The Beginnings of a Fusor Electrical System

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Rich Feldman
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

Looking good, Rex, but be sure to leave enough work for Duncan to figure out himself. :-)

If the resistor's location would permit it to be near cathode potential, then it might be a bias resistor.

In Coolidge tubes with three terminals at filament end, the isolated one doesn't go to a hot cathode (in my experience).
It goes to a metal part behind and around the filament, that goes by names like focusing cup, grid, or Wehnelt electrode.
That part can be optimized for zero bias (for simplicity), or for bias 100-200 volts more negative than filament-cathode.
In the second case, bias can be developed with a resistor, giving some HV current regulation as well as sharper focus.
wehnelt_bias.jpg
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My own mains-frequency XRT has that resistor and filament winding under the wrapping of one secondary winding. Took a while to figure it out from external resistance measurements.

Commonly, VF and VH are both 60 Hz AC, for "self-rectifying" Coolidge tube operation as Rex speculated. There's no HV current in half of each mains cycle. The average HV current is DC through the secondary windings, amounting to more than a few ampere-turns and shifting core flux toward saturation on one side. Sometimes (I don't know enough to say usually) compensated with a diode and power resistor in parallel, in series with the primary winding (and could be outside of HV tank).

Also, as a reminder, nominal voltage of X-ray transformer secondaries is generally peak, not RMS. The nameplate and meter on radiography systems say kVp.
Oh, and my old reverse-engineering post is still up, with image order reversed: viewtopic.php?f=11&t=4805&p=27643
Last edited by Rich Feldman on Sat Dec 31, 2016 2:38 pm, edited 1 time in total.
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

On closer reading of Rex's post:
There is an odd thing that I haven't quite figured out. Looking at picture 2, terminal 4 is the white wire from the left-side coil. This is wired to a strap around the middle of the core (possibly ground?) and it also goes to a 10K fairly high wattage resistor that then connects to terminal 7, that is the connection to the white wire from the right-side coil. So far I can't guess what this resistor might be doing.
As you surmised, the two HV windings have their Start (inside) ends connected to the grounded core. Except one is connected through a resistor, for monitoring the actual current in the one and only HV circuit. In Duncan's transformer's original application, 10 mA would yield 100 volts.

Some designs bring that secondary wire out of the oil tank, as a terminal called Current Monitor, for use with an external milliammeter and/or resistor. If not connected externally, you would probably get a 10 mA arc from that terminal to ground.
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Re: The Beginnings of a Fusor Electrical System

Post by Richard Hull »

Rex has given one of the best thoughts on this yet. The multiplicity of wires can be daunting. You can go to the bank with the assumption that anything on that terminal bar is low voltage only. A good, non-autoranging ohmeter and about 10 minutes with it in the hands of a skilled electronics buff, who knows HV transformers, would have the basics of what this is down pat. I despise auto-ranging anything. I can turn a knob to select a fixed range. Inductance will not fool a fixed range, old, crude DC ohmeter and a smart user. Too many generalized assumptions are made in auto-ranging systems and can leave the average user often wondering what is going on.

I still have, and occasionally use, one of a pair of old d'arsonval metered VOMs. They never lie and warrant an easily read, averaging of a fluctuating voltage that would have a digital meter going nuts.

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Re: The Beginnings of a Fusor Electrical System

Post by Bob Reite »

Ah yes! A good old Simpson 260! I personally still have the Heatkit MM-1 VOM that I built when I was in Jr. High.
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

Happy new year, everyone.

Duncan, I know you haven't told us any ohms readings without also telling us the meter range settings. Autoranging issues are a red herring in this thread, and are probably my fault.

Back to your assignment, here's a first step that ought to be pretty educational. But before that, if you don't have a set of clip leads on hand after Christmas, get one. Maybe cashier will regard it as colorful Christmas lighting stuff, and give you the end-of-season discount. :-)
th.jpg
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At the transformer, let's begin with white and yellow wires that emerge around middle of paper wrapping that covers each secondary winding. Connect a reasonably fresh 9 volt battery between the white wire and one of the yellows. Now with your meter on a DC voltage range, measure the voltage of each battery terminal with respect to the grounded core (accessible on terminal strip).
If you get values around +4 V and -4 V, then measure and report V on all other terminals. I expect interesting values at the 10K resistor and at the red wire next to the two yellow wires at mid-secondary.

