Power Supply from an "interesting" XRT

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Pablo Llaguno
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Power Supply from an "interesting" XRT

Post by Pablo Llaguno »

Hello,

I just bought an interesting 65kV transformer from Joshua Guertler viewtopic.php?f=20&t=13104&p=85386#p85386. The plan is to build a 0-30kV power supply that can produce some neutrons in the fusor. Here are some images from the transformer that Joshua took (I haven't got it in my hands yet, I need to pick it up at the US):
XRT specs.png
XRT.png
So based on my experience with neon sign transformers, and by reading the HV FAQs, I made this diagram of the electrical system of the power supply:
Power Supply Diagram.png
One main problem of this diagram is that I don't know yet if the transformer is centre tapped so that's something I'll have to check once I've got the xfmr in my hands for testing. The electrical components will consist of a 20A variac that I already have, some diodes, a ballast resistor and some resistors for measuring current and voltage.

By the way, I found it hard to locate some of these components, specially the ballast resistor. I read that some here use a 100W 60kohm wire wound resistor and found this supplier: https://www.mouser.com/ProductDetail/588-L100J60KE, while it is a bit expensive I don't have any hamfests where I live so for anyone looking for a 100W ballast resistor this one will do for $25. Also the 300m ohm resistors can be found here (thanks Rex Allers): https://www.ebay.com/itm/Ohmite-208-300 ... 2749.l2649, these are 5W and the specs say it holds up to 20kV.

The only part I am missing are the HV diodes, but I will wait to see if the XRT is centre tapped or not, because that would change the design of the rectifier. I was thinking of ordering some 20kV 100mA diodes of ebay and build 2 strings of 3 diodes each. At 65kV, if its centre tapped, each coil would produce 32.5kV, a weird value that makes me think this might not be centre tapped.

One more thing. Joshua suggested to put the thing under oil and using a pump to cool it. Since the XRT is rated at 10mA for 2sec in air, it may be able to give 10-15mA for 5min under oil. The only thing I am worried about is if it will produce enough current, since I have a rather large chamber:
Demo System.jpg
I will update you when I have the transformer in my hands, meanwhile any input would be greatly appreciated.
Pablo
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Rich Feldman
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Re: Power Supply from an "interesting" XRT

Post by Rich Feldman »

Good luck, Pablo. Lots to learn from experimenting with any transformer. I have an dental XRT about that size that's still on the workbench, not yet used by me for high voltage.
Electrical characterization reported here:
viewtopic.php?f=11&t=4805&p=27643

Here are a few comments regarding things people have said in this thread.

The 2-second limit on x ray head is from the little Coolidge tube, not the XRT itself. Tube anode, compared to transformer, receives a 10 times bigger fraction of the input power, and weighs 100 times less. As Richard said, a normal session is a few shots a minute apart, each lasting a fraction of a second. Tube and transformer were designed accordingly.

As for duty cycle of the transformer itself, it's misleading to think in terms of a watt-seconds limit.
Power loss in core is a nonlinear function of voltage & has nothing to do with secondary current.
Power loss in windings is proportional to square of current, and has nothing to do with the voltages. Easy to figure out from winding resistance.

The oil is for high voltage insulation, not cooling. Search the forum for stories about using vacuum to eliminate air bubbles in secondary windings. It's best to apply the vacuum before admitting oil to tank. That's more trouble, but is how professional HV transformer makers/installers and X-ray repair techs always do it.

I have never heard of anyone actively pumping oil in an XRT tank, but suppose it could be done. Might be easier to arrange tank for good natural convection in the oil, and blow air over the outside if you ever run long enough for whole tank to get hot.
All models are wrong; some models are useful. -- George Box
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Rich Feldman
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Re: Power Supply from an "interesting" XRT

Post by Rich Feldman »

Forgot to mention some details peculiar to XRT nameplates.
  • Primary voltage value is RMS.
  • Secondary voltage value is peak (indicated by the symbol kVp), because that's what matters to radiologists. It's common, but sloppy, practice to use kVp and VRMS and VAC as if they represent units of measurement, which they don't.
  • Secondary current is average DC value in a half-wave rectified (self rectified) HV circuit, in this case. For practical convenience, heat capacity of X-ray tubes is often given in heat units (peak kilovolts * average mA * seconds) instead of watt-seconds or joules. Value is significantly different for operation on DC (from high frequency power converter) compared to half-wave rectified 60 Hz sinusoids.
I bet your transformer secondary is center-tapped, actually two separate windings. Usually the start of one winding is tied to the grounded core, and the start of other winding is brought out for current monitoring. An obvious modification for your application is to tie both "start of winding" wires together, as center tap, and put current sense resistor between that and the grounded core. Could all be done inside the oil tank.

