Questions about HV rectifier

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Dennis P Brown
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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
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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.

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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 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.
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 »

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

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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 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

Post by John Futter »

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.)
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 »

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.

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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
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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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Dennis P Brown
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Re: Questions about HV rectifier

Post by Dennis P Brown »

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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Dennis P Brown
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Re: Questions about HV rectifier

Post by Dennis P Brown »

Your diagram raises a question: why are you using a full bridge? Besides requiring more diodes and adding complexity that doesn't really buy you much as Rich pointed out.
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Re: Questions about HV rectifier

Post by Tony Lai »

Thanks for your suggestion Dennis.
I prefer the four diode bridge primarily due to the fact that I have purchased a bunch of those HV diodes anyway, but I'm concerning about the insulation problem as I plan to leave the circuit in air. It might be disastrous to figure out how to insulate that in air so probably I will just use a single diode in the end.
For the grounding issue, sadly it is determined that all wall plugs in my room share the same ground, it's also unfortunate that I can't really install one of those 'ground poles' as the window situation of my room won't allow that.....
As my setup will be rather simple, is it possible to just disconnect all the ground wires from electronics except the transformer? (ground nothing else than the fusor circuit) I believe that for the pumps the grounding is rather a safety feature to stop people from being electrocuted and disconnect them seems to be fine as I will be wearing full protection anyway.
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Richard Hull
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Re: Questions about HV rectifier

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The circuit you show will do fine.

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Re: Questions about HV rectifier

Post by Dennis P Brown »

Do use the star ground as you diagramed it - please, don't allow any device to float (most vacuum pumps and electrical devices have three prong plugs so are case grounded, anyway.) Do ground all devices as shown in your diagram - this will offer the best protection.

As Ricahrd said, all is fine with your diagram I was just curious why you wanted a bridge system. If you want to use the diodes do not leave them in air - above 25 kV corona issues get serious. Under oil is the safest and easiest method. Synthetic car oil works great.

When you say you will wear full protection - that concerns me. These voltage and currents are lethal and regular electrican gloves simply are not safe (these voltages are more like sub-station levels) - also, wearing gloves makes any work more dangerous, not less. The basic requirement in building a fusor is using correct insulated wire, proper shielding (both electrical and against x-rays) and being safe around this deadly device - never use insulated tools or gloves on hot devices with these voltages. Always turn all high voltage devices off before any service is performed (I assume no caps.) You get no second chances.
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Re: Questions about HV rectifier

Post by Chris Giles »

Hi Tony,

As Dennis said, you must ground the chamber and indeed all metal components. In order to achieve this, put the current shunt resistor in the PSU, NOT in between the chamber and ground. I've shown the three different rectifier configurations. This results in the positive side of the rectifier circuit floating from ground but *not* the fusor chamber and not the transformer core. The terminal labelled "I" in the diagram is your current measurement wrt ground.

The current shunt resistor should be a low resistance value high power type - I see you have 10 Ohm in your diagram. 20W is perhaps a bit overkill but no harm in doing that! (Power = I^2 * R). You could choose one which has a tight tolerance to get an accurate reading as these are cheap parts. Provide further protection to the voltmeter using a Transient Voltage Supression diode in parallel the shunt (not shown on the diagram).

Please also bear in mind, as per Rich's earlier question, you must ground the transformer core unless you are absolutely sure this is insulated to the full secondary voltage rating.

Regards
Chris
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Re: Questions about HV rectifier

Post by Tony Lai »

Thanks for your advice.
I'm reluctant to be electrocuted so I will be wearing proper 35kv rated working glove, also I'll be using 40kv rated wires for wiring. For the grounding, put aside the roughing pump that I even planned to just rip the grounding wire from the three prone cables, I'm quite sure that 'ground' pin in the schematic of my Edwards ext70 turbo is not connected to the power supply. Oddly the ground pin of the turbo is just neglected and left by its own on the schematic, its power supply is grounded. Can I refer this to a safety feature? To stop the ground loop from damaging the equipment?
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Rich Feldman
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Re: Questions about HV rectifier

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To "Stop the ground loop from damaging equipment" sounds like a worry from someone who would be hard pressed to explain the problem.
You referred to a video that mentioned destruction of a turbo pump, but was that because of a ground loop?

I think there are two factors that matter in groundING topologies. (excluding neutral side of AC power, aka groundED conductors).

There's a safety thing: Protect against electric shock. Electricity on conductive equipment cases, introduced by a fault, is diverted to the grounding network. Normally the supply circuit breaker then trips, unless fault current is much too low to produce hazardous voltage on grounding network. Consider interruptions due to plugs being unplugged, wire connections coming loose, etc. You need to learn more before you can properly justify disconnecting the 3rd prong of any AC power plug connection, even if the associated equipment is bolted to other grounded apparatus. How 'bout test runs of, say, a backing pump when it is temporarily unbolted from the metal cart?

The other is small voltage drops from point to point, due to currents in the grounding network, or induced by EM fields.
In some cases they could be a nuisance for instrumentation, but I think unlikely to hurt anyone or damage equipment. Tesla coils might be an exception.
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Dennis P Brown
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Re: Questions about HV rectifier

Post by Dennis P Brown »

Let me clarifry a bit - ground loops affect micro-electrontics - not transformers, mechanical vacuum pumps or any moderate current system. This isn't an issue for a turbo controller or pump - the only device that might be an issue would be a sensitive amp for a detector.

The issue is - are your plugs connected to a proper ground? A simple plug in device will confirm whether your ground is valid and also the polarity. They sell these - this is just one example of many and they are rather cheap:

https://www.staples.com/Ideal-GFI-Recep ... 5oQAvD_BwE

More to the point, a direct plasma short to a turbo could harm the controller but via a common ground (if it is valid), never. Apparently I wasn't clear - waxing on the subject too much on my part.

I cannot say this enough - the use of safety gloves isn't a good idea nor something you should depend upon (a microcrack in it would allow a lethal shock.) The only safe method is to never work on a hot (i.e. energized) high voltage system. If you need gloves to operate your fusor then something is very wrong.

Chris and Rich are offering a lot of good advice.
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Bob Reite
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Re: Questions about HV rectifier

Post by Bob Reite »

Never defeat the safety ground! Make it part of your grounding system.
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 »

Thank you Rich, Dennis and Bob for reiterating the safety issue. I would definitely prioritise the safety over anything.
I know that it is simply daft to neglect the grounding of any electronics. I have never thought about the issue of killing electronics through the grounding circuit, just terrified by the fact that there is an example. I'm really not sure about the exact reason of how that member has killed his turbo as he did not shot the video specifically for this issue. I own one of those ground testers and is sure that my grounding circuit is good ----- that is being fully checked and optimised back in the days when I dived deep into those hifi speakers&amplifiers, probably I will just put this issue aside and focus more on my own safety......
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