. Safer, but still VERY dangerous.
You can get high voltage capacitors from TVs by the way, connect them up and charge them. If you short cut them, the power is so high that it makes a
massive spark and evaporates half of the wire!
Ever heard of Leyden jars (sp?) by the way?
[Edited on 10-6-2004 by chemoleo]
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gwalters
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ok then....say i connnect a bunch of caps. in series:
-cap+-cap+-cap+
like that...the voltage will add up but will i have to charge it with the max voltage or could i charge it with say 4000 volts until it is charged?
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chemoleo
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Well thats what I did a long time ago.
Connect them all in series, and charge them with an excessive voltage (doesnt matter if it is a bit more than needed  ). I attached a voltmeter to it, so I could monitor the voltage rising... after a few
minutes they would reach nominal/calculated voltage... and then it was time to shortcut them!
Be warned though, this is very dangerous. Plus, after treating the capacitors a few times like that, they refuse to operate any further... so clearly
that's not the standard way to go about this!
Also, I remember our central heating being replaced. Guessing that the oil is ignited by electric sparks, I thought, let's have a look at the old
central heating system. Indeed, very soon I found a lovely transformer that would amplify 220 V to 15000 Volts! (the engineers didnt mind me
scavenging the transformer  ) How did I find that out? Well obviously I connected
it to something like 20 V and measured the output on the second coil, and extrapolated the final voltage from it! With this I made many arcs, and
jacobs ladders. My most impressive experiment I remember doing is to cut a piece of obsidian apart (which is a volcanic black glassy like mineral),
whereby the sheer heat generated by the current flowing through the obsidian melted it so that it would conduct current.
After that I tried all sorts of stones... and they all fell apart. Neat stuff ... at least for the crazy mad scientist 
Again, be warned. I once managed to shortcut this through my fingers (despite precautions... it went through several cm of insulation)... and it left
two little black craters in my finger tips, which were painful. I was lucky my heart didnt give out!
[Edited on 10-6-2004 by chemoleo]
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Turel
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Parallel, Not Series
Run them in parallel, not series, if you want to accomplish anything. Running them in series will be a waste of capcitors.
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I am a fish
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| Quote: | Originally posted by Turel
Run them in parallel, not series, if you want to accomplish anything. Running them in series will be a waste of capcitors. |
You're wrong.
If you want to generate high voltages, the charged capacitors must be connected in series (just as batteries must be wired in series to deliver a
higher voltage). Connecting them in parallel will merely create a bigger capacitor, which will still be charged to the same voltage as the component
capacitors.
1f `/0u (4|\\| |234d 7|-|15, `/0u |234||`/ |\\|33d 70 937 0u7 /\\/\\0|23.
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axehandle
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If you want voltages in the 1MV range, you should feed a Tesla coil from an MOT or an NST. As others have said, transformers aren't made for such
high voltages. Powerlabs have many nice pointers.
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Geomancer
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Preferably, you charge the capacitors in parallel, then discharge in series, thus avoiding the need for a high voltage charging supply, and associated
corona losses. The switching devices are traditionally spark gaps. This setup is known as a Marx generator.
Be very careful when taking apart CRTs. They can retain deadly voltage for weeks, not only in the obvious capacitors but also in the tube itself.
While we're at it, microwave ovens are bad, too. Read the appropriate bits of the sci.electronics.repair faq conglomeration before proceeding.
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hodges
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Microwave oven transformers are dangerous because they are capable of supplying relatively high currents. They can easily kill you. I've made
fulminating silver before, which is something that is supposed to be foolish to do, but I'll never mess with a microwave oven transformer.
The safest thing to play with is a "negative ion" generator. These are often found in air cleaning appliances. They have needles attached
to them to electrify the air with charge. You can find an old air cleaner, or else check electronics junk dealers and you should find one for under
$10. Some will even run off a battery. They produce about 10KV but the current is low enough that it can't seriously injure you.
Lacking that, or if you do need slightly more current, look for circuits using either an automobile ignition coil or a TV flyback coil. I just did a
quick search and found this circuit:
http://members.misty.com/don/igcoilhv.html
These circuits are not really that hard to build, even if you have not done much with electronics before, and they are fairly safe. If you just want
to play around a bit and don't want to build a circuit you can always just buy an automobile ignition coil from a junk dealer. Connect it
through an ignition condenser to a 12 battery or power supply. By shorting/unshorting the capacitor you can generate high voltage sparks of an inch
or more in length.
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Cyrus
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Why don't you use your vandegraaff generator?
What's that, you don't have one?! 
http://www.amasci.com/emotor/vdg.html
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gwalters
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i tried to make a van degraff generator b4... didnt have the correct materials. Anyone made an emp before?
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darkflame89
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Try Tesla coil?
Ignis ubique latet, naturam amplectitur omnem.
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Marvin
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Wiring 2 transformers as you described wouldnt work and not just because of insulation problems. It simply wouldnt output the voltage you calculated,
they arnt black boxes that multiply voltage, you have to take into account the impedence of the inputs and outputs. 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.
