The Beginnings of a Fusor Electrical System

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Duncan Wilkie
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Re: The Beginnings of a Fusor Electrical System

Post by 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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Richard Hull
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Re: The Beginnings of a Fusor Electrical System

Post by 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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Rich Feldman
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Re: The Beginnings of a Fusor Electrical System

Post by 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

Post by 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?
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Rich Feldman
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Re: The Beginnings of a Fusor Electrical System

Post by 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

Post by 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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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie » Thu Mar 30, 2017 12:46 am

Quick question: I would use the full-wave rectification circuits for single-phase 120V 60hz input (standard mains electricity), grounding the positive load-end and using the negative rectified polarity with the instructional circuit seen in the schematic included? Not 100% sure, and I don't want to move on this until I have a plan.
03444.png
03444.png (10.57 KiB) Viewed 762 times
Also, are there any specific diodes or diode types I need to deal with fusor-level voltage flow?
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Rich Feldman
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman » Thu Mar 30, 2017 1:10 am

RTFF.
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Re: The Beginnings of a Fusor Electrical System

Post by Dennis P Brown » Thu Mar 30, 2017 11:04 am

As for specific diodes, you just need to be certain they can handle the surge currents (which can be much greater than the normal continuous current.) Of course, the voltage limit of the diode should be around twice the fusor max voltage (remember RMS?)

Aside: I fuse the input voltage of my x-former so that if the primary exceeds an "acceptable" level of input current, the fuse then blows preventing my primary or secondary from exceeding a safe current level that could damage those wire.

I bought 1 amp 20 kV diodes (under $3 each - so I got 20 total for various projects) and used two in series for each of my current paths. These diodes in series should withstand a max voltage of 40 kV. My x-former can easily handle 100 ma for a few minutes so my surge currents can be very high ...since I do not know how high, I error'ed on the cautious side and got those diodes (also, couldn't resist that great price.)

I placed my diodes under oil to improve their cooling but also because I could then use normal wiring and connectors with the diodes to save $$$ and avoid corona problems.

Aside: DO NOT add diodes to a system if you have not, as yet, determined proper wiring of the x-former; that can lead to a bad day for the didoes and it achieves nothing in determining wiring configuration. What people here have told you is outstanding advice - use a very, very low AC input if in doubt about the wiring configuration. AC voltages can start to become deadly very fast. Air grounding occurs quickly and then makes these voltages deadly. Follow HV safety procedures (i.e. one rule is: always assume any source is hot and deadly until proven otherwise!)

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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie » Fri Jul 21, 2017 9:37 pm

So, I finally acquired all of the components necessary to test my supply. I've spent the last few months working, and I've bought the components I believe will enable me to reach the demo phase. So, what I'll do in this post is verify my plan and pose a few questions I have.

The main question I hold is a general uncertainty about connections under oil. Can I use screw terminals? Can I use normal solder joints? My plan for encasing all connections in oil is to use a 5-gallon fish tank I had laying around and just submerging all my components in a configuration kind of like this:
IMG_20170721_150932.jpg
This is the configuration for my supply. The rectifier of 4 20kv 3A diodes is on top of the Plexiglas sheet and below it is my resistor chain of 8 50M resistors. All components will be totally immersed in oil.
To test the transformer, I have a variac and a 120vac-12vac step-down which I will connect to pins 5 and 7 (per Richard Hull's post). Then I'll connect 3 and 4 to my diodes and connect pin 8 to the positive reference of my meter. I'll connect the output of the pin 4 diode to the output of the pin 3 diode. The combined outputs of the diodes are the negative potential I'll run into my fusor or in this test the negative input of my metering circuit. I'll put two of my 50M resistors to step the voltage down to something measurable by my DPM.
IMG_20170721_150915.jpg
My variac/step-down setup
IMG_20170721_151223.jpg
Close image of my diodes and resistors
My last big question: Will my meter and resistors be sufficient as a load to the voltage?
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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