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The Beginnings of a Fusor Electrical System

Posted: Wed Dec 28, 2016 2:04 am
by Duncan Wilkie
A couple of days ago I received the preliminary components of the HV portion of my fusion system. I bought my X-ray transformer from a user of these forums. The electrical portion is the only area of the project on which I am a little iffy, and would appreciate guidance in these matters. I have perused the FAQ's and such, and the transformer I have appears to be smaller than Richard's faq post suggests. I will attach images of the transformer. It is quite greasy, suggesting earlier insertion in mineral oil. It was said to output 60kv at 10mA.
I couldn't get the img codes to work so I just linked the images. Sorry.

Here's the component bit; no idea what the little blue pot is. Some kind of relay, I guess.

I also will most likely get the diodes, capacitors, resistors, etc. within a few weeks. I would appreciate guidance as to where to proceed first.

Re: The Beginnings of a Fusor Electrical System

Posted: Wed Dec 28, 2016 2:41 am
by Bob Reite
The blue item is not a "pot" but a thermal cutout to prevent damage to the transformer from overheating. The transfomer will need to be immersed in mineral oil again before full voltage is applied to it.

Re: The Beginnings of a Fusor Electrical System

Posted: Thu Dec 29, 2016 4:45 am
by Richard Hull
In a 60 hz sysem, iron mass and copper mass determine whether the transformer can run continuously or for short periods only. Small iron mass, small physical size means short duty cycle.

Massive iron, (1 cubic foot), means the transformer is ready for hard action. Remember, X-ray transformers used to be made big, but then someone got the idea that most x-ray machines operate for a few seconds, at most, followed by a comparatively long down time and the iron mass got smaller. Finally, in the modern age, high frequency systems and fractional second exposure times meant that a well made oil immersed HF transformer could be the size of your fist and weigh a couple of pounds. These little guys are just totally unsuitable for fusor work.

Your item looks like a medium duty transformer and could be fusor capable. How much does it weigh out of oil?

Richard Hull

Re: The Beginnings of a Fusor Electrical System

Posted: Thu Dec 29, 2016 7:39 pm
by Duncan Wilkie
The transformer weighs about 15-20 pounds. How long would the run time be on a mid-duty, fusor-capable supply?

Also, could you verify my opinions on wires? (Haven't tested with a voltmeter, only been few days) Here goes: The yellows are negative in/out, the red is positive in, and the white is ground.
How exactly would I test the device? I'm still trying to read up on transformer theory and learning how to build the rest.
Also: How would I go about immersing my connections in mineral oil? Would that include the wire up to the feedthrough? The transformer itself is tanked.

My transformer:
60kv @ 10mA, according to seller. (Is that 600 VA or 6 million?) It's C-type with a small resistor labeled "10K" and a thermal cutout.
Here is the image if the wires off the coils.
I have found this guide on transformers helpful: and would recommend it to anyone that has a prior understanding of really basic circuit language. The transformer calculator is helpful.

Re: The Beginnings of a Fusor Electrical System

Posted: Thu Dec 29, 2016 9:04 pm
by Richard Hull
There are no plus and minus or negative wires on a transformer. I can't help you. I'm here and you are there. You need to ohm-meter the thing. Forget voltmetering it until you ohm-meter it. That will tell you what wires are what. Ohm-meter all wires against the core iron as well. One may be hooked to the core and that would be ground.

Richard Hull

Re: The Beginnings of a Fusor Electrical System

Posted: Fri Dec 30, 2016 9:52 pm
by Duncan Wilkie
I measured 333 between the red and white wires on my multimeter's 2000K setting and a highly varying measurement between the yellows on the 20 setting.

Re: The Beginnings of a Fusor Electrical System

Posted: Fri Dec 30, 2016 10:58 pm
by Rich Feldman
Coils with large inductance can be problematic for auto-ranging digital ohmeters.

You can always descend to fundamentals for resistance measurement. Put the unknown element and a known resistor in series with a DC voltage source, so all three elements carry the same current. The passive elements will have voltages in the same ratio as their resistances. This exercise should be child's play if you have (or want to get) enough electrical knowledge to be playing with x-ray transformers. Many transformer windings can deliver small shocks even when energized with nothing more than a 1.5-volt battery or an ohmeter.

