FAQ - Metering a fusor for voltage and current readings

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Richard Hull
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FAQ - Metering a fusor for voltage and current readings

Post by Richard Hull »

Let us assume you have built your own variable power supply for your fusor or that the supply you have purchased has no metering. All fusors must have full metering, especially neutron producing or fusing fusors.

This has been diagrammed before in our all in one advanced fusor diagram and in other construction related postings in the past.

Measuring current: You must choose either circuit #1 Analog with a common 50ua meter or circuit #2 which uses a digital read out.

DANGER!!! In both current metering schemes shown, the fusor body and shell are tied to ground through low ohm resistors...The analog meter shunt and the 100 ohm digital metering shunt. It is important to use solid wire resistances here and to make perfect, solid, low resistance connects to the meter and fusor connections!! This means the fusor body and shell are a fraction of a volt above ground in the analog circuit and as much as 10 volts in the digital circuit. Thus, all connections to the meters, resistors, ground and the fusor must be perfect.

Metering Current - Analog meter circuit #1

There is little need for any current metering capability beyond 50 milliamperes, (50ma). Few fusors will use more than 20 ma. Never, ever use a properly rated analog d'Arsonval meter!!!!! You must use a 50 ua (50 microampere) meter movement! You MUST shunt this with a custom wound piece of number 24 gauge copper wire to make up a "meter shunt" such that the full scale reading is changed to read 50ma.

A full tutoring in electronics will not be given here with related math. Suffice it to say, you need to rig up a 1.5 volt "D" cell and a 50 ohm potentiometer connected in series on your bench. You will need a length of bare 24 gauge copper wire and a DVM with some clip leads.

1. Adjust the pot to maximum resistance.
2. Now set a DVM to its current range of 100 ma.
3. Connect the meter in series with the pot and the battery. You should read about 30 ma.
4. Turn the pot until you read 50 ma then disconnect the DVM from the circuit. NOTE do not touch or move the pot from this set point!
5. Now cut a 3 foot length of # 24 bared copper hookup wire. Wrap one end of the wire around the negative post of your 50ua meter and tighten it really well to the post. Now, connect this negative post to the negative end of the pot-battery circuit.
6 Leave the positive terminal of your 50 ua meter connected to NOTHING!
7. Connect the free end of the long wire to the positive end of the battery pot circuit.
8. Now, at some arbitrary point far out on the long bare wire, touch it to the positve 50ua meter terminal. The meter will deflect.
9. You will want to move the bare wire along on this positive post until it reads 50ua. Bolt the wire down to the positive meter terminal now at this precise point on the wire.

This precise length of wire has now shunted your 50 ua meter such that it is now a 50ma meter!

Cut the wire off at the positive terminal that goes out to the circuit connection. The wire remianing connected to both terminals is what you need. Loosen the wire and wind it into a loose coil with no turns touching. This compacts your shunt. Bolt it back across the meter. You now have a blow-out proof 50 ma current meter for your fusor.

Break your fusor's ground connection. Connect this meter in the ground circuit, running the old ground through the meter and the meter to the shell of the fusor. (note: the negative meter lead goes to the fusor shell and the positive lead goes to ground, ground being the same ground your negative HV supply uses (its positive end)

You will now be able to read 0 to 50 ma of current through your fusor without fear of the fusor body or your analog meter becoming electrically lethal since your supply will not be capable of burning out the 24 gauge wire shunt even in dead short conditions.

Note: all of the above holds true for wire made of stainless steel or nichrome. Using these types of wire will result in a much, much shorter length of wire for the shunt. Just be aboslutely sure that the wire is of at least 24 gauge or bigger! Your shunt must never be capable of burning out like a fuse if the HV supply is shorted. Likewise, the shunt must be firmly bolted and affixed across the meter terminals if the shunt opens up or comes loose in service then the meter body will KILL YOU if you touch it!!!!!

Metering current - Digital metering readout circuit #2

You may use a simple low cost battery powered digital multimeter to read your current set it to the 20 volt range and just connect it across the 100 ohm resistor. to go to ground. [b]Note: 10 ma to the fusor will read as 1.000 volts just remember to multiply the voltage by ten to read milliamps!!![/b]

If you chose to use one of the .2 volt panel meters you are on your own as a good bit of figuring must be done. First, you must chose a floating ground input type panel meter. This means the negative lead to the input must not be at the same potential as the negative of the power supply or battery. The PM129A is recommended. You must now set the meter via range resistors, ( which you must figure), to read a maximum of 99.99 volts. (9999 - (4 digits). Set the decimal to a fixed point of 999.0 in this manner when across the 100 ohm resistor and with 10ma flowing through it it will read 010.0 (10ma). This seems very abstract and tedious, and it is. The simple, floating multimeter set to 20 volts recommended above for digital readout is highly recommended. Such meters are bought at Harbor Freight for under $10.00

Metering voltage -

Here, you must also be careful. I recommend another 50 ua meter movement here. for 0-50kv reading. Such meters are referred to as being "20,000 ohms per volt" Thus, the required resistor for 50,000 volts would demand a series resistor of 5 x10e4 X 2x10e4 =1 x10e9 ohms. Such a resistor, while big, is not an impossibility to construct (note: it must be constructed as a series string). Ideally, you will need (10) 100 megohm resistors in a series string on a G10 fiberglass board. I would put the board in a plastic tube to protect from dust and supply some insulation. Leave the tube open to air flow. Connect one end of this tube to the high voltage output terminal of your HV supply. Keep this board and circuit out of human reach and well away from possible arc points. Connect the other end of this super resistor to ground through a 1 megohm 2 watt resistor that has an NE-2 neon lamp across it.

