Reversing High Voltage Polarity

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.
bk8509a
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Reversing High Voltage Polarity

Post by bk8509a » Mon Dec 07, 2009 8:49 pm

Hey All,

I'm having problem's with my power supply's negative polarity, let me give you some background about the source.

I have recently started testing the 30KV DC power supply that I have via a high voltage divider circuit. Its fairly old (1979), but has been sitting dormant for I would say a decade or so. The power supply has a positive, transit, and negative polarity switch, adjustable input voltage which regulates the output voltage, a DC Limiter, and meters for both voltage and current.

Upon testing under the positive polarity I found both the current meters and the and the voltage meters are accurate. I've also taken the source up to around 9 thousand volts, in 700 voltage increments or so to test the linearity of the variac. If the linearity holds I'll get around 28KV from the supply, probably the full 30KV. The positive polarity and corresponding meters seem fully functional.

When I change the switch to negative polarity I get fishy behavior. My source puts out positive voltages at the same input variac voltage/output voltage ratio as if it was still on positive polarity. The Voltage meter on the device though (which only goes from 0-30000) starts trying to move into the negative direction, which it cannot do.

I have a few questions:

First, what is Transit polarity? Is it just there so I can switch from positive to negative polarity while the supply is in operation?

What is a DC Limiter, it goes from 0 to 10 on my device. Does it just limit the current? How?

Second, Any ideas of how to switch the overall polarity of the device? I do not care about having a positive polarity. I wish to just switch the device from positive to negative. It should be as easy as reversing two wires, right? Could I reverse the wires of the input voltage on the variac?

I can post pictures of the device, but sadly I cannot obtain any circuit diagrams as the machine is old. It is made by Universal Voltronics and is 30KVDC 70mA. Any help would be greatly appreciated.

-BK

Tyler Christensen
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Re: Reversing High Voltage Polarity

Post by Tyler Christensen » Mon Dec 07, 2009 8:55 pm

Pictures of the inside might be helpful to see how the multiplier(s) are setup. It sounds like the switch is only changing the polarity of the monitor and not the actual supply, so it expects negative monitor signal but gets positive so it goes the wrong way then. It might not be as easy to swap as the switch makes it seem. It's quite rare that a supply in this power range is fully switchable.

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Re: Reversing High Voltage Polarity

Post by DaveC » Tue Dec 08, 2009 12:06 am

Universal Voltronics - if memory serves me correctly, a company started by Dr.Karl Schussler...(sp?).

Tyler is pointing in the right direction, I think. You need to check out the HV transformer connections to the rectifier stack (or stacks). The front panel polarity switch probably is the reversing switch for the meters. What you need to look for, is the way the rectifier OUTPUT leads are supposed to be reversed. Is there a second HV output receptacle or terminal?

There could be some type of HV ball switch operated by a solenoid, that simply switches the connections to the output terminals. 30 kV is not too high for a fully electromechanically operated HV switch to be found.

But it might also be a manual, mechanical lead interchange that you have to do. It makes sense either way.

A couple pictures of the interior would be helpful.

My only Idea on "Transit" is that this might be where you were supposed to leave things, when moving the supply. If that's correct, it would suggest quite strongly, that there IS a fully electromechanical HV polarity switch inside. The Transit position is to keep the electrodes from banging about when the box is being moved... Might have been in a mobile test rig, once.

Dave Cooper

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Re: Reversing High Voltage Polarity

Post by Doug Coulter » Tue Dec 08, 2009 12:28 am

I'll start by seconding Tyler and Dave -- both are saying smart stuff, and we can't know enough from your description to tell.

The first thing that entered my mind about that transit position was that's where you leave the thing while one polarity discharges so you don't fry something suddenly trying to switch fully charged caps around in there....if you had the book it probably would say "wait a few minutes in that position" if that's the case.

Or a switch could be external for that, but hmmm, that's problematic indeed if the case, as this would mean that everything in there can stand to have either output lead at ground -- unlikely.

30 kv is just in the range where there ARE some polarity switchable supplies made, or were.
Just about everyone now wants you to have another multiplier stack you switch to get that now.

If it's old tech, and the stack isn't potted, you can of course switch it once manually.
Why guess when you can know? Measure!

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Re: Reversing High Voltage Polarity

Post by bk8509a » Wed Dec 09, 2009 1:08 am

Alright. I took a bunch of pictures and hopefully you can see whats going . I'll try to explain them all to you. I know...I know...Its old and dusty. If you need me to dust it off and tell you which components are which, I can.

Picture One:
This is a picture of the complete back of my power supply. The top part contains all the controls and the variac while the bottom has the transformer and multiplier (I think). If you're observant you will see that it still uses tubes, as do all the power supplies that I have.

Picture Two:
There are four knobs. I found the supply with main output line coming off the one knob. I cut off the old cord and put on the red high voltage wire that you see now. The red line is connected to my load which then is connected to ground. There is a solenoid switch that activates when the machine is on that connects the top two knobs. When the machine is turned off, the switch is in the position that you see now which pushes any remaining charge through a discharging resistor. Really handy/safe!

