Questions about HV rectifier

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Tony Lai
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Questions about HV rectifier

Post by Tony Lai »

Hi, there
I have noticed that the majority members of the community utilise the rectifier design of two diodes facing away from each other. However I have seen some people using a design of conventional full bridge rectifier, wiring the positive output to the ground. I'm a bit confused by the difference between those two designs, for my simplish mind they both accomplish the same job of a 'full wave rectifier' but I'm unsured. Is there any performance difference between those two designs?
Also, I have seen sources states that voltage drops drastically after the rectifying stage. I'm a bit shocked, shouldn't it just output pulses of voltages with a magnitude equals to the input voltage?
Thanks for any help
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Dennis P Brown
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Re: Questions about HV rectifier

Post by Dennis P Brown »

The four diode bridge when combined with a proper cap offers a smooth DC supply from an AC source. For a fusor, this issue is really not a significant factor and so most people use the two diode arrangement - cheaper and easier to build. Yes, the voltage drops (no cap) periodically (we all use AC sources) but this is just an non-issue. Fusors are more about handling a very touchy plasma and not about a smooth, continous voltage like many devices normally require.
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Chris Giles
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Re: Questions about HV rectifier

Post by Chris Giles »

That is full-wave vs half-wave, I'm sure you can find decent references explaining it on the web.

Note that in a basic HV PSU (rather than a commercial unit with a 'multiplier'), it is not outputting DC but rectified AC (no smoothing cap required). These differences matter for correctly estimating how much power is supplied to the fusor. However, it is more important when considering the insulation rating of the transformer secondary. In the half-wave design, one side or a centre-tap is grounded and the other side has the maximum HV potential between it and ground (i.e. the transformer core). In the full-wave, both ends of the secondary see the full potential vs ground and need to have insulation rated for that.
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Rich Feldman
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Re: Questions about HV rectifier

Post by Rich Feldman »

Tony asked about the difference between full wave rectifier with two diodes, and bridge rectifier with four diodes. In both cases the output has 120 pulses per second when AC power frequency is 60 Hz.

Sometimes the latter is called full wave bridge, but you don't need 4 diodes to have a full wave rectifier. Half wave rectifier has just one diode, and delivers only 60 pulses per second. Adding to the confusion, the advent of high frequency switch-mode power conversion has made "half-bridges" a thing, I think not applicable to mains-frequency rectification.

The most important and unavoidable difference is a factor of two in output voltage!
Right behind that is what Dennis and Chris mentioned: lack of high voltage insulation between the midpoint of HV winding, the core, and the primary winding. In transformers not designed for use with bridge rectification.

Let's consider a NST (or XRT) whose secondary winding center tap is internally connected to core and case. Two symmetrical HV terminals alternately reach +10 and -10 kV, with opposite phases, with respect to the core.

Case 1: Two diodes, and center tap connected to ground. Rectified output reaches -10 kV at the peak of each half cycle. At the same instant, the other HV terminal is at +10 kV, so the nonconducting diode has 20 kV across it. One half-secondary has no current, and the other half-secondary carries the output current at that instant.

Case 2: Four diodes in bridge configuration, with "DC+" terminal grounded. Secondary coil's center tap (and thus core and case) must be floating.
At the peak of each half-cycle, the rectifier's "DC-" terminal is at -20kV and the center tap is at -10 kV. Both half-secondary windings carry the output current, and their voltages add. Both nonconducting diodes are blocking 20 kV. The NST case is at a high voltage, enough to jump across a small air gap to bite you, and there's abnormal electrical stress on the dielectric between primary circuit and the core and case.
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Chris Giles
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Re: Questions about HV rectifier

Post by Chris Giles »

Much better explanation Rich :)

Tony, do you have a transformer already?
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Richard Hull
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Re: Questions about HV rectifier

Post by Richard Hull »

The bottom line is no one uses a 4 diode bridge on virtually any HV fusor supply. This is true for any and all who use any HV transformer with the secondary mid-point grounded. (Virtually all fusor mains transformers are midpoint grounded, thus, full wave rectification is a 2 diode only arrangement). Newbies tend not to read the FAQs which show this. Most all newbies use old iron core neon sign transformers in demo devices. Those sticking with mains, iron core transformers to do real fusion use very old and rare x-ray transformers that are all midpoint grounded.

