## FAQ: Measuring Fusor Voltage - Part I - An Overview

Richard Hull
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### FAQ: Measuring Fusor Voltage - Part I - An Overview

This is part 1 of a 3 part FAQ that will instruct you in making a voltmeter for the high voltage supply of your fusor.

Part I is an overwiew of how the high voltage metering system is put together with a diagram of the general hookup at the end.
Part II will talk about obtaining and modifying a low voltage metering system that can serve as a basis for a high voltage metering system (with photos).
Part III will talk about the critical high voltage divider network to be hooked to the meter created in part II.

The voltage potential extant across the fusor is a very important data point in a research grade system.

This voltage reading must be taken from the fusor shell (ground) to the HV, negative hot terminal leading to the inner grid insulator on the fusor.

For steady state work where the continuous current to the fusor is low, (<50ma), this voltage may be taken at the power supply output, itself. (NOTE** for pulsed fusors much more heroics are required - not covered here).

To read the high voltage, a voltage divider resistor chain is required. This requires a minimum of two resistors. One is of very high resistance ( equal to or greater than 100,000,000 ohms - 100 megohms 5% or better tolerance). The other resistor is of a much lower value, but of some precision (~1%). You MUST obtain the high ohm dropping resistor in a special high voltage capable form which can also dissapate the lost energy placed in it. Such resistors can be rather costly \$30.00 to \$100.00 and are best found surplus or made up from two or more series connected HV resitors required to make up the desired value. Series connections must be done with common sense and great care.

The best way to make a composite 100 meg resistor is to connect, in series, 20 - 5 megohm, 1 watt, 2% flameproof metal film resistors. Place them floating in air between two insulating posts on a strip of plexiglass or G-10 fiberglass circuit board. Better still, and for more voltage standoff, use 50 - 2 megohm, 1 watt, 2% flameproof film resistors in series.

NOTE** the appearance of the average HV rated resistor always takes the form of a spiral wound, flat carbon track on a ceramic body. Often these are covered in shellac or a clear coating. Metal Oxide, "MOX" resitors are the ultimate but extremely expensive. This construction reduces corona and equilibrates the distribution of electrostatic potential along the body of the device. Avoid high ohm common carbon composition resistor strings. You should have and use only this special type of HV resistor. (Series strings can use common modern film resistors but must contian at least 20 or more in series.

Regardless of your final resistor choice, the HV divider resistor is best housed safely and well insulated within the HV power supply cabinet as the resistor itself is a lethal component when energized.

Analog metering - (meter with a needle pointer)...........................................................

For an analog metering system you can rely on, select a good 100ua (micoramp) panel meter. your dropping resistor should use 100 megohms per 10kv desired to be read. Thus, a 0-40 kv metering system would need a 400 megohm high voltage reistor that could actually have 40kv impressed across it without arcing or value disruption by surface currents or corona. This will produce a 100ua current in the HV resistor. Thus, a 100ua meter movement is ideal. (the scale must be changed to read 0 to 40)

I would suggest four large, 2 watt 100 megohm HV resistors in series (each only bearing 10kv of potential across each resistor body.) This high voltage resistor or series string must be elevated and on insulators high enough and far away enough from other components so that no arcing or even corona is seen. A wad of HV silicone grease at the terminals of each resistor junction will help suppress corona into the air. One end of the high ohm resistor or series string is hooked to the HV minus or negative HOT lead of the supply. The other end of the chain on an insulator is for connecting your second lower ohm dropping resistor to ground and indicator metering.

You should just connect a common 1 megohm 5% 1 watt resistor across a 100ua panel meter and alter the scale to read 0 to 40. This would obviously require you to pop open the meter, remove the face or scale plate and devise a new proportional scaled face 0 to 40.

