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#5 FAQ - Grounding in a fusor system

Posted: Tue Feb 17, 2015 8:50 pm
by Richard Hull
The only ground you should consider is a "STAR" ground. I attach an image of how it is done.

You should chose a point more or less equidistant from your pumps, instruments, fusor and power supply. At this point, install a bolt on a metal frame. a standard SAE 1/4 X20 is fine. This bolt should have a fairly heavy guage wire cable going via the shortest route to your house electrical ground. (lower left in the image)

Now, all grounds of all metallic instruments, pumps, fusor, etc., in your system need to have a separate cable going to that "STAR bolt", as in the diagram attached. It is understood that many instruments and pumps have a ground in their electrical cable's plug already. This may be quite sufficient. To be sure, run a separate ground to the star bolt as noted.

Many folks use a metal cage or gather all their fusor materials on a metal cart or in a tightly confined space. In this case, a bolt placed on the metal cage or cart's metal body will be your star ground point. Good practice places all of your electronics as close together as possible. A good fusor system might have all your gear and the fusor in a volume of about 1 meter by 1 meter by 1.5 meters tall.

Normal star grounds are made with heavy wire. For a fusor, #12 gauge wire ought to suffice. You will find a crimped-on ring connector will add in the hooking of stacked ground wires o the bolt a lot easier. I personally crimp such heavy rings on the heavy and then back fill the crimp with solder using a good heavy gun type soldering iron of at least 60 watts and 63-37 rosin core electronic solder. (60-40 is fine)

Richard Hull

Re: #5 FAQ - Grounding in a fusor system

Posted: Wed Jul 05, 2017 8:22 pm
by Joshua Guertler
In order to attach this to the outlet to ground the high voltage, could I take a three-pronged plug and remove the neutral and live prongs and then plug it into the outlet?

Re: #5 FAQ - Grounding in a fusor system

Posted: Thu Jul 06, 2017 5:48 am
by Rich Feldman
Sure you could, but it would be a waste of a plug. Why not use an old metal fork, with all tines but one bent out of the way?

More practically,: Outlet cover plates are secured by one or two metal screws that connect to ground. Same ground as your NEMA 5-15 plug would get, even if the outlet-mounting box inside the wall is a nonmetallic type. Terminate your ground wire with a ring lug & put that under one of the screw heads. An ordinary plug could be pulled out if someone tripped on the wire. Screw-attached ground should maintain a safe connection, even if it's more likely to bring the tripper (trippee?) crashing to the floor.

What are you planning to do about the ground pins on power cords for your vacuum pumps and HV PS?

Joshua, you don't seem to have made the obligatory post to our "please introduce yourself" forum. It's one of the commitments stated on the registration page. Without it, readers have to guess about you. So far, it looks like you should learn more about electricity, before asking for help here about fusor electricity.

Re: #5 FAQ - Grounding in a fusor system

Posted: Thu Jul 06, 2017 4:53 pm
by Richard Hull
One should choose any large, metal framed instrument chassis that has a good solid 3 prong plug whose ground is attached to the body of its chassis and bore and tap a 1/4X20 hole in the chasiss. A two inch long bolt can be secured to the chassis as a star point. All other non-grounded items, specifically all electronic instruments, power supplies, metallic vacuum system plumbing and the fusor can be manully grounded to this point via a heavy gauge wire with spade or ring lug attached attached to each item.

Make sure that the area round the bolt hole is first sanded free of paint, grease, etc. A star washer will also help make good contact with the metal chassis and the bolt. Really long gounding wires should be avoided. (Put most items as close to each other and the grounding lug as is practical in your system's mechanical layout design)

Richard Hull

Re: #5 FAQ - Grounding in a fusor system

Posted: Fri Oct 26, 2018 10:35 pm
by Richard Hull
Important!!! I have altered the diagram a bit in that the ground line to the fusor will have to have the fusor's current meter in it, obviously, to measure fusor current. I have stressed in other posts that the shunt resistor be of a wattage that it will not blow like a fuse if it has an internal short or a huge surge of current. If it blows the fusor shell could be at the full HV potential!

Richard Hull

Re: #5 FAQ - Grounding in a fusor system

Posted: Sat Oct 27, 2018 1:51 am
by Rex Allers

These FAQs are a great resource but I think the current sensing part shown in the main star topology diagram that is in series in the ground return from the chamber is still a bit confusing or misleading.

The diagram shows the meter (labeled Ma) in parallel with the current sensing resistor. If I didn't know better I might assume that I should get a meter for measuring mA to use in this place. In reality we want a voltmeter (V) in this position to measure the voltage across the shunt resistor that is created by a current flowing through the resistor. The relation, of course, is Ohm's law: V = I x R .

An ammeter or mA meter normally would have its own internal shunt resistor and have a very low resistance across its terminals. A volt meter is a high resistance device which is what we want here; we want almost all of the current to flow through the shunt resistor, not the meter. If someone misunderstood and put a mA meter across the indicated shunt resistor it would carry most of the current and isn't what you intended to show.