All others who do fusion must go to a switching supply and voltage multiplication. This is a far different animal with new and more complex requirements unless purchased very much ready to run.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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
Tony Lai
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Re: Questions about HV rectifier

Post by Tony Lai »

Chris Giles wrote: Tue Sep 01, 2020 3:51 pm Much better explanation Rich :)

Tony, do you have a transformer already?
I do have placed my order on a 30kv transformer several days ago. I ended up just calling up one of those industrial suppliers that produce transformers and ordered one conventional transformer that is really without any features like center tap etc.... Just a transformer with one primary winding and one secondary winding (I believe this is the kind of transformer they put in those small iron crates on wire poles), good thing is that it comes with benefit of being epoxy insulated, so no concern to get buckets of mineral oil.
It kind worrys me bit, but I believe that the vaildity of both rectifier designs shouldn't be affacted by the fact that this transformer isn't center tapped? I plan to ground the fusor casing directly to wall plug ground, would that be safe and doable? Thanks
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Rich Feldman
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Re: Questions about HV rectifier

Post by Rich Feldman »

Tony, it sounds like you might need to learn more about electricity. Seriously, it's been a long time since we had a fusioneer who skipped the learning segment that contains your answer.

Given a transformer whose HV winding has only two terminals, do the rectifier configurations in your original post both work?
Please don't look for the answer by asking on a forum.
Draw a complete schematic diagram of each power supply configuration. Show the sign and direction of voltages and currents at one peak of the AC cycle. Write back if you are still having trouble at that point.
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Richard Hull
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Re: Questions about HV rectifier

Post by Richard Hull »

You must have a gang of 2020 money! Did you specify a secondary current capability? Very, very important. Is it a solid iron core or shunted? Did they wisely question why you demanded a single 30kv secondary? It will make them go nuts on the core and insulation requirements!! Did they state the weight or volume? Sounds suspect, stupid, and smells horribly fishy to me. You give no real data just blather on about stuff that I am intimately familiar with, as are a number of other electronics engineers and technicians here who are in the know.

28 years ago I ordered a custom, non-center-tapped, purely copper wound, 220 volt primary and a secondary of 20kv from a pole transformer manufacturer that would run DRY!!!! I told them, as I should, that it would have to supply 1 amp short term (2-3 minutes max) and 500ma continuous for less than 20 minutes with no more than a typical power duty cycle of .003% over any 24 hour period! Yes, they freaked!! Mostly on the dry requirement and the near zero operational period and duty cycle. They said that they would wind it on 20kva iron to allow for the special insulation required. They charged me 1,550, 1993 dollars, which I gladly paid for the 205 pound naked core which they wisely heavily hot lacquered. Note: the company was local and designed, restored and rewound pole and custom industrial transformers as their only business.

If you did not go armed with or were not queried related to all these specs., they are phonies and can't be even thought of as a moderately competent transformer company. Note: a neon transformer company might undertake this, but the costs again would be tremendous and they would demand a shunted core which would be limited to lowered voltages at rated currents. It is how they do business.

Please, for all in attendance to this thread, give careful and complete details related to the above claim related to your transformer on order. I am sure you must have them at hand. Inquiring minds want to know. (current output, size, weight, cost)

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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
Tony Lai
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Re: Questions about HV rectifier

Post by Tony Lai »

Richard Hull wrote: Thu Sep 03, 2020 2:23 am You must have a gang of 2020 money! Did you specify a secondary current capability? Very, very important. Is it a solid iron core or shunted? Did they wisely question why you demanded a single 30kv secondary? It will make them go nuts on the core and insulation requirements!! Did they state the weight or volume? Sounds suspect, stupid, and smells horribly fishy to me. You give no real data just blather on about stuff that I am intimately familiar with, as are a number of other electronics engineers and technicians here who are in the know.

