High voltage Fusor grade x-former score (Temporary)

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Dennis P Brown
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High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

Well, someone I know had a 1200 watt, 32 kV max voltage and is what I'd call a standard transformer (60 Hz and only four wires total). It uses 120 volt AC input (two leads) and has two high voltage wires as output. Only the core has a ground (not center tapped and the core ground is to the base plate.) They won't sell it but are happy to let me use it for the next year as a temporary loan.

So, I guess I either measure current through the high voltage side to the fusor or use a high ohm resistor. Also, I guess I add diodes to each HV output wire and a use a HV cap to filter this output (I have a 50 kV cap and 0.3 mfd.) Looks nearly new but is rather old since it is so simple (I guess) but rather heavy (20 - 30 lbs.) Does not use oil (dry.)

Even has a data sheet with operational parameters (max 1 hr @ 32 kV and 25 ma and max 10 amp load on the low side - I already added a 10 amp fuse/wall cord coupling for the 120 volt input; hard to see but the fuse unit & wires are to the right of the x-former.) Quick floating "hot" test showed it works.

A picture of the bare unit is below and the two high voltage output leads. I have a nice enclosure with the cap and special high voltage wries with a 60 k ohm resistor (in oil) all installed already with a HV lead that can reach my fusor. So, Guess I will install the x-former this weekend and test it with the fusor.
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Rich Feldman
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Rich Feldman »

That's awesome, Dennis. Can we see what you put in the rectifier box?

If the transformer were in my hands, I would connect the inner secondary winding to fusor ground through a robust current sense resistor, say 100 or 1000 ohms, as described in many fusor power FAQs.

I would use the outer secondary winding with a HV diode stick (for half-wave rectification) and ballast resistor. Monitor the primary current, preferably with an RMS-reading meter, because substantial DC current in secondary circuit moves the core toward saturation. Self-rectifying X-ray generators often mitigate that with a resistor-diode network in series with the primary.

Configuring a bridge rectifier would quadruple the HV diode requirement and render the inner secondary winding "hot", to what end? Changing ripple frequency from 60 to 120 Hz? Are you sure the inside of secondary winding is insulated for 32 kV? Be a shame to ruin a borrowed transformer.

I would start with no HV capacitors. They do nothing to increase the peak voltage. To the extent that they reduce ripple, they also generate a distorted current waveform that calls for derating the transformer (unless it's already rated for rectifier service instead of sinusoidal load current). http://powervolt.com/techan01.html
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

Rich, all really, really good ideas. I like your second one (both because it is easier and I understand it better (lol).) Can't say if I should or shouldn't filter but your cautions make good sense (esp. since the person wants it back working!) The unit is, of course, single phase input.

Below are the original drawings (really only one and the second image is just a repeated) of the device it was installed in. As can be seen, it used a diode bridge for polarity output (where each of the diodes are individually filtered for better performance; I assume the resistor values were very high but have no idea ...the caps I guess are there to handle 'ringing' - just realized that the resistors are bleed offs for these caps ...duh. Hits hand to forehead) and one side of the HV output of the x-former was ultimately grounded through a resistor as well to, I guess, a common circuit to the bridge (?) Since the manual for the original unit claims that 32 kV is safe as the output after the diodes (either polarity) I assume the x-former can handle that level. While old, the unit does appear new and all parts appear in excellent shape.

As can be seen on the drawings, a HV reading for a meter feed off the HV ground through a resistor as per your post. Also, the x-former appears "de-rated" considering it has posts 1 and 4 connected rather than 1 and 5. So I assume the x-former's non-bridged voltage is a good bit below 32 kV (somewhere in the mid 20kV? )

Like your link to the x-former site; used the center tap full wave setup for my NST and it works well but does pull the x-former up to 10 kV rather than its original 7.5 kV continuous voltage (see the normal RMS voltage). This x-former isn't center tapped as can be seen from the drawings and supports a full bridge rectifier arrangement. They do suggest in the manual a filter cap for the output but don't require one ... so, maybe best to be wary of that. Again, like your idea of no filter for initial operation and maybe test that setup later with a very high resistance dummy load. I have a 60 k ohm resistor, under oil, for the fusor input already. A sense voltage from this using a high value resistor would be easy especially since I have a HV readout probe to calibrate any meter. I assume I am more than likely stuck with running the full voltage through a milliamp meter for initial calibration studies .

