FAQ - current metering - digital readout

[Richard Hull]

Chose a digital voltmeter preferably a junk meter new for $6.99 at Harbor Freight. These meters are 1 megohm impedance on all ranges. As such, the meter need not enter into any calculation regarding the value of the shunt resistor.
You will be using the voltmeter range. (not the current range!!!)
This example will be for a 0-20 ma meter for your fusor.
place your meter on the 20 volt range.

Now we want a shunt resistor across the meter to read 20 volts across the shunt in the ground leg of the supply (positive voltage for the fusor) and thus indicate 20 ma full scale. R=E/I 20/.02 = 1000 ohms.

Locate a wire wound 1000 ohm resistor of 1-10 watts. Note: 20ma in this design demands a 2 watt resistor. 50 ma needs a 4watt, 75 ma a 8 watt, 100ma must have a 15 watt. (Rated conservatively to avoid heating.) Figure your wattage based on your realistic max fusor current....Most folks will never run 30ma thus, 4-5 watts is plenty of margin for a 1000 watt wire wound. power =I^2 X R for 30ma, P=.0009 X1000 = 0.9W figure 4X for no heating 3.6 watts so 4 oe 5 watt will run cool at this 30 ma fusor current.

You must put the meter on 20 volt range across the 1000 ohm resistor and this is your SAFE 20ma fusor current meter.

Bolt the resistor into the grounding current leg of the metal shelled fusor. This is usually the real ground path to the fusor body. If this resistor comes loose or makes poor contact or opens up for any reason, the meter and the fusor metal shell will have nearly the full fusor hv potential across it and inside it!! be sure the resistor is part of the ground leg for sure!!! If the meter comes loose from across the resistor, no problem at all. You will just read 0.0. All is fine with the fusor....

Observe proper polarity when installing. Naturally, when you are pulling 20ma in operation, the fusor metal shell will be 20 volts above ground. Still safe.

With a 100 ohm resistor across the meter, the meter will read 0-200ma on the 20 volt range, but you will have to interpret a reading of 8.25 on the 20 volt range as 82.5 ma. The shell will only be 8.25 volts above ground. Switch to the 2 volt range and 1.38 volts will be 13.8 ma

If you figure on using the 200 volt range with the 1000 ohm resistor above, when it accurately reads 82.5 ma the fusor body will be 82.5 volts above ground, shocking!

It is simpler and safe to just go with the 1000 ohm resistor and read accurate 0-20ma on the 20 volt range and if you need more current readings over 20ma, switch to the 200v range to read accurate 0-200ma. Rarely will anyone use 40ma in a fusor. At 40ma, the shell is still safe at 40 volts.

Final note: Ideally, the fusor metal shell is wired directly to a hard ground, (shell forever at zero volts regardless of accidents). The ground leg current resistor is back at the power supply positive terminal. (true floating supply) Here the positive terminal goes to ground through the current meter resistor to the hard ground.

Richard Hull