A milestone will be when you can draw a transformer schematic symbol that matches your XRT, based on visual inspection and electrical measurements. Should be able to see 1) all connection points, including the core; 2. which sets are electrically isolated from each other; 3) DC resistances between connection points within each not-isolated set. When you get to that point, we can show you a fun way to determine the winding polarities, so you can add phasing dots to your schematic symbol.

Re: The Beginnings of a Fusor Electrical System

Posted: Sat Dec 31, 2016 2:25 am
by prestonbarrows
Rich Feldman wrote:Coils with large inductance can be problematic for auto-ranging digital ohmeters.
In case you didn't catch this Duncan, this is because modern DMMs usually measure resistance by outputting a fixed small current through the leads and measuring the resulting voltage. This assumes there is no (or negligible) reactance in the circuit. When applying the leads across a large inductance like a transformer coil (or large capacitance) the reactance will certainly not negligible. The current source from the DMM will store energy in the L or C and the voltage across the DMM will change over time. This can lead to ringing in modern autoranging DMMs if the time constant of the circuit being measured is on the order of the time constant of the autoranging feedback of the DMM.

For example, say the DMM starts with a high current at the 1k scale and sees no voltage so one second later assumes the resistance must be on a lower decade and switches to a lower current and a 100ohm scale. Now the inductor is charged up and when the current is reduced it responds with a spike in voltage which is compounded because the DMM is on the finer setting. So it thinks the resistance is higher than its range and switches back to the 1k scale etc. etc.

Rich's advice is good. You can also look into '4 probe measurement' to get some more background on the fundamentals of measuring resistances.

Re: The Beginnings of a Fusor Electrical System

Posted: Sat Dec 31, 2016 4:25 am
by Jim Stead
I'm going to offer some thoughts on what I am seeing in your pictures, but that doesn't mean it's correct. You'll still need to test this out as discussed above.

Looking at the image, I see a small coil of wire between the two yellow wires. I suspect this is an isolated power tap, maybe for a measurement or control circuit in the original application. You won't need this for a Fusor. The red wire should be the HV output. You'll want to add a ballast resistor and a voltage measuring circuit to that before leading it out of your oil filled case with high voltage wire. See the HV FAQ's for more information about that.

In your first post you included links to pictures of the terminals. I believe these to be the inputs and ground connection. It appears each coil has a center tapped primary. Red and Black being the hot legs (maybe 220/240v), with the twisted copper pair in the middle of each as the CT. I agree that the white wire you identified is a ground.

What I don't understand, due to my limited knowledge of transformers, is why the HV output would exist on only one coil. The ground wire is wrapped directly onto the top of the non-HV coil, so I don't imagine there is secondary wiring on that coil at all. I'm guessing that coil exists just to add to the core flux.

The remaining question is the pair of cutoff black wires I see laying on each coil in one of the terminal photos. They may be from another power tap, or temp sensors, etc. I can only guess because we can't see the other end. If they are going to be unused, like the yellow wires, they should be properly terminated to avoid accidental contact/conduction.

Re: The Beginnings of a Fusor Electrical System

Posted: Sat Dec 31, 2016 10:30 am
by Rex Allers
I guess I'll take a try at guessing some things about how this transformer might be designed. I've never actually had an xray transformer in front of me, but I know a little about transformers and I think I've picked up a little more about these from hanging around this forum for a few years. All of this is guess work, so hopefully Duncan can prove me right or wrong with an ohm meter and maybe some other tests later.

First, to aid the discussion, I copied the three pictures from the original links, reduced their size to not waste space here, and I'll include these here, now.
Xformer 1
Xformer 2
Xformer 3
So it looks like we have two coil sections on opposite sides of a common circular(-ish) metal core.

Another member was recently working on a different xray transformer. For that one he had a pretty good schematic of the transformer in its circuit. The transformer basically had two high-voltage secondary sections. I forget the exact numbers but let me guess that the tube was driven by 70 KV across it. This was made by the two secondaries both producing 35 KV (out of phase with each other). So the bottom end of each secondary was tied to ground and also tied to the other. So the hot HV end of each secondary was making 35 KV relative to ground but because they were out of phase, the voltage between them was the sum or 70 KV. Just to be clear, by out of phase, I mean when one HV output was at the peak of its AC wave the other output was at its minimum. Looking at the connections on this transformer I think it might be built in the same dual secondary way. I think you said it should be a 60 KV transformer. That would mean 30 KV on each secondary If my guess is right.

