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How to increase reactance in a coil of few turns?


thrival
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Hi. I'm working on a HV project using a microwave oven transformer as the source. Rather than use the 120 turn primary winding (1:20 P2S ratio), I knocked out the magnetic shunts, removed the heater winding to make room for a new primary winding of 12 turns, upping the ratio to the secondary 1:200. Unfortunately I keep blowing fuses when I plug it in. Does anyone have an idea how to increase reactance in the primary without costing voltage at the output? (R's and caps in series cause a drop:(
Thanks!

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thrival,

Welcome to this community,

If you switch the mains voltage in to the 12 turns no wonder you are blowing the fuses! This is close to short-circuiting. You need to puls the feed or have a much higher frequency at hand if you are gone get away with it. If you want to change the ratio don

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Thanks for your reply.

As a matter of fact I was gonna pulse around 14 khz inside the half-wave 60hz envelopes. It is the latter frequency causing the fuse issue. Are you saying at the former, my circuit could still work?

Not so easy increasing number of turns on a microwave oven transformer secondary; 2400 turns is already a lot!

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All that I see is a fuse-blowing circuit:
1) The IGBT feeds its half-wave 60Hz pulses into the 12 turns primary (dead short) of the transformer.
2) The 120VAC mains is rectified and filtered with 4 RC filters, developing about 42VDC (when the 1K pot is at maximum) which far exceeds the 20V maximum rating of the gate of the IGBT. If the gate survives, then the IGBT will be turned-on all the time. But the fuse probably blows before the gate voltage becomes too high.
3) The only function of the 1M pot is to adjust the time delay of up to a few seconds before the fuse blows.
4) The circuit doesn't have a 14KHz oscillator.

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Dear Ante & Audioguru:

Thanks for your replies. Yes the is the original (& final unless you have improvements) version of my circuit.

The filter section provides steady DC to the RC oscillator (about the simplest I could find: see page 102 & 192 "Basic Oscillators" by Gottlieb.) Audioguru, you are correct the filter outputs 48V, but its a pot so I tap it at the 24V setting. The 1meg pot and 10uf cap allow me set the frequency where I want it, and the tap between the two components never reach above V/2 (about 12V) which is within the gate rating of my IGBT.

Yes, I blow fuses, the solution I hope you gentlemen might have.
Update: yesterday I shorted the secondary and tested the primary winding again, hoping the added reactance might help. Not only did I blow another fuse but the faintest spark in the secondary and now its dead. Apparently the HV side of MOTs aren't meant to handle kV's.

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Thrival,
I am sorry but I don't see your oscillator and can't find your book.
The only way that your circuit could oscillate is if the transistor is a unijunction, which it ain't.
I'm sorry to hear that your IGBT died. Without any protection, it probably would have blown if the load disconnected or was turned-off anyway. Protection against inductive "flyback" spikes in your DC circuit is only 1 cheap diode connected in reverse-polarity across the primary.

If you ever do add an oscillator to your circuit, please be careful because its HV current output will be lethal..

What are you going to use it for? Electrocuting bears? Moose?
If the HV doesn't kill them, then the shock-wave of the loud thunder probably will.

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Hi; thank you everyone, for their comments-- even the negative ones! (I make my ego transparent and they go right through me!)
Audioguru: (Your comments are helpful, made me look harder and see my secondary diode was wrong polarity.) The oscillator is the grounded RC, the 1meg R & 10uf cap, tapped in the middle. The transistor gate is the switch. FYI, I tested that part (without the MOT) and it worked fine in the 20usec range of my scope. Such simple oscillators are used in many commercial inverters, schematics of which you can find all over the net, but description of their workings is hard to find. You'll see all timer chips use a simple rc relationship to set the frequency. I'm combining the switching aspect of the oscillator with the gating function to let line current pass through the transformer, and no, I'm not 100% sure this will work till I try!

Since my original idea of stepping up voltage by reducing primary turns is a bust, I'll just use the MOT as it meant and add another coil for the output I need. (see attached circuit.) I'm building a stan meyer hydro-torch. (See: http://www.zip.com.au/~alansch/danforth/dan1.htm
http://www.rexresearch.com/meyerhy/meyerhy.htm

circuit2.txt

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Thrival,
I can see now that your "oscillator" is actually an emitter-driven half-wave 60Hz pulser.
You won't get much 14KHz out of individual flyback pulses of 60Hz, unless something resonates at 14KHz.
That's a cool application for electrolysis of water. I like the picture of the "hydrogen-powered jet engine".
Assuming that the engine's exhaust is pure water vapour, if you can condense the vapour then use the resulting water as "fuel" again, then it should run for a long time without adding water.

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Remember that the current specified on a fuse is the peak current. You can have a 3amp average transformer and still get higher peaks. The wire amperage chart is the average current. In other words it's different than the fuse. The current for a 22AWG is about 1A average. Could somebody corroborate this. I did an experiment one time and stumbled across this observation. I used bare copper wire for the ouptput of a bridge rectifier and it blew. I used two kynar wire and it was ok. This tells me I was around the current I had calculated. I know I calculated average current so the peak must have been higher. I must mention also the way I calculate current. I break down the sine wave into equal parts and manually determine the current for each part. Then I get the average by adding the total current and dividing by the number of parts. This is most accurate I believe.

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Audioguru: thanks. The water molecule supposedly resonates just over 14khz, at least according to the good folks at oupower forum.
You mentioned earlier about paltry protection against reverse spikes in my circuit. Do you have any ideas to prevent that? One guy on above board said he fried a few mosfets before wising up.

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Dear Kevin:

Well I modified another MOT, adding 12 turns of wire. I used the 20 ft coils called "primary wire" from Home Depot. It's multi-stranded, plastic coated, not sure of the guage. Anyway I hooked up an electrolysis cell directly to the house mains through my transformer. My primary is 120 turns and my new secondary 12 turns. Didn't blow any circuits but the plastic just melts off the wires. It looks like when I want to step V down I need to run the MOT in reverse, 2400 turn primary, 120 turn secondary. That way at least the reactance limits the current.

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An autotransformer is easy to calculate for the current and voltage. An isolation transformer is done a little differently. You must have enough reactance to limit the current in the primary. The secondary will also have to be able to handle the current. Try this, breakdown the sine wave into 16 equal parts. Determine the voltage of each part. Then use the rate of change to determine the impedance. This gives you 16 precise values of current. Add the current up and divide by 16. This gives you the real average current. Use this in determining the size wire you will need.

With an isolation transformer it is hard to determine current because you don't take the reactance into account. This is the problem. A low reactance will affect the transformers ability to realize the turns ratio. You then have a primary reactance that is the load. This is your problem with the wires you are using. You don't have an isolation transformer because the reactance isn't high enough to produce the turns ratio.

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