I also like your suggestion of using a high wattage, 1000 ohm resistor for the ground side of the HV output lead on the x-former. Better safe rather then sorry.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

Here is my initial design for the fusor. Have not included any metering for clarity.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Richard Hull »

That circuit is perfect!!! However, DO NOT use the capacitor. You do not need it! I have operated my fusor for years only half-wave rectified, no cap. With your superb isolated secondary you can double the power to your fusor with the 120hz full-wave system.

I use a beefy 10 ohm wire wound resistor in my grounding leg to the fusor (bolted connections only). Across this, resistor I have a digital meter to read fusor current.

Nice find and extremely rare to find this type of transformer. Companies do not like to make these. When they do, they over build them. This one is over built.

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Re: High voltage Fusor grade x-former score (Temporary)

Post by Rich Feldman »

The original application schematic is circuit is drawn in fine style,
notes.JPG
notes.JPG (13.23 KiB) Viewed 8800 times
and its use of DEPENDANT for DEPENDENT can be excused.

I see no bridge rectifier, just a place to plug jumpers 2 different ways to select output polarity. Also it looks like there's a protective spark gap and crowbar relay in the upper right corner.
reverse.JPG
The connection between left and right pages is ambiguous, because your scans are missing a critical strip. Can't see where the unselected half-wave rectifier goes.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

I will delete the cap; the drawing shows a symbolic bridge because it is a complex mechanical system that reversed the system so polarity could be reversed;however, I haven't seen the original box this was taken from so not certain. Too bad the system doesn't work and as a result, the x-former was available (but that was lucky for me.)
Sorry about the bad photocopying - it isn't easy but no components are missing between the two images.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Rex Allers »

I did some clean up on your HV supply PDF (the original circuit that you don't actually have). I merged the two pages and did a lot of contrast and clean-up to convert it to a much smaller grayscale image file.
orig HV supply schematic
orig HV supply schematic
Where the two pages join, it looks like a little bit is missing. The wire from the ground stud and one wire from the ground common at the right just ended at the join, so in my png I connected them.

It looks like this circuit is a voltage doubler. There are two diode strings (2 diodes each, with parallel caps) in series, with the HV from the secondary feeding into the middle. One end of the series diodes is grounded and the other is the output. The polarity jumper selects which end is which.

You said you have a datasheet with the voltage and current output. Is that actually for the transformer? I ask because if it is for the supply, your transformer might be half the voltage you think it is. Hope you have the tools to measure it.

This circuit always has the bottom end of the secondary grounded. (Effectively. It goes through a resistor R-105 but won't be more than a few volts from ground.) (Pin F on the left terminal is the low end of the output so, I presume is available for current monitoring, but also to be tied at or near ground when connected.) So, as others mentioned, I think you should always have the HV wire near the core on the secondary output grounded. That end is probably not insulated from the primary or the core for letting it get much above ground.

In your proposed circuit you show a full-wave diode bridge. As mentioned, this is probably overkill for a fusor. It will need 2 or 3 times the expensive diodes. More importantly, the connection you show doesn't keep the one end of the secondary grounded.

I assume you intend to drive the primary of the transformer from a beefy variac to get voltage control. The original doubler circuit doesn't look like a bad idea if you can afford the extra diodes and caps. You do need the caps for a doubler. The resistors in parallel with each diode are probably to balance the voltages between each section and also to drain the HV from the caps when turned off.

Neat stuff. In the original schematic, I like the 'N' in Notes at the lower left of the drawing. A touch you are not likely to see in any recent stuff. Date says '71.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Richard Hull »

I would ground the core iron and still let that lower HV exit wire float for a bridge and ground the bridge. I doubt it would arc. dunking it in oil would help a bit as the seondary looks like it is potted. The oil would supply a bit more insulation over air between the winding and the core. Just bathe it up to slightly above the lower HV wire. No need to drown it all under oil.

However, the doubler would allow for more voltage and grounding the lower wire but, the usable fusor current would have to come from energy storage caps. A mistake here, an arc there could be rather distructive. Your choice. The bridge above, as in you original diagram, minus the cap, would not have stored energy behind it.

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Re: High voltage Fusor grade x-former score (Temporary)

Post by Rex Allers »

As Richard said about using a doubler, "the usable fusor current would have to come from energy storage caps."

This is very true. I didn't see any values for capacitance or resistance in the original diagram. It would have been interesting to see what they used. There are design guides for doublers on the web. At especially as low as 50 or 60 Hz, the capacitance needed to get 20-or-so mA at these voltages could be a challenge to find or buy.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

All excellent details and thanks for cleaning up and analyzing the drawing Mr. Allers! I may end up now modifying the circuit I just installed but not a big deal either way. So, the photo's I have may change (a good bit!) If a doubler is required; if so, that I can do as well. I am fairly certain you are correct and the x-former is rated for 15 kV or so. I will simply test it using my existing setup since this design provides negative and positive 15 kV on the bridge (assuming the diodes survive!)