Let's go with the assumption that the output has two secondaries in series with the common center point at ground potential. I think picture 1 makes sense as the high voltage output. Why are there 3 connections on the left-side coil? I think any xray tube would have a filament on the cathode end. On the left side, two wires are the same color (yellow), so I would guess between these two yellow wires are a very few turns to produce a low voltage for the filament across them. They are wound on the outside (HV) of this coil so they don't have a problem with insulation from ground -- the filament voltage is floating at 30 KV relative to ground. The red wire then would be the cathode (-) 30 KV. So that (in my view) makes the white wire from the other coil the anode (+) 30 KV output.

The transformers only make AC outputs. The implied (-) cathode and (+) anode potentials are derived from the AC, either by external diodes (as I will show in my later circuit sketch) or because an xray tube with a filament is essentially a vacuum tube diode that will automatically only pass the AC potential, that is across it, in one direction.

So in picture 1, I think the red wire is one HV output from the hot end of one secondary winding. If I'm right the white wire on the other coil would be the hot end of the other secondary winding. But where are the opposite ends of each secondary that I think should be grounded? In picture 3, there are two black wires on each coil that are cut off. These are toward the top of the picture and under the plastic housing of the transformer. The picture is cut off so we can't see where they are coming from but they must be out of the coil sections. Where did they go? Who knows? There is a circular pattern in the plastic. Maybe there is a hole in each circular pattern they could have passed through; maybe they just came out through the square holes where other wires are passing.

Why were they cut? Ask the guy who cut them. My guess is that they passed through the center of each circular pattern in the plastic (which we can't see in the picture) but that was the easy way to get the transformer out of the tank that used to hold it. Why are there two wires on each coil? Although we can't see in the picture my guess is the two black wires that are cut on each coil both come from the same connection. Why are there two? Possibly redundancy safety on the grounding of the HV source.

Now, let's think about the primary side coil(s). In pictures 2 and 3, there seem to be wires coming out of the coils, through the square holes in the plastic and connected to the terminal strip. I think these are the primary connections. It looks to me that each coil section has the same number of wires coming out, and with the same colors on each coil. So I would guess each coil section has its own primary windings and these are probably in identical configurations on each coil. From a manufacturing point of view, the coils would be made the same except in the last step where one coil would just get a single HV output wire, while the other coil would get a slightly different HV output wire, plus a few separate turns to make a coil for the filament voltage.

So I suspect that these primary wires coming out of each coil section will have the same configuration. For one thing, this could allow selecting 110 V or 220V input by either connecting coils of the two primaries in either series or parallel. So how are the primaries configured? There are a couple hints. Looking at picture 3, there is a terminal strip with 8 connection points. Counting from left to right, the coil wires coming to pin 2 and pin 6 seem to actually be two wires in parallel. This probably means it is the center tap of two serially connected coils. As Bob pointed out, the blue thing at the left of the terminals is probably a thermal switch for protection if the transformer gets hot. It looks to be in series with the black primary wire out of the left-side coil and terminal 1. I would think this means the black wire is AC input to one end of a coil.

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.

Some smart ohm meter readings are needed on each of these primary side coil connections to figure out what's going on.

Here's a sketch I made of what I think the basic transformer looks like and the wires coming out of it. From the photos, I can't be sure if all the orientation is right -- maybe the way I drew the HV wires, the primary wire should be coming out the back side in my drawing. But that doesn't logically affect what is going on.
xfmr diagram.png
Xfrormer Diagram
And then I made a crude schematic of how I think this may have been connected with an xray tube.
xfrmr circuit.png
Possible original circuit
I think the general orientation might be right, but I clearly don't have the primary coils right because I have one too many terminals on each side vs. the wires out of the coils. I was just trying to get the general idea of how I think it might be configured.

Hopefully Duncan can figure out how to prove, with an ohm meter, if I am close or not. One part will be getting access to those black wires that are cut off inside the plastic case to see if they are the other end of the secondaries, as I have guessed.

Hope some of this makes sense and helps.