Most other accessible wires will probably read zero volts, but that doesn't mean they are connected to ground. If you leave the battery connections undisturbed, but move your voltmeter's common probe to the negative battery terminal, the HV circuit should have voltages between 0 and about +9 V. If wires that read 0 before are reading 0 again, with different meter reference voltage, then you know they are isolated from the HV circuit. That's easy to double-check with no battery, and meter in ohms mode.

There may be a small, harmless spark when you disconnect the battery. Have fun!
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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie »

I've measured 8.04 volts on the specified connections. Isn't that similar to what my multimeter does to measure resistance?
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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie »

To help clarify, here's a picture of where the wires enter the transformer body from the connection board

20170101_172531.jpg
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

Duncan, I think you missed a key instruction, because of not knowing enough about electricity. There are online learning places much more appropriate than fusor.net. It's even better if you can engage a live teacher/mentor face to face. In fact it's essential, for safety among other things.

We get people here who should be sent away to learn basics. Here's one really good starter kit, with _no_ proprietary parts and no circuits to build without understanding. Power comes from a D cell (not included) with a rubber band around both ends to hold wires in contact. My wife found these kits at Marshalls before Christmas for $15.
Mr.Electricity.PNG
The next step is to introduce a multimeter, and series and parallel circuits more complicated than any in the kit. Circuit A might be child's play. What's the voltage on each lamp in circuit B? What's the voltage between points X and Y? It is sad to think of people seeking nuclear fusion before they can confidently answer those questions by inspection.
series.PNG
You could check the answers by measurement, using your voltmeter and a few incandescent holiday lamps. The kind that come wired in series. Time to sacrifice your parents' least favorite light string. Cut and strip the green wires midway between sockets. When you grow tired of measuring voltage, measure and report the current in the wire between the first and second lamps.

If you want to get right back to your transformer, here's more detail (no substantive change) on the original assignment. Let's number the connections:
xfrmr-1a.jpg
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xfrmr-2a.jpg
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Last time I asked you to connect points 1 and 4 to your 9 volt battery. Then tried to say: hold or clip the voltmeter's black probe onto point 8. Measure and write down the indicated voltage when the red probe is on points 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12. Then move the black probe to point 1 and repeat the sweep of red probe through all 12 points. For extra credit, put black probe on point 4 and walk the red probe around yet again. You can put the 36 measurements in a 3 by 12 table with a story to tell.
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

Duncan Wilkie wrote:I've measured 8.04 volts on the specified connections. Isn't that similar to what my multimeter does to measure resistance?
Yup, but there's more. I bet the full exercise will confirm one probable topology for the complete high voltage circuit, and give you the DC resistance of three windings and two resistors, regardless of how your multimeter's ohm modes behave with multiple-henry loads. You would also be helping with a lesson for inexperienced readers, about the customary meaning of "voltage" at single points.

I was serious about using miniature incandescent holiday lights for elementary experiments in voltage and current measurement on series and parallel circuits. An additional learning experience comes when you try to reconcile the V and I results with measured resistance of the lamps. Please be sure the string is unplugged, and stays unplugged, when you start cutting the wires. :-)
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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie »

Mr. Feldman
I do have some considerable experience with low voltage electrics (Arduino, Tesla coils, etc). I had that exact electricity kit as a kid. Good stuff.
I apologize for having led you to believe my lack of experience; I simply would like to exercise extreme caution in this process. Your original post said "If you get values..." and I wished to withhold from testing the other terminals because you do indeed know considerably more than I. I assumed the discrepancy in voltage confirms that there are two secondaries wired in series, however, I wished to confirm that my result didn't pose any risk to me.
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

OK, Duncan. It wasn't clear where you had measured the 8.04 volts, which looks like the full voltage of a nominal 9V battery that's seen some use or is rechargeable.

I am guessing that your transformer, as pictured, has a resistive path all the way from 1 to 4. It might or might not include the filament winding. Then a hidden bias resistor, then the left side secondary, then node 8 and the core, the visible 10K resistor before or after node 8, and finally the right side secondary winding. Total resistance on the order of 100 kΩ. You could see what happens if you measure 1 to 4 with an ohmeter.