65 kVp is an extremely common maximum voltage in dental x-ray generators. 32.5 kV peak is plenty for fusion.
All models are wrong; some models are useful. -- George Box
Pablo Llaguno
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Re: Power Supply from an "interesting" XRT

Post by Pablo Llaguno »

Thank you for your input Rich, I always appreciate it.

I think you are right about using oil for insulation rather than cooling. Also, I think that with a big enough case, natural convection and a fan (as you said) would allow for "long" runs. About eliminating air bubbles inside the transformer, I once saw this video of a guy building a 50kV supply (impressive work) and what he did is put a flare fitting in the case and applied the vacuum with the case half filled with oil. https://www.youtube.com/watch?v=uCm1iPL ... dex=2&t=0s @min 37. I'll search through the forums to see if there is an easier way to do this.

If as you say the transformer secondary is center-tapped, then I will tie these two "start of winding" wires and use them as the center tap, which will then be used as ground and for the current measuring resistor. By the way I am searching for some HV wire, since the supply will provide 32.5kV peak I was thinking 18AWG 30kV wire might suffice? Walmart is selling 5 meters of this wire at $9 so I am tempted to buy it. https://www.walmart.com/ip/5M-Length-18 ... /244434583

Lastly, if each winding gives 32.5kV then could the diode string have in total a series PIV of 40kV? Or should I go to the safe side and aim for 65kV?
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Rich Feldman
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Re: Power Supply from an "interesting" XRT

Post by Rich Feldman »

The latter.
When one secondary terminal and the rectifier output are at -32 kV, the other secondary terminal is at +32 kV.
So diode needs to withstand the end-to-end secondary voltage.
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Re: Power Supply from an "interesting" XRT

Post by Rex Allers »

Rich,

The voltages across the diodes in this center-tapped full wave circuit is something you pointed out to me in an earlier thread.

The full end-to-end voltage requirement is something I missed after dozens of years of looking at that circuit configuration. It is important and many, like me, might not see it and think only the output voltage (half transformer secondary) is needed.

Thanks, Rich, for putting me right and for sharing again this information that I missed and others might too.
Rex Allers
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Re: Power Supply from an "interesting" XRT

Post by Pablo Llaguno »

The transformer arrived and I did a couple of tests yesterday. First I measured resistance on the wires to find the primary and the secondary outputs. It was quite easy because the primary read less than 5 ohms and each secondary read 90kohm. After that I proceeded to test the output of the transformer in air with a small input voltage. The way I did it is with a method that I read in another post (by Rex Allers if I recall correctly). It goes something like this: Plug a 12V transformer to a Variac and with that you have a precise way of controlling the input voltage to your XRT. I did it slightly differently using a power supply set to 2V and powering it with the Variac, that way I had full control from 0 to about 3V input for the XRT.
IMG_1492.jpg
IMG_1492.jpg (118.74 KiB) Viewed 6539 times
As you can see in the picture I used two DMM, one for measuring input voltage and the other coupled with a high voltage probe (basically a voltage divider) for the output of the transformer. Nevermind the high voltage probe, it was very inaccurate and since my output was acceptable for the DMM I just used the banana wires for the tests.

I'll summarize the tests with these pictures:
- Input voltage was 1V, measuring voltage between ground (core) and each output:
Screen Shot 2020-01-15 at 12.22.02 PM.png
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- Input voltage was 1V, measuring voltage between the two output wires of the secondary:
Screen Shot 2020-01-15 at 12.22.13 PM.png
Screen Shot 2020-01-15 at 12.22.13 PM.png (121.96 KiB) Viewed 6539 times
With this I now know the transformer is in working order and the next thing to do is build its case and add the rectifier and measuring systems. Rich was right in that the secondary is center-tapped, and as he said one "start of winding" is connected to the core. However the other winding is odd because it has its "start of winding" connected to the core, however it has two HV outputs. My plan would be to just leave one of this outputs isolated and solder a thick cable to the core as my center-tap for the current measuring resistor and ground to the fusor.
Screen Shot 2020-01-15 at 12.22.27 PM.png
Screen Shot 2020-01-15 at 12.22.27 PM.png (269.54 KiB) Viewed 6539 times
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Mark Rowley
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Re: Power Supply from an "interesting" XRT

Post by Mark Rowley »

I have one of those transformers but elected not to use it due to its smaller size and potentially limited duty-cycle. My Fusor overtaxed a couple that we’re about double the size so I didn’t want to risk damaging it. I hope it works out and am interested in how you ballast and power it.