28 years ago I ordered a custom, non-center-tapped, purely copper wound, 220 volt primary and a secondary of 20kv from a pole transformer manufacturer that would run DRY!!!! I told them, as I should, that it would have to supply 1 amp short term (2-3 minutes max) and 500ma continuous for less than 20 minutes with no more than a typical power duty cycle of .003% over any 24 hour period! Yes, they freaked!! Mostly on the dry requirement and the near zero operational period and duty cycle. They said that they would wind it on 20kva iron to allow for the special insulation required. They charged me 1,550, 1993 dollars, which I gladly paid for the 205 pound naked core which they wisely heavily hot lacquered. Note: the company was local and designed, restored and rewound pole and custom industrial transformers as their only business.

If you did not go armed with or were not queried related to all these specs., they are phonies and can't be even thought of as a moderately competent transformer company. Note: a neon transformer company might undertake this, but the costs again would be tremendous and they would demand a shunted core which would be limited to lowered voltages at rated currents. It is how they do business.

Please, for all in attendance to this thread, give careful and complete details related to the above claim related to your transformer on order. I am sure you must have them at hand. Inquiring minds want to know. (current output, size, weight, cost)

Richard Hull
If I wasn't wrong, the maximum current through the system will be 0.6a when a ballast of 50k ohm is included. The transformer won't really have a issue with that current for short term. I honestly stupidly thinks that the current surge won't really be that horendous if variac is controled slowly and vigilantly, but it is clear that I should conduct more studies before ever start this project.
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Re: Questions about HV rectifier

Post by Dennis P Brown »

A very minor point - please don't re-quote people - especially more than just a part of a single sentence. It eats up operation capacity for the site.

As for controlling the current in a fusor during a universally common short through the plasma, no variac controlled by a human can succeed in preventing a huge current surge; the human delay factor alone will allow a huge current flow for a not insignificant time. If the system can't handle that then the smoke produced by the transformer will demonstrate that fact all too clearly.

Understanding the (not) simple configuration of a single transformer driven circuit does require some electrical reading and study. Add diodes, and ballast resistor all feeding into a very conductive gas and then the situation gets complex fast and major failures can occur. The FAQ's are good but are not meant to be all inclusive - with wiki and other online sources there are really great places to study these topics and you should if you want to order a real transformer. As Richard pointed out, there are many factors that must be considered or the said device will likely fail in its application.
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Rich Feldman
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Re: Questions about HV rectifier

Post by Rich Feldman »

>>It kind worrys me bit, but I believe that the vaildity of both rectifier designs shouldn't be affacted by the fact that this transformer isn't center tapped?
>>I plan to ground the fusor casing directly to wall plug ground, would that be safe and doable? Thanks

No risk of too much current if you use only two diodes, each with its cathode connected to one end of your transformer's non-tapped HV winding.
Then the maximum absolute current will be zero, unless you exceed a diode breakdown voltage.
Please draw a schematic, in order to confirm or refute this free advice from someone on the Internet.

How many transformer rectifier circuits have you made with _any_ voltage and current capacity? I don't think a custom transformer is a good starter project. On the other hand, many demo fusors have been made with minimal electrical experience and knowledge.
All models are wrong; some models are useful. -- George Box
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Richard Hull
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Re: Questions about HV rectifier

Post by Richard Hull »

Dennis and others are correct. There is never a gentle bring up in any fusor, be it a demo of the real thing. It is totally off as you inch the variac up and then.....BOOM!!!..... it is on and a short circuit through the gas! The proper calculation of the ballast acts to give the sluggish human time to back off the variac without total power supply destruction, keeping what would be a short to dump the energy into the ballast, attempting to burn it out momentarily. You will see when you are deep in the "doing".

