FAQ: Measuring Fusor Current

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Richard Hull
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FAQ: Measuring Fusor Current

Post by Richard Hull » Thu Jun 27, 2002 3:36 pm

The fusor body MUST be attached to real electrical ground. Note* this is for a standard, negative hot, DC fusor power supply. It will not work on AC

A nearly indestrucable 10 ohm WIRE WOUND, 20 WATT or larger resistor must be connected between the POSITIVE lead of the power supply (often the metal shell of the power supply case) and the actual system ground (real electrical ground) Real electrical ground goes to the fusor shell.

If, however, you are using a center tapped, case ground transformer like a neon sign transformer with two rectifiers for fullwave, half voltage rectification, you MUST NOT ground the case directly, but instead hook the transformer case to ground through the above 10 ohm resistor. (case floats above ground) This is OK as the case will never be more than about 1 volt above ground. This will give you only 1/2 or less of the transformers rated voltage, but will be full wave rectified.

If you are using a single end, grounded case transformer, in half wave, full voltage mode, you would also hook its case to real electrical ground through the above resistor.

A diagram is attached on just grounding for current measurement.

If you are using a bridge for full wave rectification to get full wave, full voltage rectification from a neon transformer you must electrically and physically isolate the transfomrer!! This will, unfortunately, allow its case to float to a leathal 7 KV or more!!!!!! You must then ground the positive output of the bridge through the above resistor (This last method is NOT RECOMMENDED AT ALL). It is a scenario for death and disaster.

Regardless of supply hookup, all CURRENT metering is done across this 10 ohm resistor!

The easiest method is to take a standard, industry wide, 0.1999 volt digital panel meter and place its input across this resistor and set its decimal point so that it reads 19.99 or xx.xx Having done this, you now have a 0-20 ma reading digital current meter hooked to your fusor. For those wanting a 0-200ma scale (xxx.x), the 10 ohm resistor should be changed to a 1 ohm 20 watt resistor and the decimal point wired on in the third digit.

Note** I prefer the 1ohm resistor for the much higher 200ma reading capability.

Digital panel meters found at

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

Digital panel meters require a 5 volt or 9 volt supply of their own for power and you will have to provide this.
NOTE* one supply can power several of these meters. Marlin P. Jones can supply these meters in LCD or LED display. I highly recommend the LED meters as they can be read in the dark. Their website is

http://www.mpja.com

Regular, analog, panel meters could be used, if desired, but you would have to cobble up your own shunt and make it super beefy!! An ideal candidate would be a 50ua meter with a shunt designed to allow it to read 50ma. Obviously this arrangement would REPLACE the above resistor in the current circuit between the supply positive or transformer center tap to ground.

WARNING>>>> NEVER EVER use a "correct reading" meter! i.e., Never use a fixed 0-20ma analog meter or 0-50ma meter or a 0-100ma meter. If the meter's hair fine movement wire burns open, the entire supply grounding could float to lethal levels.

ALWAYS USE A BEEFY SHUNT across a very sensitive analog meter if you choose this latter route for current indication. This shunt should be of heavy gauge wire wound nichrome wire.

Knowing fusor current is a MUST for real fusor operation and data collection on a research level system. Reading the system current in a manner which retains full safety grounding is ultra important.

Farnsworth's team at ITT measured their HV supply current directly on the hot lead!!!! You can sometimes see in old photos that they had a large hollow aluminum terminal atop the fusor HV insulator and inside of the closed hollow sphere was a normal meter movement set back inside from a hole. A telescope was setup at a distance to look into this hole and read the meter by eyeball. It doesn't get anymore brute force than this.

Richard Hull
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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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Martin Hathaway
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Re: FAQ: Measuring Fusor Current

Post by Martin Hathaway » Sun Aug 31, 2014 4:08 pm

Why is it that when you isolate a center-tapped NST the case only floats one volt above ground while when you isolate a non-center-tapped transformer the case ends up floating to 7kV? And why is this so bad?

Martin Hathaway

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Richard Hull
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Re: FAQ: Measuring Fusor Current

Post by Richard Hull » Sun Aug 31, 2014 10:46 pm

I was not talking about isolating the transformer, but adding a 10 ohm resistor to ground for current measurement. The case is still, effectively, at ground potential. A 100 ma draw (impossible on a neon sign transformer), would raise the case to 1 volt above ground.

The isolation was regarding difficulties encountered when you rectify using a bridge in an attempt to get full voltage, full wave rectification used on a center tapped neon transformer or on a non- center tapped neon. Truly floating the case would leave both types of transformer cases hot and not allow you to ground the shell of the fusor making it very dangerous and deadly to touch.

If you understand bridge rectification you can readily see this. Your current metering and voltage metering would also be deadly to touch or be near.

Bottom line, never bridge rectify any neon transformer or every part of your fusor is lethal. The best you can hope for with a properly grounded center tapped neon transformer case is one half the transformer's rated output under most any load and probably a lot less, due to that loading. example: A 15kv neon transformer that is rectified and case grounded, will never supply 7.5 kv DC to any demo fusor. Probably not even 6kv with the plasma glowing.

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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