The first photo the standard bridge is shown (and the x-former.) The negative output of the bridge is connected to a HV lead out; the positive side of the bridge has not, as yet, been connected to anything. I did not want to 'float' the case until I have a second, physically separate ground wire for the case (to a water ground) and not depend on the wall outlet plug alone. The core is grounded (via a wire to the case) so even through it is mounted on wood, that isn't an issue.

The second photo is the nice case I have; the two digital meters can be used for voltage and current once I install appropriate resistors. The varic is beefy and can handle the load easily (it is rated for 15 amps and the x-former only for 10 amps.) I have a 10 amp fuse in line with the "hot" side of the x-former input power.

The last photo shows the entire internal system (less oil on the diodes.) The main output resistor (50 kilo-ohms) is under oil for cooling - the diodes really don't need oil if the max voltage is 15 kV or so. The "positive" side of the x-former has a 100 ohm resistor to protect the x-former from any surge to the bridge.

I will carefully review what needs to be done after I test the x-formers voltage in the bridge (my HV probe only reads positive voltages ... .)

All major parts are modular and can be removed/changed as needed without causing issues for the overall setup since the wires have dis-connections built in.

The diodes are cheap and are rated to 20 kV but have been proven only to 15 kV; so I (hope!) they can stand the 32 kV as doubled sets (if it can really do that ... .)

Took only a few hours to put this all together but between having an available case, some spare diodes/electrical parts/extra HV wire and more junk parts on hand, managed to do it today. Hope the results aren't explosive for the diodes. The resistor I would think will provide protection to the x-former if the diodes fail.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Rich Feldman »

Rex called it right, about voltage doubler in original schematic, and questioning the secondary voltage of transformer itself.

Thank you for figuring out how to bridge the gap between right scan, say abcdefghij, and left scan, abcdeghifj. The mismatched vertical spacing had deterred me on pass 1. Duh!
Here I have re-drawn the circuit for clarity, assuming that the wire from J103 goes to an external mA meter.

Code: Select all

+-(mA)--|<----+--|<--+--HVout
|       D1    |  D2  | 
|          coil      | 
|    C2    |    C1   | 
+----||----+----||---+ 
|
Ground
That ASCII rendering was adapted today from one I drew a long time ago, for another forum.
The configuration might be called a "symmetrical" voltage doubler. Both ends of the secondary are hot, but one is hotter.
You can make this doubler functional even if you have only one HV capacitor.

All three (?) of my high-frequency XRT's have multiple secondary windings, for multiple instances of this kind of doubler in series.
dscn0257.jpg
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Re: High voltage Fusor grade x-former score (Temporary)

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The x-former does in fact produce 32 kV without any doubling; direct measure from the high side of the diode bridge (negative voltage) was just over 30 kV with loses from the exposed wires (a lot of crackling ... I need to reinstall the exposed hot lead back onto the fusor.) It was tricky to set up my voltmeter but have done that in the past without issue - the meter is calibrated. And yes, the diode bridge was grounded.

Since the original device had massive diodes (not sure what they are made of) but no caps that were of any significant size leads me to think that 1) that it is unlikely that the system doubled voltage since the original voltmeter scale didn't go above 35 kV or maybe 2) that drawing might not be exactly correct even through a lot of the stuff in it matches exactly to the drawing. Pity the original system doesn't work or I could have just used that and seen what/how it operates - through rather primitive (lol - but I bet very well done because I suspect that those huge and strange looking diodes make the modern semi-conductor ones look like the weak under rated units I am using.)

Considering the x-former size I have little doubt that it can handle 25 milli-amps but I will test that under vacuum (a few microns) in the original fusor (currently down due to its glass window having cracked; I sub a plastic one that allows 10-6 torr operation but hardly proper for a real fusor test with deuterium.) Until the new port window arrives, I will run tests at a few microns in the fusor using trace air to get a current meter to operate either directly (at the fusor and isolated so it can handle the full voltage) or with resistors using one of the digital displays. A voltmeter isn't really needed since the variac scale tracks perfectly with voltage (33% = 10 kV; 66% = 20 kV and 100% = 30 kV) but that is an easy one to install so I may just get the digital display for that working.

