FAQ: Measuring Fusor Current
Posted: Thu Jun 27, 2002 11:36 am
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.
I submit a paper on the subject in this PDF
Richard Hull
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.
I submit a paper on the subject in this PDF
Richard Hull