As an example for those who might not be following, say we have a voltmeter that is 1 volt full scale and we decide we want full scale (1V) to happen at 100 mA of current through the shunt. From Ohm's law R = E / I (E is the voltage, I is the current). So the resistor we want for a shunt is 1 / .1 = 10 ohms.

Without going into details (not the subject of this thread) I'd say a swagged rated wattage of 10W might be pretty safe for this example with 10 ohm shunt resistor.

I think there is another FAQ that covers current sensing in more detail.

Re: #5 FAQ - Grounding in a fusor system

Posted: Sat Oct 27, 2018 6:26 am
by Richard Hull
Indeed, there is another FAQ that advises the use of a digital volt meter of .2 volt, 1 megohm input and a 10 ohm multiwatt resistor. The Ma indicates a current meter block and not a milliamp meter movement.

All voltmeters be they digital or D'Arsonval are true current meters, as built. The only true, natural voltmeter is an electrostatic voltmeter using a moving capacitive plate against a stationary one.

The other FAQs deal with making a suitable current meter/voltmeter suitable for safe use in a fusor. These involve the use of another current meter typically configured as a Hi-Z, very low voltage, voltmeter or a true 20 or 50 ua D'Arsonval meter movement placed in parallel with the aforementioned low ohm, high watt resistor in the fusor ground line.

All fusor builders are on their own in manufacturing the critical, safe current meter that will not leave them dead on the floor. These, as mentioned, are in other FAQs.

Richard Hull

Re: #5 FAQ - Grounding in a fusor system

Posted: Tue Oct 30, 2018 3:42 am
by Bob Reite
I defined my "star point" as the fusor shell and put my current sample resistor in the positive output line from the power supply to the fusor shell. This has the advantage that should the sample resistor go open circuit, the fusor shell will remain safely grounded. However depending on the power supply design, bad things might happen to it, and the current meter will certainly be destroyed. I address additional protections for the current sample in another FAQ.

Re: #5 FAQ - Grounding in a fusor system

Posted: Tue Oct 30, 2018 9:09 pm
by Richard Hull
Star grounding from the fusor as the focus of the star ground is a great idea. However, many power supplies of very high voltage have one pole of the output voltage case grounded as a safety measure. This forces you to do some internal wiring reworking to link the current system in prior to grounding of the case. It is a matter of choosing where you wish to measure the current.

There are a many issues involving grounding and current transducing, (measurement in the fusor).
1. How much power/current can one's power supply deliver at fusion voltages?
2. If capable of over 2kw power, do you have a ballast resistor in the line? (we hope so)
3. Most folks have a ballast resistor in a 1KW capable supply.
4. A 0.1 ohm to 10 ohm 10 watt resistor or smaller in the current metering circuit will just not burn out.

In any conceivable fusor based short circuit in the average supply with a 50kohm 100 watt ballast resistor, will see all of the voltage appear across this resistor instead of the .1 ohm to 10 ohm resistor. If your supply can indeed burn out the ballast resistor without blowing its fuse or destroying the supply itself, the HV line will simply open and the fusor will remain grounded.

Demo fusors typically use a neon xfrmr and you will never burn out the metering shunt as the transformer output will fold to nothing via the magnetic shunts.

Non-shunted 60hz multiplier systems will fold via limited 60hz capable filtration in the multiplier ladder.

Most modern switching supplies will simply electronically "trip out".

Naturally, the adroit electronics buff could easily fabricate an instant acting dead short detector that would shut the entire supply down if he had a 5kw high voltage power supply.

In the end, each of us will measure current based on the knowledge of what kind of supply we have. This is just one of those instances where a technology must be mastered to a degree that safety is warranted. There are several technologies that must be dealt with in assembling and operating a fusor system. Self-directed learning is key here. If you don't know much about an issue........Study it.... Practice it....Own it.

Richard Hull

Re: #5 FAQ - Grounding in a fusor system

Posted: Wed Nov 20, 2019 6:39 pm
by Richard Hull
Finally, all d'Arsonval voltmeters are really current meters in the final analysis. I show the mA meter in the ground current measuring line. I tend to use a 50uA meter here and it effectively measures mA. The resistor is a typical current shunt taking the bulk of the current past the 50uA meter allowing the full scale to be altered to 50mA. We are never reading voltage across the resistor with a magnetic meter movement.
A digital or electronic voltmeter will, indeed be used to measure voltage across the resistor, but unless it is an electrostatic voltmeter, we are always, in effect, reading only the current sourced by the resistor as a voltage. Simple electronics. We are measuring a flowing current producing a voltage across the shunt resistor.

Current shunting can use a 1 ampere d'Arsonval current meter to measure 10,000 amps. the shunt will force 9,999 amps around the meter to read full scale 1 amp of current. Such meters are magnetic and the degree of deflection is based on current flowing in the meter's moving coil.

Richard Hull