Playing with this new power supply and the fusor unit should bide me over until fall when humidity will drop enough so my Van de Graaff can be fired up and tested; even with a de-humidifier the room is just too humid to even consider a run. Until then I can try for some neutrons using this fusor; at least until I get the deuterium accelerator to operate (if only.)

I hope the 25 kV+ power supply can achieved its rated power and deliver 15 - 25 ma in the 20 kV+ range and this will provide me with enough neutrons to 1) confirm this occurrence and 2) try to operate my original experiment that was designed for the accelerator. Of course, this assumes I have no issues going forward.

So, to protect the x-former the secondary of the unit is first fed through a 200 ohm, 50 watt resistor (incorrectly said 100 ohm in the first post) before the bridge (on the positive side of the bridge where the ground wire is located); in the photo that bridge output lead was just floating (wire with connector hanging in the air) and hadn't as yet been connected to a ground. That was, of course, fixed before I fired the unit up. Floating a bridge is a sure way to burn out a x-former!) And on the negative side of the bridge that output runs through a 50 kilo-ohm, 100 watt resistor in oil that, in turn, then goes to the fusor. I'd think with the core grounded and these resistors the loads will not hurt the x-former. Also, even if the diodes fail, the 200 ohm resistor on the secondary and the 10 amp fuse (rating as called for in the spec's) for the primary should protect the x-former during any such run away condition. Any other protections for the x-former that I am overlooking? (aside; any one know why the core of a x-former needs to be grounded, over and above if a wire shorts to it the core doesn't reach those voltages.)

I will not use any caps as per everyone's advice. As a side issue, I didn't realize that the bridge does double the frequency from 60 Hz to 120 Hz. So, thanks Richard for pointing that out - obvious to many but not to me. I guess in hindsight I should have realized this fact but never thought of that ... .

I am not ready to sink the x-former in oil; it operates well in air and I am a bit worried oil might attack/weaken the x-former wrapping's; the owner wasn't real thrilled with that approach either when I mentioned it as a possible way to protect the unit ... .

I will top off the oil for the diode bridge system and resistors (aside. I will use synthetic oil since a paper I read proved this offers as good and even slightly better performance for HV systems compared to standard oils) and then I will close up the PS housing (the cabinet does need a better fan - it is aimed at the x-former.)

I will redesign my fusor with a current meter and maybe a voltmeter. I should be able to start tests this coming weekend on current loading/voltage for 5 microns (an arbitrary vacuum but one that is in the range of interest for fusion.) The main objective being to confirm current loading performance of the x-former; and secondarily, designing a good current meter system for the fusor (an important longer range goal.) Also, allow me to better develop a feel for the current/voltage/loading on my chamber. I do need a new cathode so, after the window comes in that will be my first project before trying some fusion. I will be careful of x-ray issues.

I will mothball my 40 kV supply (negative voltage output; works nicely and for under $100 (my std max price for any high end item - lol) I couldn't resist grabbing it when it was available. It is a Glassman unit.) But since it was good only up to 10 milliamps (with extra forced air cooling) I realized that there was no way it could reach even that level in the 20 kV range so this was just a backup PS. Maybe test that unit in a future day (and really push it to see exactly what it can do) when I finish my major experiments with this x-former and after I return this x-former (and had better be working!)
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Re: High voltage Fusor grade x-former score (Temporary)

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Now that the system/x-former can provide negative 30 kV (and hopefully, I will determine if the 20 - 30 ma range is ok), I've ordered the new glass circular view window/electrode feed thru for the fusor (8.5 inches by 5/8 inches thick.) Also, topped off the oil for the bridge and replaced the missing front panel and re-installed the top on the power supply cabinet. Still have to address the issues in the previous post but that is for the future.

Decided I will add a deuterium low pressure fill tank to the fusor and install its own leak valve; in this manner I can operate the fusor away from the accelerator (currently a fill line connects the two units.)

As such, decided to move the deuterium fill station off the deuterium accelerator as well and make a separate, independent enclosure. The accelerator main hemisphere has it own deuterium mini-tank/needle valve.

I will use this deuterium fill station to fill either of my two low pressure deuterium holding tanks (each to about atm after a high vac pump out) from the high pressure deuterium lecture bottle. This will simplify operations of either system and allow them to be separate a good deal helping me work safer - too many high voltage systems in my research lab room in the house - the Van de Graaf is very large and the accelerator is a complete bench. Add my work benches and now a fusor with that rather large power supply and crowding is getting bad ... life as a home experimentalist ... the significant other wasn't happy when I took the spare bedroom to store more lab equipment.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Richard Hull »

The oil was a mere suggestion as overkill protection core-to-windings.

