The Beginnings of a Fusor Electrical System

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Re: The Beginnings of a Fusor Electrical System

Postby Duncan Wilkie » Fri Mar 10, 2017 8:10 pm

Rich-
Please correct me if I'm wrong, but this is the calculation I did to get the values.
For the coil with a resistance of 160K ohms at .02A, voltage drop is derived from V=IR, so V=. 02*160000,so V=3200V. Power Dissipation is P=IV, so P=.02*3200, so P= 64A.
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Re: The Beginnings of a Fusor Electrical System

Postby Richard Hull » Sat Mar 11, 2017 4:07 pm

This is a DC reisitance of a coil and relates to the secondary's "source impedance"' and is a complex issue based on the load type and value. DC figures can't be used accurately on a coil of wire supplying AC in a transformer system. Much also depends on the core and how it is made. To noodle out the complex issues, good instruments and a bit of investigation is needed.

DC Coil resistances only give a clue as to what "might" be their function, and little to do with there actual in-circuit performance, which is based on many other factors.

DC circuits and networks can be taught and learned in a one semester college course. All the ramifications of AC, reactive and HF power circuitry can consume 2 years of upper level college study.

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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Mon Mar 13, 2017 10:58 pm

Duncan, your answer to my latest numerical question is correct, except for a petty typographical error.

Drawing 20 mA from an XRT secondary winding that has 160 kΩ of DC resistance will cause the voltage to drop by 3,200 V, and will generate 64 watts of heat in the winding. Those numbers hold for DC or RMS AC. The resistance and copper loss will increase as the winding heats up.

As mentioned before: if you configure a full-wave rectifier, using one HV diode on each secondary winding, each winding would provide half of the average current and get half (not 1/4) of the heating calculated above. Each diode needs to withstand at least twice the fusor voltage. (Same applies to full wave rectifier circuit on a NST in demo fusor FAQ).
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Re: The Beginnings of a Fusor Electrical System

Postby Duncan Wilkie » Tue Mar 14, 2017 5:32 pm

Would the next step be to variac the primary way down and see what the ratio of power increase is? Or is there more still to learn before I plug it in?
Some say the glass is half full. Others see it as half empty. I say it is twice as big as it needs to be.
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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Wed Mar 15, 2017 7:01 pm

>> ... next step be to variac the primary way down and see what the ratio of power increase is?
>> Or is there more still to learn before I plug it in?

I'm still concerned about your very junior level of knowledge about electricity. Transformers do not increase power, in fact they invariably lose some. What's the name (or even just the initials) of your adult mentor who has experience with electrical connections and measurements at voltages over 100 V? Over 300 V?

You could do what I did when I had an unfamiliar x-ray transformer. It's slightly different, in some important ways, from your proposal copied above.

Step 1. Set up instrumentation that can measure AC voltages up to a few thousand. This can be tested, and sensitivity verified, with regular house power. Could be an analog or digital multimeter on an AC volts range, with simple external attenuation as described in FAQs.
Alternative: Set up to measure DC voltages up to a few thousand, and connect it to the _rectifed_ output of your XRT under test. You'll need that anyway for your demo or real fusor.

Step 2. Get your Variac, but don't use it to directly feed any XRT primary, with or without ballast.
Put a step-down transformer in between, that normally reduces house voltage to 12 volts or less. These are common for low-voltage outdoor lighting, and indoor lights that use low-voltage halogen bulbs. Generally have their own fuses on low voltage side. I'd find one rated for at least 50 VA, as opposed to (say) a doorbell transformer, or something out of an inexpensive cordless tool charger. The filament winding of a MOT (2 or 3 volts) might serve, with suitable precautions.

Then you can use most of your variac range, for much better resolution and repeatability, without having to put your XRT properly under oil. You could still die from touching an XRT secondary connection, even with primary voltage much less than 10% of nominal. But it won't jump through (much) air to get you.
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Re: The Beginnings of a Fusor Electrical System

Postby Duncan Wilkie » Wed Mar 15, 2017 10:38 pm

Thanks for your response, Rich. I did mean the voltage increase when I said power. Sorry for that careless mistake. I do have a guy, Matt D. who works at the local coal power plant who has helped me in the past and will likely help me with the overall system (i.e. grounding, other things specific to my setup).
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