#4b FAQ - Wiring for real fusion fusor - part two

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Richard Hull
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#4b FAQ - Wiring for real fusion fusor - part two

Post by Richard Hull » Tue Mar 02, 2004 7:19 pm

This circuit is for the much more common single ended type high tension transformer. In this design, one end of the secondary is connected to the frame ground of the transformer and a single high voltage knob is found on the output.

Such transformers are common among fault testers, insulation testers, and x-ray transfomrers going up to over 100kv!

Due to their construction, only a half wave rectification system is really possible and a capacitor might be a good idea to hold up the voltages at higher currents. Again, this is a lethal supply, as are all real fusing fusor supplies.

Digital panel meters found at

http://www.mpja.com/Digital-Panel-Meters/products/52/

Richard Hull
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Re: FAQ - Wiring for real fusion fusor - part two

Post by fruitytuter » Mon Jul 04, 2011 2:17 pm

Mr. Hull, Being that the electrical current measurement is done on the outside of the HV transformer case, I assume that the case must be isolated from ground via plastic stand offs ect..
Would this be correct? Also why is the 10 ohm resistor rated at only 5 watts while the 60kohm HV resistor is rated at 100 watts? If this is a circuit shouldn't the power rating be similar throughout? I have some experience with electrical circuits but perhaps I am missing something. Internal resistance in the transformer?
Getting something wrong at these power and voltage levels could be lethal so I want to be absolutely certain of what I doing.

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Re: FAQ - Wiring for real fusion fusor - part two

Post by John Futter » Tue Jul 05, 2011 9:42 am

Josh

a little thought before posting is in order

do the sums I^2 x R

Richard has worked all this out with safety margins in mind
in normal operation the 10 ohm resistor might see 36 milliwatt but if it fails then the panel meter will get fried and rise to dangerous potential with possible flashover to the transformer primary

the 60k ohm will see under arc over conditions albeit transitory 20.3kilowatts at 35kV this only being there until the capacitor is discharged--- note the transformer will not supply this power unless it is very large due to core saturation----fuses should blow before the ballast resistor (if it is wire wound)

other types of resistor might require an even higher wattage to cope with short transients without damage


for all of this you require to know the behaviour of all componets and what their overload and high voltage characteristics are

ps I would put transil, transorb type diodes across the DPM meters to protect them and yourself

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Re: FAQ - Wiring for real fusion fusor - part two

Post by Richard Hull » Tue Jul 05, 2011 2:15 pm

John's got it right. The 10 ohm will never suffer high currents mostly due to the 60K ohm (which is under oil in with the transformer and is a large wirewound.)

In normal use, with a fusor, the 60K ohm will have a continuous 20 ma max running through it. Thus, it will drop ~1200 volts and dissapate ~24 watts. It stands by to drop more voltage in case of short transients and dissappate much more energy due to its gross overating. Normally I run 15 ma full tilt and the resistor will dissapate only ~15 watts. Sometimes, in glow clean mode, I run 35ma and then it has to handle ~75 watts

The diodes and much more will blow long before the limiter resistor goes. I have had a few external insulator arc overs and the diodes have survived, thanks to the limiter resistor.

Note, I do not use a filter cap. I run full wave rectified only on Fusor IV.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Wiring for real fusion fusor - part two

Post by Dan Tibbets » Tue Jul 05, 2011 7:35 pm

A couple of questions.
Have you considered using parallel resistors at the current sensor/ ground. The redundancy might provide some more protection against resistor failure. I'm guessing that if one failed, the only indication might be that the amp /volt measurement wouuld change by a factor of two.

As far as diode protection of the amp determining voltmeter, what voltage tolerance is needed. Would a second independant ground to the transformer with perhaps a 100-1000 Ohm resistance provide any backup protection without introducing too much error in the amp sensing circuit? Would there be additional thermal loads on this resistance?

With 1 diode (string) the current to the fusor is pulsating/ 1/2 wave rectified as stated. If full wave rectified, would the fusion output ~ double? Or, is there enough capacitance in the wires and fusor that there is already significant filtering of the pulsating DC current?

[EDIT] Sorry, I confused your schematic with what you (R. Hill) said about your Fusor IV. It is full wave rectified without a capacitor. The schematic is of a 1/2 wave rectified and presumably '1/2' voltage doubled with the diode + capacitor arrangement. (eg: if 12 KV NST the ac voltage would be 6 KV ac and 8.4 KV somewhat filtered dc after the diode/ capacitor stage).

Dan Tibbets

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Re: FAQ - Wiring for real fusion fusor - part two

Post by Richard Hull » Tue Jul 05, 2011 9:35 pm

Any number of backups of backups are possible, triple and quadruple redundancy with transorbs, diodes, extra resistors and extra ground out safety paths, but for an old fusor runner like me one simple, overly beefy resistor is good enough and I use a digital meter, not an analog which would fry instantly if the resistor went. but with all the ground connections in the meter, a fighting arc would scorch and smoke to alert me to what has happened..... A great indicator.

Now, if I were using a reversed pole transformer with large Macklett 100kv 1 amp rectifiers much like I powered my 15KW coils, then I would rethink everything related to fusor current measurment. A simple 20ma rated x-ray transformer with 100ma HV silicon diodes has enough weak points to where the 10 ohm resistor will not blow out before something else buckles.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Wiring for real fusion fusor - part two

Post by Doug Coulter » Wed Jul 06, 2011 4:27 pm

I note something in Richard's circuit that many might miss. His filter cap is grounded separately. A good move. This means that in the event of an arc -- even one that arcs across the beefy ballast, that this stored energy has a way to get to ground without going through the current sense resistor.

In our experiences here, it's the stored energy in the stack or filter that's the big problem. Avoiding having that go through things that can't take it is a fairly big deal and a very good move indeed. It's also the reason we favor the use of higher frequencies, as that means you don't need as many stored joules to give decent ripple filtering. In a CW multiplier, the top diode has to carry any peak currents in an arc from the AC side caps....so the smaller the better, for them as well.

A lot of times "the devil is in the details" and "the wires that aren't there" are the ones that matter.
Why guess when you can know? Measure!

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