FAQ: Measuring Fusor Voltage - Part I - Analog voltmeter
Posted: Thu Jun 27, 2002 12:50 pm
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
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