I hope you let the transformer folks know you would be grounding one leg of the transformer secondary even if it is through a bridge, that way they can put a red mark on the secondary "start winding" lead nearest to the core. Safety First! (old railroad motto)

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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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Re: Questions about HV rectifier

Post by John Futter »

Tony please do not repeat what others have written in your replies
we can all read what has transpired before your post
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Richard Hull
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Re: Questions about HV rectifier

Post by Richard Hull »

John and Dennis reiterate what I have said for years. Everyone here is learned enough and competent enough to have followed the thread through its entirety. No need to recap.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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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Re: Questions about HV rectifier

Post by Bob Reite »

What one really needs for a fusor is a supply that will current limit when what was an open circuit becomes nearly a dead short when the gas finally ionizes and you get a plasma. The quick and dirty way is a series ballast resistor. My home brew supply did not use a ballast resistor in the output. The load sharing resistors on my amplifiers also serve as current limiting.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Questions about HV rectifier

Post by Tony Lai »

I really appreciate all of your valueble opinions.
Just done a double check with the transformer supplier that the transformer itself will be fine even if the consumer tries to draw those 30kv arcs across the secondary winding, now with that in mind I sort of have the confidence that HV winding won't be fried up that easily with a ballast in place.
I have just finished a flimsy schematic for the circuit. The current measuring component has really put me off, I even starts to think about just measuring the voltage across the ballast resistor with a HV meter to calculate for the current. Given that the transformer have nothing else than two input terminals and two HV terminals, and all grounding points in the schematic will be gathered to make a ‘STAR’ that grounds directly to a wall plug, where exactly should I place the current metering block?
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Rich Feldman
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Re: Questions about HV rectifier

Post by Rich Feldman »

Yay for schematic diagram.
Is the transformer core electrically inaccessible, insulated by epoxy from the transformer's mechanical attachment details?

Before we discuss current metering,
please show (e.g. with a highlighter pen) one complete, closed loop for current in the HV circuit, at an arbitrary peak of the AC cycle.
The loop is expected to include parts like wire, diode, and resistor, but no arcs except inside the fusor block.
You could start at the fusor case, through plasma to fusor HV terminal, then follow the current upstream all the way back to ground.
Remember that voltage sources and current sources are two-terminal elements.
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Richard Hull
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Re: Questions about HV rectifier

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Great Rich! There is a big OOPs here I am sure folks like Bob, Jon and others here caught it.

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Fusion is the energy of the future....and it always will be
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Re: Questions about HV rectifier

Post by Tony Lai »

Now I can really see where the issue is, my insufficient electrical knowledge traumatised me so much.
The core of the transformer is insulated and there is no point for connection, just two HV leads. It just feels so wrong that the circuit draws current from the ground although it seems fine to me at first to 'emit' electron towards the ground.....
I can see that most transformers have a 'center tap' and the cuicuit can be completed through that point, but it really overthrowns me to establish a correct curcuit in my case.....
I'm thinking about probably ground one of the HV leads together with all the grounding points and utilise only one HV lead with one diode for negative HV supply. That to me seems more correct however it now really becomes a 'half wave rectified' situation where half of the energy is wasted...... Or should I really use those 4 diode bridge and connect the positive output ground?
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Re: Questions about HV rectifier

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Tony
Excellent you are learning!!
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Rich Feldman
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Re: Questions about HV rectifier

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>>I'm thinking about probably ground one of the HV leads together with all the grounding points and utilise only one HV lead with one diode for negative HV supply.
>>That to me seems more correct however it now really becomes a 'half wave rectified' situation where half of the energy is wasted......

It's not that bad, Tony.
In your schematic you drew a series pair of diodes "facing away from each other", i.e. with their anodes tied together.
Draw another pair, right beside the first pair, but "facing toward each other" (cathodes connected in the middle).
The outside connections of D3 and D4 join the outside connections of D1 and D2, connected to the transformer secondary terminals.
Finally, draw a connection from the inside (common) connection of D3 and D4 to ground.

Now you can highlight a closed path for the HV current through fusor, at one peak of AC cycle, and a slightly different path for the opposite peak.
Got it?

