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FAQ: Measuring Fusor Voltage - Part I - Analog voltmeter

Posted: Thu Jun 27, 2002 4:50 pm
by Richard Hull
This FAQ has been completely re-written 9/27/19 R.H.

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).

An analog meter is one with the classic needle that moves upscale as voltage is increased. For this discussion you must obtain a 100ua or 50ua meter movement. ua is "microamp"

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 dissipate the lost energy placed in it. Such resistors can be rather costly the modern MOX styles are ideal but are expensive. Surplus 100 megohm resistors are usually seen with a black spiral wound carbon track like a barber pole visible. The import thing is that with a 100 ua meter, one 100 meg resistor in series with the meter turns it into a 10,000 volt meter. Two 100 meg resistors in series with the meter will make it a 20,000 volt meter.
Each added 100 meg resistor with increase the meters reading by 10,000 more volts. Series connections must be done with common sense and great care.

The best way to make a composite 100 meg resistor if you can't find one is to connect, in series, 10 - 10 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.

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 equalizes 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 contain at least 10 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.
Remember at 40kv a 3" arc in air is possible and corona will foul your readings. keep the resistor string ends far apart see the diagrams below.

One image shows one of the many fully made up fusor voltmeter and ammeter systems that I use to offer for sale in my trading post. Never made any money at it so I gave it up. In one image I am calibrating one of my finished Analog metering systems. you see and example of a stacked divider using (5) 100 megohm factory made resistors. Note the spacing and insulated standoffs and non-conductive stand.

Richard Hull

Richard Hull

Re: Measuring Fusor Voltage

Posted: Thu Jun 27, 2002 5:12 pm
by guest
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.)

Re: Measuring Fusor Voltage

Posted: Thu Jun 27, 2002 5:22 pm
by Richard Hull

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 10 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

Re: Measuring Fusor Voltage

Posted: Fri Jun 28, 2002 4:03 am
by guest

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.

Re: Measuring Fusor Voltage

Posted: Fri Jun 28, 2002 3:33 pm
by Richard Hull
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

Re: Measuring Fusor Voltage

Posted: Fri Jun 28, 2002 4:01 pm
by Mike Veldman
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.

Re: FAQ: Measuring Fusor Voltage

Posted: Wed Jul 03, 2013 6:57 pm
by Richard Hull
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

Re: FAQ: Measuring Fusor Voltage

Posted: Wed Sep 11, 2013 11:03 pm
by charlie_mccartney
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.