The meter, with the permenantly shunted resistor mentioned, is connected with the positive terminal to ground and the negative terminal lead connected to the junction of the HV, high ohm resistor string and the lower ohm resistor to ground. Thus, we have a high voltage resistor in series with a 1 megohm resistor to ground with the meter across the 1 megohm resistor.

Finally **** IMPORTANT**** Always place a NE-2 or better neon bulb across all analog meter terminals (in shunt) to protect against high voltages appearing on the movement should the coil burnout or open up. Diagram below.........

This completes the analog metering for high voltage.

***************************************************************************************************************************************************************************

Digital panel meter - LED or LCD display of high voltage...................................

The best way to meter, and match for the preceeding current system, would be to palce a variable 0-15 kilohm (15,000 ohm) resistor across the input terminals of a common 3-1/2 digit, 0.1999 volt DC LED or LCD, digital panel meter that has been modified to read 0-20volts DC. Connect a very long 100 megohm resistor that is fully rated for your maximum fusor voltage to the Hot terminal insulator on the fusor and the other end to the Vin of the 0-20 volt panel meter which is across the above mentioned 15K ohm variable resistor. The ground of the meter goes to the shell and system ground of the fusor system. You will need to set the decimal point to xxx.x and the meter will read in kilovolts, i.e., 27.2 for 27.2 kv. Re-stated, in this arrangement, we have the HV resistor in series with a 10K ohm resistor to ground with the 0-20volt digital panel meter across the 10K ohm resistor.

There are two style of digital panel meters. Both will demand a power supply to function.

1. LED This is a positive glowing red digit display that is easily seen at a good distance in intense room light or pitch darkness. Such meters commonly demand a 5 volt DC supply capable of at least 150 ma of current, but it is easy to obtain 1 amp supplies at 5 volts for litle cost.

2. LCD This is a dark gray digit display against a light gray background that may or may not depend on the viewers angle to be seen well in a well lit room, but invisible in the dark. Backlit LCD meters are available which are much easier to read and can be viewed in darkness. LCD panel meters typically demand a 8-12 volt power supply with 9 volts DC being ideal.

Regardless of type, your panel meter must have an input impedance of 100 megohms and be of the 0-0.199 volt type. The source below offers, for \$1.00, a two resistor kit that will turn your meter into a 0-20 volt DC meter. You must purchase this kit.

Digital panel meters of all types can be found at

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

I attach a simple schematic of the digital metering system

I hope this helps out in voltage measurement for those unskilled in the art.

I will add images in the part II and Part III FAQs.

Richard Hull
Attachments
Analog meter diagram - simple, cheap, but not most accurate - Best for demo fusors under 20kv
Basic digital hookup diagram. Note the rectangle with the HV divider is best placed in the power supply cabinet.
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.

guest

### Re: Measuring Fusor Voltage

For your fusor work, would you recommend wire wound high voltage resistors, as they are easier to get a hold of?
I suppose for regular DC fusor work, that would be acceptable. But what about pulsed versions?

I'm personally more interested in the metrology of high voltage measurement than in getting a fusor to work.
(Uh-oh, I can hear moans.)

Richard Hull
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### Re: Measuring Fusor Voltage

David,

No need to moan, we are all in this for the knowledge and everyone can contribute, even no-fusorites. Your question is a good one.

I do not think there is such a thing as a wirewound HV resistor.

Power rated wire wounds usually go to only about 100k ohms, but are very rare even at that level. HV resistors START at 1megohm and most are well over 10 megohms. ALL are CARBON. You would need many, many miles of fine resistance wire to get there in a wirewound.

You would be best to follow my advice about using the 5 megohm film type 2% flameproof resistors in series. (see original posting)

Pulsed work often utilizes specially contructed, one off, systems of liquid water/copper sulfate solution, HV resistors in lengths of tygon tubing. Again, a rather serious undertaking with a lot of liquid mess, poisonous chemicals and plumbing which should not be undertaken by the casual fusorite.