All of the suggestions are workable ways of getting high voltage, but they all have different applications. You need to ask yourself what you want to
do with it. If it just big sparks, the best way would be with a van der graaf. You can buy something slightly bigger than a tabletop version that
will get you half a million volts.
Teslas are quite tricky to build, and the math requirement is heavy if you want it to work properly. You also need a fair amount of test gear to tune
everything.
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chemoleo
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Hmm, I realise what you are saying here, Marvin. I had this thought myself on occasion, but never had problems with it in practise.
I used just such a system to generate a voltage range from 500-1500 V, to charge up the capacitor system described above. In fact, I sometimes used 3
(!!) transformers (or two, with one 220V-->15kV transformer), one bringing down the voltage to i.e. 5 Volts (but high A), and then two/one reverted
transformers to bring the voltage up.
Presumably the reason why this worked was because the amount of power flowing through secondary coil of the first transformer was restricted.
The final output was high V, and low A - which was fairly un-dangerous and still useful for charging the capacitors...
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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gwalters
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does anyone know a good way to calculate the distance a spark will travel with a given humidity and voltage? I need to find a way to calibrate my
spark gaps on a mosfet (?) generator. Will wiring the caps. in series lower their working life?
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hodges
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Depends on how sharp the electrodes are (sharper = longer spark) but I think about 1kV/mm is ballpark.
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axehandle
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Sounds about right, hodges. I've gotten a 10mm spark between electrodes with a voltage of 9kV. That would support the math, w.r.t. the
resistivity of air.
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chemoleo
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Hmm, and I got a continuous spark with DC current, at 15 kV, about 3-4 cm long!
And the 15 kV is not something I make up!
To be honest, I think humidity, air pressure etc are going to matter.... but it's interesting to see there are some ball park figures to this!
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Cyrus
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I always thought that sharper electrodes would yield SHORTER sparks!
When you have sharp electrodes the electricity tends to leak off as corona leakage, when you have spherical electrodes the electricity comes off as
sparks! Thats why to test voltage by measuring spark length spherical electrodes should be used.
At least thats the case for vdgs, I'm not sure if that is the case everywhere.
Chemoleo, what did your electrodes look like?
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chemoleo
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lol, htat used to be simply the tips of wires...
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Oxydro
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If you use a constant-current source like a van de graff, which has very limited power, sharp electrodes will just mean that due to corona leakage,
the voltage will never be able to climb as high as it would for smoth-surface electrodes (basically think a capacitor with a resistor bleeding the
charge away, unless you have enough power you won't be able to get it to full charge). But for a flyback, nst, mot, etc the power is high enough
that the sharper electrodes simply have the effect of lowering the breakdown voltage of the air.
Remember, you should be measuring the point where the sparks/arcs start, not how far you can draw them out. For sparks (off VDGs, capacitors, etc)
they should be the same, but you can usually draw out an arc a lot further than the starting length. The ultimate length is dependant on current as
well as voltage, while the breakdown distance is purely a function of voltage (withstanding humidity, etc, of course).
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Marvin
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For large spherical electrodes dry air at 1atm breaks down at about 30kv/cm.
chemleo, Its certainly possible to wire transformers together, no doubt, its how the national grids of every country I know reduce pylon losses. They
design the transformer impedences specifically to do the job, if you arnt trying to do anything massivly spectacular you usually get away with it at
home. Stepping down 220 to 5 and then stepping up works very well because the low impedence part of the first transformer is driving the low
impedence primary of the secondary transformer. For the specific case of 2 identical MOTs wired secondary to primary though, it certainly cant work
and I'm almost certain the second transformer could not generate a higher voltage over its secondary than the first transformer would unloaded.
If a very large MOT was driving a much smaller MOT some stepping up would be observed, but not a lot.
MOTs arnt things Id recommend people use unless they had to, they are rather unforgiving compaired to neon sign transformers.
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unionised
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Provided that you are not trying to run so much current that the core saturates; or so much voltage that the insulation breaks down; you can run
transformers in cascade (running them in series is different).
The impedance looking into a transformer is equal to the product of the load impedance connected to the secondary divided by the turns ratio squared.
If you connect no load to the output then real part of the input impedance is (for an ideal transformer) infinite. For the setup that you are talking
about ( a pair of MOTs ) this would work for low voltages at the input. Unfortunately, the losses of the second transformer load the first so its
output is reduced.
(and, by theway, the arithmetic is wrong in the first post 120/4000 = 0.03 so the output would be 133KV)
You might be able to get away with running a volt into the input to test the idea.
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Marvin
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I think that explanation is oversimplified to the extent the answer is wrong. I'm bothered by terms such as 'ideal transformer'
because inevitably the device follows some unstated laws of physics and ignores others, and which it follows and which it ignores seems to vary
according to who is doing the math and what they expect the answer to be. I'll try to build up a realistic ideal transformer as I go.
The thought experiment, expanded somewhat, that lead me to the no greater output voltage conclusion runs as follows..