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Rich Feldman
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Re: Power Supply from an "interesting" XRT

Post by Rich Feldman »

Good work so far, Pablo! Thank you for sharing.

>>one "start of winding" is connected to the core. However the other winding is odd because it has its "start of winding" connected to the core, however it has two HV outputs.

That detail is like my similar-size XRT, except mine has three HV outputs. Either way, the secondary with more than one connection (not counting "start of winding") is for the cathode end of the x-ray tube. Your two HV outputs are probably connected internally to a Filament winding designed for a few volts and a few amperes. The x-ray tube self-rectifies the HV current, by not conducting when cold electrode is negative.

When a third HV output is present, it's internally connected to the others with a resistor of some tens of kΩ. X-ray tube would have a third terminal at cathode end, connected to the "focus cup" metal behind and around the cathode. Arranged so the HV current, passing through the resistor, develops a bias of a couple hundred volts between focus cup and cathode. Designed to reduce the e-beam spot size on anode, and also to somewhat regulate the HV current.
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Pablo Llaguno
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Re: Power Supply from an "interesting" XRT

Post by Pablo Llaguno »

Got the supply all wired up and ready to be filled with oil:
IMG_0055.jpg
IMG_0055.jpg (58.06 KiB) Viewed 6284 times
Thankfully I have a friend who will help me with filling the tank with oil. He has years of experience in the utility transformer business and has equipment to correctly fill the tank. His idea is to bake the supply (without the tank) for 48hrs at 100ºC and the apply the high voltage mineral oil. I am kind of worried about the diodes or the ballast breaking up with that amount of heat. Does anyone have experience with this to know if the components of the supply could handle the heat?

Alternatively, we could apply a vacuum to the tank and then apply the oil, however I am not sure if this would be enough as I don't know how much time this transformer has been outside its oil tank.
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Rich Feldman
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Re: Power Supply from an "interesting" XRT

Post by Rich Feldman »

100 degrees C forever won't damage any idle diode, resistor, or transformer winding. They get hotter than that inside when working at rated current, and are exposed to much higher temperatures during manufacture.

You can learn a lot from "How It's Made" TV episode about "Electric Pole Transformers", about 5 minutes long.
I wasn't going to post a URL, because the first 10 hits in YouTube search were obviously pirated. Then saw one which appears to be "official", a detail I think matters for creative works. Baking and oil potting appear 3 minutes into video.
https://youtu.be/8F9m_HN6ueE?t=185

The order shown is: bake in air, put into tank hot, pull vacuum and add oil. Order of the last two steps is unclear. If and when I pot my own dental XRT, am planning to heat it within evacuated tank using DC current in the windings (to remove moisture from 10 years of storage).

There are many reports on fusor.net about amateurs skipping the vacuum, or applying vacuum after the transformer is submerged in oil, with varying success.
I think the pros always evacuate chamber before admitting oil. The tanks (even big rectangular ones mounted on concrete pads outdoors) are reinforced to withstand full vacuum.
High frequency transformers, with fewer turns of thicker wire, might get an exception.

Suppose your finely wound XRT secondary carries tiny bubbles of air deep into the oil. When you pull a perfect vacuum above the oil, the pressure at deepest bubbles can't get below 10 torr (unless you turn off Earth gravity).
Bubbles will expand by factor of 4 or 5 in diameter, which I think isn't going to make them all escape. If the oil bubbles under vacuum even without a transformer, then it hasn't been degassed properly.

Please tell us what your expert friend recommends. Experience and practice count more than my theory and talk.
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Pablo Llaguno
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Re: Power Supply from an "interesting" XRT

Post by Pablo Llaguno »

Back with updates from the past few weeks. I learned that it does take time to make your own power supply. Rich was right in that baking at 100ºC won't damage components, however if anyone does this with their own supply, make sure your wires can take the temperature. I left it all in a bucket and my HV line (made from a RG213 cable) couldn't take it, the dielectric melted. Otherwise baking it actually helped a lot in getting rid of humidity from the coils. Here is a picture before it was baked:
IMG_0055.jpg
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After baking it for 2 days at 120ºC we put the power supply in a vacuum chamber for about a day. Then we tested the dielectric strength with a dielectric strength tester. At 5kV from the tester, between ground and the HV line we had a good impedance (in the order of Gigaohms). Not many have access to this devices, but they sure make your work a lot easier when building the tank for your supply, it also helps in making sure your HV wire can take it. Once all that was done, we put the thing in oil and tested it.