Suppose that guess is right. In normal service, the white wire and the pair of yellow wires have high voltage AC of similar magnitude and opposite phase, with respect to the core and the equipment ground. In fusor service, you would have high voltage rectifiers on the white and one yellow, with their anodes connected together and to the HV ballast resistor. Point 8 and the core would be connected to _your_ equipment ground. With that full-wave rectification, each secondary winding would carry about half of the DC plasma current, in directions that cancel each other instead of adding (which could create a magnetic saturation issue).

If the only energy source is your 9 volt battery, I wouldn't worry about damage to battery or transformer or yourself. As explained above, I would expect point 8 voltage to be about in the middle of the battery voltage. Measured with respect to point 8, the battery terminals would be around +4 and -4 volts, and the other terminals at intermediate voltages according to the resistance ratios. Terminals in isolated circuits would read zero, no matter which point in the HV loop (battery circuit) is your reference.
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Re: The Beginnings of a Fusor Electrical System

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So, I've measured 0 on everything but point 2 at the battery voltage in respect to point 4, and 0 on everything except for a 7.5V reading on point 2 in respect to 8. In respect to point 1, there are -6.6V on point 4 and -7.6V on either 8 and 10.
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

Maybe it's time for us to take this off line, Duncan. I will send you a PM.

Thanks for taking the time to try it my way. Even allowing for some ambiguities in your report, I see no way to reconcile the numbers with the model of voltage as potential, if the battery stayed connected between p1 and p4 for the whole experiment. How can p4 and p8 be 1.0 volt different when reference is p1, but not different when ref is p4 or p8? How can p8 be more negative than p4 (measured W.R.T. p1) when p4 is tied to negative battery terminal?

What's your voltmeter's input impedance?

On ohms range, is there any continuity between p3 and any other point? Is there very low resistance between p1 and p2?

The set of pins on which you found any nonzero voltage is 1, 2, 4, 8, 10. You could check that they are isolated from everything else, then we can nail down that 5-terminal network.
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Re: The Beginnings of a Fusor Electrical System

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It has been wisely suggested to make an x-y table of all the pins/wires/terminals and work your way through measuring resistances. This will speak volumes and identify all windings.

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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie »

I've worked to identify the wires on the transformer. The x-y table of the resistances is listed below, and I've laid out the separate networks in a drawing labeled with the resistances.


image001.png


Resistor Diagram XRT.jpg
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Re: The Beginnings of a Fusor Electrical System

Post by Richard Hull »

3 and 4 are your HV outputs with 8 being the center tap of the HV winding. Wire number 11 may be a high voltage metering tap relative to the center tap, (wire 8). NOTE: does wire 8 measure 0 ohms to the metal core?..It should. If not, it must be grounded by you.


1 and 2 might be the filament winding

5, 6 and 7 might be the primary with 5 and 6 being 220 volts and 6 and 7 120 volts. I would try the `120volts across 5 and 7 first. USE A VARIAC!! see what the output voltage is.
10, 12 and 9 are an identical winding to 5,6,7 and may be a companion or related primary.

All the above assumes this is a 60 hz transformer.

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Fusion is the energy of the future....and it always will be
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Re: The Beginnings of a Fusor Electrical System

Post by Rex Allers »

Duncan,

Your table of resistances is good, but I don't think what you have presented is sufficient.

You have assigned the terminals or connections the numbers 1 through 12. I think first you need a simple table that describes what each of these numbers (1-12) means. I couldn't easily figure that association out and in your table you have a color on each of these numbers that didn't make sense to me on a quick look.

Your transformer is made up of two coil sections. For working things out and labeling your terminal definitions, I'd call one coil Left and the other Right. With the transformer in front of you you can decide which one makes most sense to call Left. For the sake of this discussion, let's say the Left coil is the one that has the 3 HV output wires with colors Red, Yel and Yel. So then the Right coil would be the the one with one corresponding white wire.

Then let's call the side of the L coil with the Red, Yel, Yel wires the coil "Top". Each coil also has wires going to 4 consecutive terminals on the plastic terminal strip. Let's say each of these wires comes from the coil "End". The 4 wires from the coil "Ends" to the terminal strips each have color codes that (I think) are the same for each of the coils (L and R). I would also put these wire codes in the definition table.

So the first definition table entry might be for over-all terminal 1, and this might also be terminal strip connector 1 and be the black wire from the L coil End. Some other over-all terminal definition (Lets say #9) might say it is not on the plastic terminal strip, but is the red wire on the Top of the L coil.