Assuming an transformer is 2 coils of resistanceless wire wrapped around a common non saturating core,
Assuming its a 33:1 transfomer and we have 2 of them (they are identical) and assuming the power source is mains and has zero impedence.
Wire the first tranformer primary to 120V mains as a step up transformer. The primary coil excites the core and the field induces a voltage in the
secodary of the expected 4kv or so. So far so good.
Same experiment, only this time the mains is 4kv and I'm interested in the difference. The voltage over the same primary is higher, so the
change in current over time is also higher resulting in a higher change in core field over time and the field ultimatly reaches a much higher values.
This must be true in order to induce the higher voltage in the same secondary which depends on rate of change of flux over time.
Ok, final stage. We wire them in cascade step up - step up again with no load.
The classical voltage we expect is a little over 130kv in this example.
The first core should excite to the level we had before assuming its output is going to be the 4kv or so we expect.
In order to output more than this, let alone the 133kv though, ignoring how the secondary of the first is wired to the primary of the second.... The
second core has to be excited to a much higher flux in order to induce the higher voltage in the second core - yet the energy for this to happen comes
entirely from the first core.
Given the cores are *identical* to reach the same flux levels requires the same energy, so for the second core to be experiencing a higher change in
flux/peak flux than the first core violates conservation of energy.
In what circumstances does this not apply...
We can make the second transformer much smaller, so that the same amount of energy from the first transformer results in a much larger change in flux
over the smaller volume of core, and a larger number of turns (while keeping the same ratio) and get out the expected 130kv. The objection only
applies because we know the 2 units are identical.
Though I had nothing in the way of maths to backup my argument this was enough evidence for me to post that I doubted the output voltage could exceed
4kv under conditions described. I should point out that reason I had no maths was because nothing in books or in education covered this directly.
While I mentioned before that a transformer output impedence rises with turns ratio squared, its not relevent to our 'ideal' example,
because our mains input impedence is zero, and our output impedence is infinite.
In my second post I was talking about impedence, unionised, youve pointed out that the real part of the impedence for an ideal transformer is zero and
I agree. However the windings are massive inductors and this I think is the clue to why the conservation of energy argument and our simplified
transformer maths do not marry.
Going back to a single transformer, and imagine it being attached to an impedenceless DC source. Lets assume the primary coil has a massive
reactance, say 10H. Since the secondary is wound to the same core, and the number of turns are in proportion in our ideal setup, the reactance must
be in proportion, giving our secondary a reactance of 330H.
When we turn on the juice, it reacts just like an electromagnet, the change in current behaves exactly like a 10H inductor would (open circuit
secondary, so thats all it is), and the change in flux is correct for inducing the expected voltage in the secondary.
Now lets say we add a choke to the primary circuit, say 10H. When we turn on the power the current will rise at half the rate it did before (becuase
the same voltage is now driving 20H total), This means the flux in the core will raise at half the rate and therefore only half the voltage will be
induced at any given point in time. Its equivalent to saying half the voltage is dropped over the primary and half over the choke even though there
is no real componant to the impedence.
Right, going back finally to the cascade setup. The primary has a reactance of 10H, inducing the 4kv or so in the second winding. The seondary is
closed circuit with the primary of the next transformer. If the primary of transformer 2 was being powered from an impedenceless 4kv supply the
current would rise at the right rate to produce 130kv over its secondary. The total reactance of the secondary circuit though is both coils in
series. 330 + 10 = 340 H. We expect the current in the secondary to rise 10/340 or 1/34 what it would be, and thus the field and thus the voltage in
its secondry will be 1/34 what simple multiplication would get us. 120x33*1/34*33, or slightly less than the output of the first transformer on its
own with no load.
Its mismatched impedence causing a power factor problem. This as I understand it, is the meat to my generalisations about transformer impedence. Its
also as best as I can see it an intrinsic problem, nomatter how well you make the transformer, no matter how high the reactance (inductance) of the
windings, the ratio will still be exactly the same as the step up ratio and thus cascade of 2 identical transformers will still fail to increase the
voltage.
Rule of thumb, for this system to stand a hope in hell of working, the impedence (including reactance) of the secondary winding of the source
tranformer must be less, and preferably much less than the sink primary winding. You may assume that all the AC values in this post are RMS, not that
the values matter at all, just for illustation. In a real transformer, the core would be designed so it was just short of saturating for its working
voltage, which keeps down core size for a given power, but also makes it useless for overdriving even when you ignore insulation issues.
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Marvin
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Ok, put simply, I screwed up.
2 identical transformers in series can develop a higher voltage than the first one. In a test done by a friend 2 1:10 signal transformers were wired
secondary to primary, and a small input produced a little less than half of the expected output. On furthur testing insulation failed on the second
transformer.
My first argument on energy grounds is invalid because the argument is only true for no current in a transformer secondary. The current in the
secondary creates a field that reduces the field - while increaseing the current in the primary. While there is no power drawn by the second
transformer it is drawing current from the first and thus the idea breaks down.
I do not yet properly understand why the second argument fails, only that it does.
My apologies.
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