Old Fusor Power Supply Back Up

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Dennis P Brown
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Old Fusor Power Supply Back Up

Post by Dennis P Brown » Sun Nov 14, 2021 6:00 pm

As mentioned in another thread, I'm slowly trying to get my old Fusor back into operation to try my hand at activation (again.) I assembled (after searching a good bit for some removed parts) the old power supply and did manage to put it back together. Took awhile to figure out my original wiring inside; all the guts had been removed some years ago. At least the bridge was still on its mounting board with HV wires intact.

The diodes are 1 amp, 20 kV each so I had to use two in series for each "diode" in the full wave bridge configuration.

I tested it up to 33 kV on a dummy load (1 G-ohm) voltage meter. All went well - no arcs, smoke or component failures. The x-former can handle 28 ma continuous at 32 kV.

I decided to have the ballast resistor under oil for cooling; as such, it is isolated in a plastic housing for that purpose. The diode bridge should be fine in air (at least seemed ok in this test.) I still have to pump down my fusor and test its diffusion pump (been sitting four or five years) - I am finishing the internal plumbing for the water in the lab shed since this is a water cooled DP.

Aside: the HV return wire for the transformer has a 200 ohm 75 watt resister (hollow core) on the diode bridge mounting board. This resister is in series to further protect the x-former in the event of a short - not really necessary but I'm being overly cautious with this borrowed x-former. The HV output ballast resister is 50 k-ohms, 750 watts (if I recall) and is ceramic, hollow core.

The potentiometer on the panel is required so the 'HV' digital meter display can be calibrated.
Attachments
20211114_130853.jpg
Power Supply Front Panel
20211114_124105.jpg
33 kV Transformer, Diode Bridge, and Ballast Resister (HV fusor cable not installed)

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Dennis P Brown
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Re: Old Fusor Power Supply Back Up

Post by Dennis P Brown » Sat Nov 20, 2021 9:26 pm

Well, I ordered a new ballast resistor (100 watt, 40 k-Ohm); rather long compared to my 55 k-ohm - this had about half the resister value I want so I needed two (ebay comes through again - I hope); now I wait till I locate a suitable fusor chamber that I can incorporate into my setup ... .

I guess I can work on my He-3 detector system for now. I could also consider building the 70 kV power supply using the x-former I have (that I doubt I'll ever use such a beast in a fusor - shielding issues - but might be worth doing so I can test one - just because.)

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Richard Hull
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Re: Old Fusor Power Supply Back Up

Post by Richard Hull » Sun Nov 21, 2021 4:21 am

Smart man. Get something going at 40kv max for a while before pushing the envelope. Any work on developing a good 3He neutron detection system is time well spent.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Old Fusor Power Supply Back Up

Post by Dennis P Brown » Thu Nov 25, 2021 11:27 am

Thank you Richard.

As I mentioned in the other thread, I obtained my new ballast resisters. The 100 watt, 40 k-ohm resisters, in series (80 K-ohms), are now installed in the power supply. They will be oil cooled (they are hollow core.)
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20211125_061813.jpg
Twin 40 K-ohm ceramic resisters in their oil box

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Richard Hull
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Re: Old Fusor Power Supply Back Up

Post by Richard Hull » Thu Nov 25, 2021 7:37 pm

That resistive ballast is just at the perfect value. I would say 50K to 80K is the ideal ballasting resistance. A 10ma run on the fusor would only kick out 800 volts from you 40 kv and dissipate only 8 watts in the ballast. A small electron runaway due to the grid runaway emission might go to 60ma before you catch it and dial back the variac. During such a short burst loading on the supply, the resistors will have 3000 volts across them and dissipate 180 watts for just a small moment until you correct for the runaway. I have a 63k ohm 200 watt ballast under oil in my x-ray transformer tank since 2001.

Most such runaways occur as you try to strike the plasma at first and are very short in duration as you adjust the gas pressure to allow start to proceed. Later, pushing the fusor too hard is when runaway emission can produce a multi-second event if you aren't fast enough on the controls.

If you have a beefy supply, a ballast resistor is a must. Most supplies used by the average fusioneer are not beefy enough to power a full 60ma fault. As such, they typically just fold up and limit through their internal components, hopefully not blowing anything up.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
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Re: Old Fusor Power Supply Back Up

Post by Dennis P Brown » Fri Nov 26, 2021 12:15 pm

Thank you Richard, that is all very good to know - these are details and operation procedures that are extremely useful (newbie's take note); often (and I am guilty of this) we just say someone needs a ballast resister and leave it at that - no details; while you have already provided a FAQ on this subject, it is extremely useful to see the results as applied to a specific case like this.

As for my possible future electrode system for this larger fusor, I decided to take parts from my last electrode (made for my small fusor.)When I made it, I decided to save some work so I used ceramic adhesive (baked) to affix the electrode to a stainless steel (SS) vacuum fitting for the chamber (in hind sight, a big mistake). Now that this electrode system is not going to be used (the small chamber) all I need is the ceramic part. So I needed to separate the SS fitting bonded to it. Unfortunately, there is no easy way to separate these two bonded items - the ceramic adhesive is high temp tolerate. So, I decided to use a mason blade to grind a grove along both sides along the fitting with my power circular saw - that tile abrasive blade cut through the stainless like a warm knife through butter. This enabled me to separate the fitting from the ceramic shaft section. As a result of that process the ceramic is now a little beat up but that is fine - the damage isn't where it seals with an O-ring.

Hopefully, the vacuum integrity of the electrode is good (the region where the ceramic bonds to metal electrode) - I never bothered to test it previously. I will have to figure out how to best mate it and the SS adapter o-ring sealing coupling I have to an 8-inch ISO flange; I might need to lathe an interface plate from aluminum sheet (I will not machine a SS ISO plate to accept this electrode - that material is just too hard and isn't worth the time or danger to tooling. I've machined harden stainless steel before and that is a bear requiring slow speeds, lots of oil and being VERY careful. Working aluminum is a breeze.)
Attachments
20211126_065215.jpg
Current electrode parts I have on hand (center core is steel)
20211126_065150.jpg
Assembled electrode with SS O-ring coupling

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