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Re: High voltage Fusor grade x-former score (Temporary)

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Thank you Richard; your advice was very helpful (as always!) on many other issues relative to installing this x-former. The oil idea was good just not required.

Since my x-former is not center taped and I am using a grounded bridge, I've decided to use the standard and rather direct method of reading the x-former's current output through an analog meter at the fusor high voltage input. This should work since the meter is isolated and with its low internal resistance should be ok - my high voltage analog probe meter (since it is positive sensing I have to set it up for negative readings by placing the meter section on the "wrong side" of its giga-ohm resistor exposing it to the full potential) handles the direct 30 kV fine (of course, very low current.) But both the principle and practice are identical. If the milli-amp meter does fails, really no lost nor is safety compromised. I have enclosed a meter in a plastic housing and put an insulated "wall" between the two connector inputs for the meter. If shorting still occurs, can always lay the meter face up and fill its "internal volume", less the face, with low viscosity oil.

Not that I won't consider other methods but even if I did, a direct measurement to calibrate would be useful. Any such ideas would be appreciated.
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Re: High voltage Fusor grade x-former score (Temporary)

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Since you are grounding the bridge and the fusor, I would read the current in the ground leg of the fusor. A 10 ohm, 20 watt, 5% wire wound resistor from ground to the fusor shell would have 0.1 volt across it at a 10ma draw on the fusor's operational current, (demand from the supply). Note: The resistor needs to be firmly bolted between power ground and the fusor shell. A simple 0.2 volt digital meter across it would read up to 20ma. A 2 volt digital meter would read up to 200ma. Easy, simple and no where near the HV. Smart money always measures current in the ground leg.

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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

Again, Richard, thank you. My fusor is mounted in a plastic box and electrically isolated ...there is no way I would have thought of that simple method and to say I am laughing at how easy your method compared to mine is understatement. Only my missing this is funnier. I will incorporate that but am a little lost about "bolting the resistor". I realize it has to be between the ground and the fusor as well as being the only ground wire but can it be affixed across the voltmeter (digital)? That meter will be in my power supply case.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Richard Hull »

If that resistor has anything less than the finest bolted connection near or at the fusor, your shell will go Hot! I would run a shielded cable across the resistor at the fusor to the supply box and meter. A good safe ground and current shunt to the fusor shell is important. I would not remote the resistor. the shielded cable will carry the .1 or .2 volt DC level, noise free, to the meter. An NE-2 lamp across the resistor would be an added safety feature. If it lights, your shell is hot.

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Re: High voltage Fusor grade x-former score (Temporary)

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Wow, am I glad I listened to the wise ones here! Didn't realize my High Voltage cable had a metal shield that was too close to the hot ends of the cable. Of course in a test the cable was on a device that offered good grounding and the x-former found a near direct ground; however, thanks to the various resistors in the circuits (again, thanks guys!) and my fuse directly on the low voltage of the x-former, the fuse blew but no other harm.

Since my fusor's temporary window (plastic) leaks like anything I will wait for the new large glass window before I can test the fusor at pressure to measure available current from the new power supply.

Also, I added the 10 ohm resister to my fusor and it has a shielded cable that will be connected to a milli-amp gauge for fusor power measurement. Note the resistor is 10 watt (air cooled core and is bolted directly to the fusor vacuum chamber as instructed!) Even with water cooling the diffusion pump, the fusor (when all grounds are dis-connected) still is fully isolated and floating (reads mega-ohm between fusor itself and both the main common ground I use for all devices and the fusor power supply. Aside: most my equipment has working third pin electrical cables; however, I also have a water pipe that offers great grounding - I also, connect to this for added redundancy.) So, when the milli-amp gauge is hooked up for sole ground, it should measure the full output of the x-former.