In your resistance table, you have assigned terminals 1 through 12. I assume this is terminal strip terminals 1 through 8, plus the 4 wires on the Top of the two coils (3 on L and one on R). I don't think this is enough.

Using the coil Top definition for where the (probably) HV wires are, the the opposite "Bottom" side of each coil has two Black wires that appear to have been cut. I think you need to get access to these wires, strip their ends and add them to your definition and resistance tables. My guess is the two black Bottom wires from one of the coils are probably connected together and can be treated as one connection. If you get two these wires and find the two black wires from one coil have zero resistance between them, you can treat the two wires as one terminal, so you would only need two more terminals in your tables rather than four more. 14 terminals total vs. 16.

I hope this makes sense to you. You might want to go back to the early post I made in this thread, trying to guess what the connections might be. Until proven wrong, I'll stand by the big picture of what I think the connections were.

Oh, I almost forgot. You also have the blue thermal switch and the power resistor to contend with. Their connections and resistance need to be factored into the analysis. I think the thermal switch should be like a closed switch, so if that measures true, can be though of as an extension of the wire that connects to it (Black End wire, I think).
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

To all: Duncan's pin numbers were pictorially assigned by me in a preceding post: viewtopic.php?f=11&t=11223&start=10#p74002 We decided to save the black wires for later.

Duncan, it's good to see you back. You didn't give me credit for off-forum help with your DC resistance table, long ago. I filled in the upper triangle (by symmetry) and color coded the isolated groups (by inspection). Richard Hull was first to call for the plain resistance-table attack, etc.

For the network consisting of pins 3, 4, 8, and 11, your diagram has six paths. If you connect six resistors as drawn, the node-to-node measurements will all be lower than what you observed.
We want a simple circuit model that matches all six of your measurements and would work in an x-ray generator.
Three elements are sufficient, in a series string (as opposed to a wye configuration). Looks like what you have is:

Pin 3: HV end of secondary winding for cathode (close to filament winding pins 1 and 2).
*R1* 160 K ohm coil resistance
Pin 8: LV end of cathode winding, core connection in picture, and (implicit) external ground.
*R2* 10 K ohm current sense resistor
Pin 11: LV end of anode winding, and (implicit) external current indicating meter
*R3* 160 K ohm coil resistance
Pin 4: HV end of secondary winding for anode.

Verstehen Sie?

Now go figure the voltage drop and power dissipation in one HV winding, from a DC current of 20 mA.
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Re: The Beginnings of a Fusor Electrical System

Post by Rex Allers »

OK, I follow the numbering now, and agree with most of what Richard and Rich have said.

To make it clearer to me about the physical meanings, I interpreted things as I mentioned in my last post. Here are two pictures showing that.
Top view
Top view
Terminal End view
Terminal End view
So in my labels TL=Top Left, TR=Top Right, EL=End Left, and ER=End Right. I also added the transformer wire colors to my labels. The resistance node map provided was very helpful in visualizing the connections. To help my understanding, I edited the image to add my physical labels. After looking at it, I also added red lines on the map to show the actual transformer windings and one orange line to show the power resistor. Here's the result.
Node map of connections
Node map of connections
On the primary side there are two matching sets of two series coils on the Left and Right. I would expect that the left and right sides would be connected in parallel for 110V or in series for 220V (ish). Not sure about using all or part of each side series coils. I'd start with the maximum coils or across 5-7 (left) and 9-12 (right). So for (eventual) 110VAC input I have one AC wire to 5 & 9 and the other AC wire to 7 & 12. Other opinions welcomed. As others suggest, I'd start feeding it with a very low voltage and measure outputs for sanity check.

Maybe others have a good approach for figuring out how much of each side's primary to use. I've suggested starting with AC across 5 & 7 (both coils in series). It could also be 5 & 6 for just one coil. The second would make higher output voltage but too little primary would saturate and do bad things. I'm not sure how to work out the proper connections for an input voltage.

There's also that thermal switch to eventually get correct. I think it is between terminal 5 and the black wire of the left transformer coil. If my parallel connection suggestion is correct, the Thermal opening would only cut out one coil. Something needs to be changed to have this thermal switch opening, switch off all primary.

Assuming the 10K resistor is to measure current in the original operation with load between HV outputs of 3 and 4, this wont be right for fusor operation with the two HV outputs feeding push/pull into two diodes for a full wave rectifier. Shouldn't 8 & 11 be connected together with some sense resistor between this node and ground?