These marginally conductive liquid power resistors are also often found used in giant capacitor bank energy storage systems as charging circuit current limiters.

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.

guest

### Re: Measuring Fusor Voltage

Richard,

I was looking at some Hosfelt HV resistors that I had ordered: part No. 47-165, which are 100Meg Ohm, 15kV (2.6W) resistors. They have "non-insulated ceramic construction". On examination, they appear to be cylinders with leads at each end, and what I had thought was wire wound around it, but was actually a black wrapping around it. In actuality, it looked like carbon that was scraped away to give a wire-wound effect.

For HV wire-wound resistors, I have an article called "Special Shielded Resistor for High-Voltage D-C Measurements", by J.H. Part, Sept. 26, 1961, that appeared in the Journal of Research of the National Bureau of Standards -C.Engineering and Instrumentation Vol. 66C, No.1, Jan-March 1962.

In it he describes a nice high voltage resistor divider (a little overkill for the low voltage fusor devices currently in vogue) that was built and tested up to 100kV, and made of a 200-MegOhm unit, consisting of 200 one-megohm wire-wound resistor units made up of , I believe, either Karma or Evanohm wire (trade names for special alloys - NiCr + Al+Fe -> having high resistivity and low temp. coefficient of resistance). Nichrome wire-wound resistors was also tried but found to have a large temp. coeff. of resitance. The paper mainly talks about how to shield the resistors to prevent corona from affecting measuremtns, and to use both positive and negative coeff. resistors to reduce the drift due to current self-heating.

Richard Hull
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### Re: Measuring Fusor Voltage

I would absolutely defy anyone to find a catalog offering 1 megohm wire wounds today!! They would cost a fortune!

In 1962, (article date), tubes were still king and highohm wirewounds were still in common use and in every manufacturer's catalog. Today, I found only one company offering 100k wire wounds. The 20 watt units were \$110.58 each!

I religiously buy every large power wire wound I see at every hamfest over 50kohms. They, like old hams are a dying breed......and....there is nothing else like them.

The Hosfelt 100meg 2.6W carbon HV resistors are superb units. They would be a deal at \$10.00 each! I have over a hundred on hand. I don't think they are available anymore. I may be wrong though.

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.

Mike Veldman
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Location: Stillwater, Oklahoma

### Re: Measuring Fusor Voltage

In case anyone is interested, I noticed in the Fair Radio catalog which arrived this week some 40M ohm 2.5"L x .6"dia @\$5 ea. and 80M ohm 6.5"L x .6 dia @ \$7 ea. No wattage specified. I ordered some just because I haven't seen many at the hamfests around here in a while.
mike
I tried to contain myself, but I escaped.

Richard Hull
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### Re: FAQ: Measuring Fusor Voltage

That's the spirit! Think "film resistors" of at least 10 meg each and series string them.... Regardless, the key to HV resistors is to make sure they are long such that an air flashover end-to-end in your string is just not possible.

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.

Richard Hull
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Posts: 11427
Joined: Fri Jun 15, 2001 1:44 pm
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### Re: FAQ: Measuring Fusor Voltage

I have updated this posting and will add a take you by the hand... Let's make a meter FAQ based on this FAQ. with full photo images.
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.

charlie_mccartney
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### Re: FAQ: Measuring Fusor Voltage

Thank you for this Richard, I am been searching tirelessly for a mentor yet, one had evaded me. This is perfect and I could not ask for a more in depth and detailed explanation.
Charlie McCartney
charlie.mccartney1104@gmail.com
charliespersonalproject.weebly.com

Liam David
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Real name: Liam David
Location: Arizona

### Re: FAQ: Measuring Fusor Voltage - Part I - An Overview

I am assuming that in the digital measurement circuit, the 100megohm resistor, 9.1K, and 5K 10 turn resistor are used no matter the max voltage, meaning that I don't use a 200megohm resistor for 20kV. Is this correct?
Thanks,
-Liam David