Picture Three:
This is the main transformer. I don't understand why there are five prongs on it. Anyone know why?

Picture Four:
This is to the right of the transformer. I have really no clue what it is, but is suspect it to be the multiplier.

Picture Five:
This is the meat of the machine. At the top you can see the discharging resistor. I have no clue what that positive negative acrylic thing is. Anyone know? Is that the full wave rectifier attachments?

Picture Six:
The front of my machine!

Picture Seven:
Close up of the front!

If anyone can give me any information that would enlighten me at all that would be great. If anyone could give me advice on how to manually switch the polarity, that would make my project 50% complete. This supply is awesome, cept for the polarity.

I know there's smart dudes on this forum, please post with any ideas you have! Educate a novice!

-BK
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John Futter
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Re: Reversing High Voltage Polarity

Post by John Futter » Wed Dec 09, 2009 1:24 am

The Plastic pos /Neg acrylic thing is the polarity switch for the rectifier. You have to change this and the front panel switch to get it to work. As you surmised earlier the front panel switch reverses the meters and probably the control sense.

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Re: Reversing High Voltage Polarity

Post by Wilfried Heil » Wed Dec 09, 2009 4:28 am

The red sticks next to the transformer are very likely selenium rectifier stacks in a full bridge configuration. The black kludge above them is a more recent silicon rectifier which replaces some or all of them. The orange parts in a Plexiglas cage on the right will be a smoothing capacitor for the output, comprised of six individual caps. The black stick across them is probably a resistor to discharge them safely.

So the transformer must produce the full 30 kV peak voltage, with a single HV output wire and the other end of the secondary winding grounded. The extra lugs on the primary will be additional taps which can be used to select other output voltages, through different transformer ratios. Otherwise they could be used for different mains voltages with the same variac. They must not touch each other as that will short out the primary. Looks a bit lenient how this was assembled. The blue wires from this transformer go to the variac and to a current regulator or limit switch.

A nice find, but potentially very deadly as it uses a large storage capacitor which can discharge in an instant. Be careful with that beast until you get a good understanding of how it all comes together. Draw up a full diagram of its circuit.

Note: you can double the output voltage by configuring the diodes and capacitor in the standard half wave doubler circuit which is used in nearly every microwave oven. It is possible that someone already did this with the two makeshift silicon rectifiers.

My guess on "DC Limit" is that it is part of a comparator which cuts off the primary current when a preset value is exceeded. There seems to be a regulator (thyratron circuit?) using two electron tubes for this purpose.

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Re: Reversing High Voltage Polarity

Post by Richard Hull » Wed Dec 09, 2009 10:24 pm

Historical note: Farnsworth's team used only Universal Voltronics supplies for their successful efforts. According to Gene Meeks, they had two smaller 60kv models, but the big boy was a 150kv monster with the business end of the works in a 55 gallon oil drum.

These were only around after 1962 when ITT took the effort seriously enough to give them some real money to buy the professional supplies.

Prior to 1962, as they farted around with the worthless electron based fusor, they used a cobbled up 40kv supply made by hand by Gene.

Just a bit of lore.

Richard Hull
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Re: Reversing High Voltage Polarity

Post by DaveC » Wed Dec 09, 2009 10:36 pm

I was about to write "John ! You've got really good eyes!"

But when I enlarged that picture, I see what he's referring to... and I'm certain that's the correct interpretation.

Those cutouts must show whichever polarity iit's set to give. Simple, and convenient.

Be careful Brian to route your power input cable outside the HV transformer enclosure. No need to HiPot your house or lab wiring.

Dave Cooper

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Re: Reversing High Voltage Polarity

Post by bk8509a » Thu Dec 10, 2009 11:25 pm

I feel like an idiot. You guys were completely correct....as was I in a sense. I knew it was just going to involve switching wires, but I didn't see this coming! Just rotate the acrylic.

I did tests today up to around 70% and it looks like the system isn't exactly as linear as I thought it was going to be. between 5-10% input voltage it goes up a lot quicker than after 30% or so. Looks like I'm going to get around 18KV DC out of this. That should be good enough for a pretty impressive demo.

Are there any changes in the plasma that occur with higher voltage(other than fusion)? It seems as the Jet and Star modes are mainly pressure related.

Also, I discovered and used the DC limiter today. It's a really nice safety feature that shuts the system off if the current goes beyond a certain amount. The only bad thing is that from what I've seen, it seems like the current through plasma gets very choppy at points and that circuits break and start pretty often. I was wondering if this would trigger my DC limiter. Its kind of odd because my milliampere reading goes all the way to 70mA+ but the limiter is at 10mA. I wonder why? I guess I'll find out in time.

I'm curious about the voltage too. The machine is titled the BAX-30-70-NRL as in 30KV 70mA but it seems like it only wants to go up to around 20KV. Theres a red line that marks this limit on the Voltage reading on the machine but there's room to go to 30KV. Do you think if I gave a higher input voltage to the transformer, say 220, I could get this think up to 30KV?

All-in-all, Looks like I have a solid power supply for fusion. Next stop, vacuum chamber which I believe I will be ordering from Sharon Vacuums.
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