If It was me, I'd also fish out at least one (for starters) of the cut black wires on the bottom of the coils and see how it relates to the other nodes. They must be there for something.
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Re: The Beginnings of a Fusor Electrical System

Post by Richard Hull »

Sad,sad,sad. I am fortunate to own metering and systems up the wazzoo. AC or DC metering high and low voltage for each....Variacs that are fully metered in currnt and volts, etc. I would have had this puppy noodled out and turned on in minutes. Bottom line....If you don't have the gear needed, you are looking somewhere between time tortured guess work and and a possible early death in the family as you play "guess the windings". Some are obvious using an ohmeter. Many good and perhaps correct answers abound here, but when the rubber has to meet the road, only good instrumentation, in hand, will figure out what you really do have.

Fire it up and tune for minimum smoke......

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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie »

Rich-
Please correct me if I'm wrong, but this is the calculation I did to get the values.
For the coil with a resistance of 160K ohms at .02A, voltage drop is derived from V=IR, so V=. 02*160000,so V=3200V. Power Dissipation is P=IV, so P=.02*3200, so P= 64A.
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Re: The Beginnings of a Fusor Electrical System

Post by Richard Hull »

This is a DC reisitance of a coil and relates to the secondary's "source impedance"' and is a complex issue based on the load type and value. DC figures can't be used accurately on a coil of wire supplying AC in a transformer system. Much also depends on the core and how it is made. To noodle out the complex issues, good instruments and a bit of investigation is needed.

DC Coil resistances only give a clue as to what "might" be their function, and little to do with there actual in-circuit performance, which is based on many other factors.

DC circuits and networks can be taught and learned in a one semester college course. All the ramifications of AC, reactive and HF power circuitry can consume 2 years of upper level college study.

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Fusion is the energy of the future....and it always will be
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

Duncan, your answer to my latest numerical question is correct, except for a petty typographical error.

Drawing 20 mA from an XRT secondary winding that has 160 kΩ of DC resistance will cause the voltage to drop by 3,200 V, and will generate 64 watts of heat in the winding. Those numbers hold for DC or RMS AC. The resistance and copper loss will increase as the winding heats up.

As mentioned before: if you configure a full-wave rectifier, using one HV diode on each secondary winding, each winding would provide half of the average current and get half (not 1/4) of the heating calculated above. Each diode needs to withstand at least twice the fusor voltage. (Same applies to full wave rectifier circuit on a NST in demo fusor FAQ).
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Re: The Beginnings of a Fusor Electrical System

Post by Duncan Wilkie »

Would the next step be to variac the primary way down and see what the ratio of power increase is? Or is there more still to learn before I plug it in?
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Re: The Beginnings of a Fusor Electrical System

Post by Rich Feldman »

>> ... next step be to variac the primary way down and see what the ratio of power increase is?
>> Or is there more still to learn before I plug it in?

I'm still concerned about your very junior level of knowledge about electricity. Transformers do not increase power, in fact they invariably lose some. What's the name (or even just the initials) of your adult mentor who has experience with electrical connections and measurements at voltages over 100 V? Over 300 V?

You could do what I did when I had an unfamiliar x-ray transformer. It's slightly different, in some important ways, from your proposal copied above.

Step 1. Set up instrumentation that can measure AC voltages up to a few thousand. This can be tested, and sensitivity verified, with regular house power. Could be an analog or digital multimeter on an AC volts range, with simple external attenuation as described in FAQs.
Alternative: Set up to measure DC voltages up to a few thousand, and connect it to the _rectifed_ output of your XRT under test. You'll need that anyway for your demo or real fusor.

Step 2. Get your Variac, but don't use it to directly feed any XRT primary, with or without ballast.
Put a step-down transformer in between, that normally reduces house voltage to 12 volts or less. These are common for low-voltage outdoor lighting, and indoor lights that use low-voltage halogen bulbs. Generally have their own fuses on low voltage side. I'd find one rated for at least 50 VA, as opposed to (say) a doorbell transformer, or something out of an inexpensive cordless tool charger. The filament winding of a MOT (2 or 3 volts) might serve, with suitable precautions.

Then you can use most of your variac range, for much better resolution and repeatability, without having to put your XRT properly under oil. You could still die from touching an XRT secondary connection, even with primary voltage much less than 10% of nominal. But it won't jump through (much) air to get you.
All models are wrong; some models are useful. -- George Box
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