Below are the pics of the resistor, and my standard universal ground (will be dis-connected when ever the amp gauge is acting as the full ground.) Aside: I will add two redundant grounds that will work if the milliamp gauge circuit ever fails. The first will use two small copper spheres set just 0.5 mm apart. One will connect to the fusor the other to earth ground. If all other systems fail, the fusor will easily jump that and keep it's voltage at a safe level. Also, the standard neon light by-pass on the gauge circuit. I have ohmed all current grounds and they measure out well under an ohm.
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Ten ohm resister and shielded cable attached to the main fusor chamber
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Bob Reite »

I would have the shell of the fusor as your "real ground" Define it as your "Star ground" as mentioned in the FAQ. This ground is connected to the house power system
"equipment ground", even if only through the green wire of a three prong power cord. The 10 ohm resistor goes between the well grounded fusor shell and the positive output of the supply. The whole HV supply should be inside an enclosure. Your call if you choose to double insulate the enclosure or or have a metal enclosure, case of which is also connected to the star ground. With this arrangement the only thing that becomes hot (that is not normally at high voltage) should the resistor open is the positive wire coming out of the power supply enclosure and any metering connected across the resistor.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Dennis P Brown
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

Excellent advice Mr. Reite and thank you; the issue of grounding the positive x-former thru the resistor is well worth considering and I will see what is involved but can't think of any reason why this wouldn't work and be easier. I always like comments on alternative methods - most times they are better! Another reason this forum is so very helpful.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

Since I have a 2 M-ohm, 300 - 400 watt air cooled resistor, I can hook this up to the fusor PS negative out put and run that to ground. I'll hook the milli-amp meter to the positive side of the x-former's diode bridge and run the ground wire to the other side of the amp gauge. Running my system up via the variac to no more than 15 kV should get the unit up to 7.5 milli-amps and keep the total power under 120 watts (and not require oil for breakdown issues.) My system should easily handle this and allow me to check the amp gauge's polarity response, current response, and confirm over all performance before I hardwired in the gauge.

Of course the fusor will keep its normal master ground connected.
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Richard Hull »

You can only do this because you have a floating, isolated secondary. Most folks will never have this luxury of choices. Like Bob said keep this all in a grounded power supply box.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: High voltage Fusor grade x-former score (Temporary)

Post by Dennis P Brown »

You are correct about my secondary; and yes, the case is grounded at my common point for all equipment (this is the 'star' system. This ground is independent of my power plug's third ground line/prong.)

I did the power test with the resistor (no fusor.) I ignored a shunt across the gauge since I wasn't worried about the gauge -if it failed - all that would happen would be my bridge would float and since I was monitoring the voltage, I'd catch that issue right away and not raise the variac further. Besides, this was just a one minute test to confirm my gauge polarity and current capabilities. The case for my PS was grounded (and yet, I overlooked an important safety issue. More on that at the end.)

Experimental results: so, with 2.42 M-ohms (2.36 Mohm + 60 kohm), the max current I obtained was 0.005 ma; by V=IR this should result in 12,100 volts. I obtained this current load near 15 kV. I guess that is close. What I am confused about is why that maxed out the x-former? That is the max current occurred at half the variac's setting and rose no higher as I raised the variac past its mid-point. Did that resistance/current at that voltage saturate the core at that point? I would think that if the x-former can reach 28 kV than the current would be 0.012 ma; yet, the voltage pegged at 15 kV.

This experiment makes me wonder about the fusor with a plasma (varying resistance depending on parameters); that is, the varaic setting for voltage (ionization), gas flow, gate valve creating a given pressure (available atoms) all determines the resistance (hence a current is set and the voltage is limited/controlled by the variac setting(?)), Then if one adjusts the variac for max voltage then I assume the fusor's resistance is then adjusted - gas flow and total pressure are then optimized for max current ...? But if the conduction jumps, the current/voltage would go all over the map ... . Does the system just max out the x-former (core saturation?) Or does one select a lower x-former voltage and then try and max out the current by raising the gas pressure (lower the resistance of the fusor)?

This experiment I just did raises more questions then I realized - translation: I really don't understand this relationship for a fusor (and especially with a plasma.) For this fixed resistor I was surprised by the voltage (it was higher) but that value was its max no matter the variac setting ...is that what is supposed to happen? I am not clear on all this. Any thoughts?

A side issue that was bad; my HV cable (after I finished the experiment and moved the system for one more test) failed in a spectacular manner with soot flying up into the air as an arc struck the floor. Must have metal re-barb under it. So, that cable is shot. I have a replacement but that is a bit of work and really should not have happen. That cable is rated for 30 kV ... wait, I think I know why (see previous post; ugh.)

I did learn a lesson that one MUST ground the shield for the HV cable! That shield was floating and this proves why that is a very bad idea. Next time, I run one side of the shield for the HV cable through a good resistor (high wattage) to ground. That should both protect the x-former and me! Again, learning the hard way. Not grounding the shielding on the HV cable is, as I now see clearly, very dangerous and could be fatal. My errors added up to a dangerous situation - need to think issues out on a larger scale.
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