Power Supply Completion

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
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Sam_Samida-Pugh
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Power Supply Completion

Post by Sam_Samida-Pugh » Fri Jul 07, 2017 1:44 am

I have acquired the majority of the parts for my Fusor power supply. I'm using a 120kV X-ray transformer controlled by a 20A variac and ballasted by a MOT primary. I have also purchased four 40kV diodes which I will put in series (2 each) to get a full wave rectified DC supply. I also hope to acquire a better housing container for the transformer soon.
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Dennis P Brown
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Re: Power Supply Completion

Post by Dennis P Brown » Fri Jul 07, 2017 11:58 am

Nice for you to get it operational and tested!

Your two diodes in series will perform only up to the RMS of their exposed voltage; ie. well under 80 kV (so, to first oder, 57 kV should be your max applied voltage for two 40 kV diodes in series (for half wave; that is two diodes in the forward direction and two for the reverse); so for the max usable measured voltage one uses 0.707 times the max rated diode voltage for the applied AC.)

You will want more diodes for a full wave system if you want higher voltages and do place them under oil (I have a full wave bridge for my x-former; that required four 40 kV diodes and my max 32 kV and that is a bit over the limit for these diodes but there is a safety factor that I use (and really shouldn't); so, if you do go full wave, your max voltage for your 40 kV diodes should be no higher than this value. Also, I have resistors for both sides of my bridge - while not really needed, I like to play it safe with my x-former.) Remember, when arcing the resistance can drop and the current could exceed the diodes ability to handle, so, do have a proper resistor in the circuit (I use a diode bridge system

You may want to get a cable rated to 120 kV at least so you can have future growth but in no event, exceed the voltage rating of the cable you do use. I've discovered that getting right up to the rating limit can lead to failure for cables that are not new (lol.)

For your design, consider a grounded case (housing with the x-former under oil (along with your diodes, resistor, and cable connection) would be a good and safe method.)

Keep up the good work and be safe.

Rex Allers
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Re: Power Supply Completion

Post by Rex Allers » Sat Jul 08, 2017 8:45 am

Sam,

If I understand what you intend to do, I think you have a reasonable plan.

These transformers should have two HV coils in series with their center connection intended to be grounded. So this makes 120 kV between the two opposite phases of the two HV coil outputs but each of them is at 60 kV relative to ground. I think you are planning to keep the center ground of the transformer and make full-wave rectified 60 kV supply. That should be more than enough for a fusor and you probably want to stay lower until you get familiar with controlling the fusor.

It makes sense to me rather than trying to get the full 120 kV which could get into over-volting parts of the transformer that were intended to be at ground potential.

So the simplified circuit would look like this:
circuit simp.gif
Simple full-wave circuit
In your implementation each of the diodes in the schematic would actually be 2 40 kV diodes in series. I think this should be fine. I assume you will have some kind of big HV divider to measure the voltage output. Just make sure that you don't turn your variac up (if it is possible) to an output voltage that exceeds the diode ratings.

I'm not sure if you need to worry about rms vs. peak voltages. I don't really know how x-ray transformer voltages are rated, but I would think that the x-ray tube cares about peak voltages so the 120 kV rating of the transformer may already be a peak rating. Anyone else know more about this?

I'm not sure why you have a MOT ballasting the primary circuit. Where did you get the idea to do this? I could see it may be good as kind of a safety series load while trying to figure out the transformer connections but I don't know why you'd need it in normal operation.

The idea of putting the transformer into a grounded metal container sounds good but I'd also focus on getting a good mechanical setup that keeps the transformer centered with some spacing around anything that may be at high voltage.

On that theme, your HV wiring in the pictures looks to be done with low voltage wires that are not well supported. I hope you do a better, safer job as you work toward a final configuration. You need to figure out how to mount your diodes safely in the oil too.

Hope you get it all sorted out. Be careful.
Rex Allers

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Rich Feldman
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Re: Power Supply Completion

Post by Rich Feldman » Sat Jul 08, 2017 1:35 pm

Here are some small additions to Rex's excellent comments and diagram.

1. As he surmised, x-ray system and transformer voltage ratings are _peak_ values, commonly stated in kVp. Been traditional for about a century. Reduces the disparity in time-averaged spectrum between rectified-AC and constant potential (DC) x-ray generators. Sort of a color temperature metric for radiographers.
kvp.jpg
kvp.jpg (4.12 KiB) Viewed 2875 times
2. If that full wave rectifier is run at rated voltage, each diode needs to withstand 120 kV. At the moments when upper winding and rectified output are at -60 kV, the lower winding is putting out +60 kV.

3. A popular place to monitor current is the connection between ground and the grounded end of secondary winding(s).
All models are wrong; some models are useful. -- George Box

Sam_Samida-Pugh
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Re: Power Supply Completion

Post by Sam_Samida-Pugh » Sun Jul 09, 2017 4:28 am

Thanks for the insight everyone.

I am currently in the process of getting a proper metal casing for the transformer, my current housing and setup was just temporary for testing and such, I will have a proper casing and hookups very soon.

As Rex showed with the diagram, I am planning on putting together a full wave rectifier. Using the four 40kV diodes and putting two in series for each string, I should be safe until I start approaching 80kV. This won't be a problem as I only plan to fire it up to 60kVp during my initial Fusor tests. However, in the future when I crank it up more I will be acquiring higher rated diodes.

I do plan on monitoring voltage levels with a divider and multimeter which I have already procured.

I decided to go down the inductive ballast route with the MOT as I don't currently have a resistor which would be suitable to be used in the secondary circuit. I was hoping that this would be a reasonable way to ballast the supply for my initial testing.

Sam.

Rex Allers
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Re: Power Supply Completion

Post by Rex Allers » Sun Jul 09, 2017 4:30 am

Oh my. I've been looking at that kind of full-wave circuit since the 60's and it never clicked that the full end-to-end voltage (120 kV in this case) is across each leg of the rectifier.

Thanks, Rich, for pointing out this important oversight in my message.
Rex Allers

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