Now, using a length of RG-59 shielded cable connect it across the 1 megohm resistor. The shield goes on the grounded end of the 1 meg resistor and the central lead is connected to end of the 1 megohm where it connected to the super resistor.

Run this cable (however long) to your 50 ua meter that is to read fusor voltage. Place another NE-2 neon lamp across the meter terminals and connect the end of the cable to the meter terminals.... shield to the positive meter terminal and the central lead to the negative meter terminal.

You are now setup to read both voltage and current on your fusor.

See my diagram below.............

Note: Tyler Christensen noted that if your HV supply has both terminals isolated from ground, (rare), it would be better to place the current meter in the power supply ground connection, leaving the fusor shell firmly grounded with large cable. I agree. Should you have such a supply, I would move the current metering into the supply grounding circuit. Just be sure before you do it. The diagram and instructions I give will work with all supplies.

Richard Hull
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Re: FAQ - Metering a fusor for voltage and current readings

Post by Tyler Christensen »

For people who run off of a home-built multiplier: I found it works just as well, and is vastly safer, to put the current meter on the base of the multiplier where it connects to ground. This assures that the chamber is always connected to earth through large size wire, and if the meter shunt were to somehow fault, the worst that could happen would be the base of the multiplier could go high, and possibly arc over, but you shouldn't be near that anyways. Keep the logic earthed along with any control pots, and no harm done.

This could theoretically be done to a retail multiplier supply also, but you'd want to be a lot more careful to not damage it since putting a bias voltage on the base of the multiplier could throw off factory metering, feedback, etc.

Tyler
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Re: FAQ - Metering a fusor for voltage and current readings

Post by Richard Hull »

I agree with Tyler. Current metering is best done in the supply ground, (provided, and only if, your supply return is not hard wired internally to electrical ground or if both terminals are truly floating), this is very rare. Still, should the meter shunt fault open, even in Tyler's recommended configuration, the meter body would be deadly. No way around that, save to use the good strong shunt as I note. The shunt is the life saver regardless of hookup. Make it strong. No HV supply normally used by us can burn out 24 gauge wire. More likely is a poor, ill made connection on the shunt coming adrift. Put all you effort into making a firm, permanent connection across the meter terminals.

I have appended and credited Tyler's suggestion to my original

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ - Metering a fusor for voltage and current readings

Post by DaveC »

Adding a stout MOV across the current sensing resistor will help in the event that something either comes loose, or burns open.

I have always found, that the signal in the HVPS's current return is pretty noisy. With DVM instrumentation it is usually necessary to filter the signal developed across the current sensing resistor, to get something resembling reality. I usually shunt a 100 ohm metal film current sense resistor (or 1 k ohm for light load circuits), with a 2 stage RC filter and read the current signal across the capacitor.of the second stage.

With the direct reading shunted D'Arsonval meters, filtering usually isn't necessary, since the meter reads an average current.

The following has been said before, but may be useful in this context, -To just pass on a little experience gleaned over the years.......

When I do the initial stage check-out and Hipot tests, I read the filtered current signal with a 6-1/2 digit DVM, which gives useable readings to 0.1 uA, generally. This allows all of the associated HV components to be checked and leakages dealt with if the numbers are out of line with expectations.

When you are building a new HV multiplier and then potting it in solid dielectric materials such as RTV -11 the leakage current will have a somewhat non linear voltage dependence, in the early tests - say few days after the material "cures". (Curing always takes a lot longer than the data sheet says, if you are doing the potting for High Voltage applications.

As the cure completes, leakage will steadily decline and the point at which the V-I characteristic goes non linear, will steadily move to higher voltages until the design range has a nice linear (i.e.: ohmic) response. The potting compound can now be considered cured. This typically takes a week or sometimes more at room temperature..

The above is for solid cast-type potting compounds.. For oil immersion, the same nonlinearities of current vs voltage will be observed usually the first time the supply is fired up. After the ionic species have moved wherever they're going to go, the currents generally get pretty ohmic until the voltages get high enough to bring you to edge of an internal flashover (aka. fault).

So a few minutes checking the supply out, before attaching the fusor is usually worth the extra work.

If your system has a good high vac pump (Diffusion, or Turbo) then pulling the chamber down into the low -5's or -6's or below, allows you to connect up everything and check out the whole setup up to full operating voltage.

Dave Cooper
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Richard Hull
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Re: FAQ - Metering a fusor for voltage and current readings

Post by Richard Hull »

Bumped this FAQ to the top if the pile with new diagram for 2022.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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