So how did it perform? Well, Mark was right in that these small size transformers are quite limited. At no load the thing does output -32kV, however with a load it is another story. My chamber is rather large as it is a Kurt J Lesker PVD system (a large tank with tons of cf flanges) so this might be a reason why my output was limited to just -12kV. I did create a plasma though:
IMG_0071.jpg
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I put a fuse between the line from my variac to the power supply, and it kept blowing 1A, 3A, 5A and even 10A fuses. Ultimately I decided to leave only the fuse from the variac (20A). I think I pushed it too far, because I also blew that one. That is 2.4kW of input power, on the output I had -12kV and some 30mA at around 20mTorr. I assume a lot of power was being lost in the windings/core of the transformer.

Sadly, silly me made a huge mistake. When realizing the supply was limited under the plasma load, I decided to test the power supply to see if it still achieved 32kV under no load. I placed the line in an oil tank (as a insulator) and started raising the input voltage with my variac all the way to 32kV. Then, realizing I could still raise the voltage a bit more I pushed it to about 38/40 kV. Then something blew up and burned the oil and the thing is behaving really weird. Btw burnt transformer oil smells horrible and it sticks in your nose!!! Here is a picture of the mess:
IMG_0078.jpg
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Notice the dark oil and also how some of the plastic deformed during the baking of the tank.

Due to a certain virus my university is cancelling classes, so I am not sure when I am going to be able to inspect the supply as it is in the lab. I am hoping that I blew the diodes and not a secondary coil.
Here is the whole thing
Here is the whole thing
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Pablo Llaguno
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Re: Power Supply from an "interesting" XRT

Post by Pablo Llaguno »

Back with some updates, the XRT is basically done.

I opened up the supply and pulled the XRT, removing the diodes, ballast and measuring resistors. Initially there was hope, as with 1V input I was measuring 400V between both secondary windings. However, as I increased the input voltage I noticed input current was way too high under no load. At about 20V input, there was some smoke and I could see the traces of contaminants from the failure point under the oil. Took the XRT out of the oil, removed the plastic stripping that act as insulators and identified the failure, a burnt secondary winding.

I am still not sure how I burnt a secondary winding. All operation was under oil so I am guessing it was either extended runs with the fusor or when I pushed it to 40kV (probably the latter). While I could wire it up as to use only a secondary winding and a half wave rectifier, or rewind the damaged secondary, I don't think it will be worth it. What I learned from this is that as Richard and others have said, you need a heavy XRT to achieve fusion on these larger sized fusors.

Also, one can conclude that the stress on these smaller sized transformers when operating with a HV load is quite significant. Rich Feldman`s viewtopic.php?f=11&t=4805&p=27643 really shows how much power is heating these kind of devices. As I learned during Fusor operation. Also, using any radiator or pump to cool it down will not leed to success, as much of the heat is trapped inside of the windings.

As to the future of my fusor, this was certainly a huge blow, but I've learnt a lot. I hope I can find a heavy weight XRT as much of the process of building the tank and wiring it up would be similar. I am also considering making a HF supply.
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Mark Rowley
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Re: Power Supply from an "interesting" XRT

Post by Mark Rowley »

As a group we should not offer hope that these smaller XRTs work for Fusor use. I also traveled down this rabbit hole of smoked secondaries and was met with the same results. The entire forum is littered with similar failures. Yes, one does learn some things but it's hardly worth the wasted time and money spent in the process.

Pablo, I'm saddened to hear about the failure but there is no doubt you will forge through with a working power supply. Obviously, keep your eyes open for the unobtanium heavy weight XRT, but dont count on getting one. Pursuing a high frequency arrangement is your best bet.

Looking forward to your soon-to be Neutron Club membership!

Mark Rowley
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Re: Power Supply from an "interesting" XRT

Post by Richard Hull »

If it is a portable, swing out over the patient x-ray head with no external supply, it will not work well or at all for a real fusing fusor. Period!

The x-ray supply you want should not be able to be lifted or moved without at least a hand truck! (the reason: Transformer iron and gallons of oil)

Only special high frequency, modern HV systems can get away with near zero mass in the transformer and absolutely no oil anywhere in the system. $$$$

Richard Hull
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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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