By the way, half wave rectified situation doesn't waste half of any energy.
Most mains-frequency x-ray generators have half wave rectified HV circuits. Saves lots of extra-high-voltage diodes.
It ought to work just fine for a fusor. Secondary current rating would apply to RMS value of resulting non-sinusoidal current. Voltage rating might be compromised because DC current biases core toward saturation in one half-cycle. (There is a standard remedy for that in half-wave x-ray generators.)
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Richard Hull
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Re: Questions about HV rectifier

Post by Richard Hull »

One reason I suggested that you ask the company where the start winding was is so that if you chose the half wave single diode route, you should best ground that end of the secondary. Of course, if the transformer company really went all out with the secondary insulation, which I am sure they did, the insulation would be good enough so that either HV lead could be grounded for half wave rectification.

The bridge is the real reason you want a non-center-tapped, well insulated HV transformer. Rich told it the way it should be. You have two choices.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Questions about HV rectifier

Post by Tony Lai »

Thanks so much for help.
I have finally finalised one viable schematic, and now I really have a understanding about my initial question, that is the difference between a 'two diode' and a 'four diode' rectifier.
I was initially planning to connect all my electronics (vacuum pumps, controller, transformer) to one power stripe (eventually to the same wall plug). However, yesterday I happened to watch a youtube video from one of the neutron club members, who unfortunately fries his turbo pump up by grounding it together with his xray transformer. In his video he suggests that the transformer might have arced through the secondary of the turbo, therefore shorting the circuit and destroying the pump. That truly terrifies me as obviously I don't want to risk frying up my turbo pump that costs a fortune. Should the electronics and the transformer be connected to different wall sockets? It seems odd to me as those wires eventually come together anyway.
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Re: Questions about HV rectifier

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As to your question, there is a long and a short answer. First the long answer:

If you don't know what circuit each outlet uses in a room relative to the breaker box (there are two independent power circuits of 120 AC in all breaker boxes; thats how you get 220 v in a home), using two seperate outlets in a room does not necessarily isolate the two grounding points.

I overcome this issue by using two seperate circuits for my ground points - each goes seperately to the breaker box. However, do realize that most breaker box circuits then use the same commonn ground point (the master return wire which connects everyone to the system - talk about a possible noise source); hence, one isn't necessarily fully isolated/independent via that method alone. To overcome this problem I used the fact that I have a well for water. This extra master ground point connects my breaker box circuits to the ground system provided by the well water. This can be done for city water if the inlet pipe is metal but that is an electrian's job (mine, wisely, did this on their own without me even asking - definitely they were both experienced and thinking.) Whether this is done routinely in most houses I do not know.

Some here simply run a ground wire to an external grounding rod but that requires hitting the water table with the rod besides running a wire through a wall/window. The whole issue of "ground loop's" and dealing with them is an annoying issue (Ham radio people have chapters devoted to this issue and on finding real independent grounds.) That is why most people accept and ignore this entire problem and for good reason.

Another thing I do is provide my turbo with a wire grid cover over its inlet port (some turbo's come with these - I got one on ebay); I did this primarily to protect my turbo blades from anything that might fall into them but also to aid in electrical screening (assuming the case is properly grounded.) Using a right-angle bend is another methodolgy that works (the connecting pipe must be grounded - o-rings can isolate a connector to some extent.)

So that is the long answer to a simple question and one that you have to solve via the best method and amount of effort/cost you finally decide to put into it. There is, like many fusor issues, an easy way and difficult way to solve the problem depending on what you decide.

As to the short answer what you can do is determine if the room's outlets are common. If the room does uses two seperate circuits then that could be exploited - determining that requires reading the tags on the breakers in the box and then determining if the room is serviced by the two seperate ciruits (really phases but that is another issue) and if the wall outlets are as well. Not really too difficult to determine (likely need to turn breakers on and off to really determine) but using the two seperate circuits helps more on load issues rather then on a better ground (as mentioned previously). So not really all that useful unless the breaker box uses another external ground point besides the power company's common ground wire ... . I think I looped back on the whole point here.
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