High Voltage Multiplier Issue

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.
User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Mon Oct 16, 2017 9:38 pm

Thanks for the posts.

I think I follow most of this - I am just making sure I am not building something that will lead to issues later.

I will go with the "floating" VM stack and since the x-former's center tap and the case are both grounded, I am not worried about ground loop issues creating problems for the x-former nor issues within the stack.

Did learn a lesson about the x-former voltage being really doubled when the stack is floating (i.e. both NST terminals are driving the VM.) Also, the NST handles the load created by the VM without dropping out in voltage much - that is nice to confirm.

Burned out a diode (I hope that is all) since I use one that can only tolerate 20 kV (kind of a built in fuse.) I forgot that I don't have ten stages ...so, the diodes saw far higher voltages than I realized. Might need to consider that since more stages will, for a fixed output, mean lower voltage seen per stage.

Niels Geerits
Posts: 55
Joined: Tue Jun 13, 2017 10:25 pm
Real name: Niels Geerits

Re: High Voltage Multiplier Issue

Post by Niels Geerits » Mon Oct 16, 2017 10:39 pm

One more thing. You could get get 200kV referenced to ground. To do this you would have to float the centertap and ground the bottom connection. If floating the centertap results in a floating case this is a terrible idea of course.

The reason I mention this is because 200kV "floating" could generate a signal around your supply that messes with micro controllers and such. You can test this by holding one of those screwdrivers with a neon light inside close by. When my supply was floating the neon light would light up. I think the reason is that even an isolated supply is coupled to ground via a small capacitance. Relative to ground your floating supply is not necessarily DC (thus creating the signal that can mess with close by electronics). Just a theory based on what I have seen though.

John Futter
Posts: 1233
Joined: Thu Apr 22, 2004 2:29 am
Real name:
Contact:

Re: High Voltage Multiplier Issue

Post by John Futter » Tue Oct 17, 2017 6:18 am

Dennis
you should use a full wave CW multiplier on the one using the outer windings and centre earthed
otherwise both are similar with the supposed full wave having losses due to being half wave

thanks for the pics it made it clearer

User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Tue Oct 17, 2017 11:54 am

Thanks Neils and John for the responses (making those drawings were a pain, lol.)

Neil; again that is useful information. Since I will keep the center tapped x-former case properly earth grounded that should both reference the stack and provide a universal ground point preventing the x-former to be anything but zero voltage. Unless I upgrade my caps (I do have 40 kV ones) and diodes (would need those) I will abandon the 200 kV goal (pointless, really.) Guess I will repair this VM system and run around 140 kV (and output the stack thru a 5 G-ohm special HV resistor (very long) that will be under oil to make the output of the VM in the safe micro-amps range (under 200 uA); its current performance is far too high for what I want it for - a simple electrostatic source.)

John, since I am going to just use this VM system to accelerate deuterons to generate neutrons this only requires that the VM achieves positive 100 kV or so (which I now have significantly exceeded that value for almost pointless reasons but that is another story.) All it will be tasked with doing is "charge" a large metal globe - leakage current to the air will all it has to overcome. So my half wave VM is best since it is a far easier build.

What concerned me was my ignorance relative to VM and the grounding issue on the output stack; it almost appeared as if that was absolutely required for it to work (but mine worked far better without this grounding) and I was confused by this issue. That one could ground or not directly ground the VM stack was never discussed nor even touched upon in all the texts I had read - for me, an extremely bad oversight by writers. Knowing this is a totally non-issue for my x-former (with its properly grounded center tap) is good to know and a relief. I do see a simple x-former with a stack that is not grounded would float and create a strange voltage reference relative to any system "hooked up to it".

So, bottom-line, since my NST x-former is center tapped grounded (to the case, which in turn, I give a proper earth ground), this appears to satisfy that issue for my system very well and allows my existing VM system to double my overall voltage output for the fix number of VM units - nice.

User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Wed Oct 18, 2017 12:39 pm

For those reading this thread and thinking of building either an accelerator or a smaller ion gun but want a voltage multiplier (VM) for the high voltage, I will add a few minor build points that might be useful.

Apparently, for a fast build, using a NST will work (but is only a 60 cycle system - so a lot of stages will not be a good idea. For large number of stacks, a higher frequency will work better; however, remember some electronic components like diodes can have issues handling higher frequencies efficiently.)

Also, most NST's are center grounded removing the need to ground the output side of the VM stack (see my circuit drawings in the earlier part of this thread for both types of VM's.) This allows the full NST voltage to be exploited for charging a VM. Hoever, if one does not have a center ground (a few special NST's don't) you must ground the stack or else as pointed out in the previous post, the x-former will float at a dangerous voltage.

Of course using a full wave VM will provide more current but for an ion gun, this may not be an issue for that type of application. As for the Ground Fault Interrupter (GFI) in my NST, this has caused me zero issues for a VM.

Full synthetic motor oil is excellent for using in a VM column/tank - it is readily available, inexpensive, and has been shown (by researchers) to be equal in performance to most x-former oils in preventing electrical breakdown.

I build the column tank first and seal it very well. That is, I take the plastic tube, install/seal the x-former electrical feed-thru connectors at the 'base' of the tube. Then I connect these extra long high voltage wires to the lower connectors that are the main feed-thru's (these wires are as long as the tube and just exit the top of the tube on the opposite side of where I will seal the base plate.) Only after that do I then install and double seal the lower base plate to the tube (I hate leaking oil problems.)

So, for this method (i.e. one side of the column already sealed), to install the stack I use these extra long high voltage wires to connect the x-former input wires to the base of my VM stack (which is still outside the tube.)

I then carefully allow these wires to coil at the base of the column as I lower the VM stack into place. Then I add a high voltage output wire/connector system and install the top cover. For many, adding the base plate nearly last in order to allow the stack's lower connectors to be accessed/connected to the input terminals might be the easier method.

Again, see my pic's of a completed unit in this thread:

viewtopic.php?f=18&t=11933

and details of my VM stack using doorknob caps here.

viewtopic.php?f=12&t=11902

Remember, the diodes will see an RMS peak voltage, not necessarily the measured voltage (I forget that from time to time and kill a diode too often ... .) Door knob caps will carry a lethal charge so be careful if you uses these types of caps (a bleed resistor is a good idea.)

For those interested in a smaller (safer) build, here is the thread containing the circuits/design for a high frequency VM that that person uses to accelerate deuterium for neutrons:

viewtopic.php?f=12&t=10791&start=10

User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Sun Oct 22, 2017 2:50 am

Well, made some progresses early this day on the original voltage multiplier - I added an additional two stages to its top; these are smaller caps since I have no more door knob caps. If all goes well a real test will occur tomorrow - the dry test went well (but the first diode concerns me) and the system reached 45 kV with a 30% variac input setting (essentially identical to my first build but hopefully, less stress on the diodes with more stages.)
Last edited by Dennis P Brown on Sun Oct 22, 2017 4:59 pm, edited 2 times in total.

User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Sun Oct 22, 2017 1:28 pm

After I did a proper voltage test (reading BOTH ends of the VM output resistor) I realize I made an error on the preceding post (and carelessness on my part) - the voltage on the output of the VM for the top of the VM stack wasn't 45 kV but a dangerous 135 kV! (I am luckily I didn't cause damage to my diodes/caps since this was a dry run (no oil in the stack.)

My error in that test run was that my high voltage probe, which directly reads kV on a display scale, is based on a Giga-ohm resistor. The issue (and cause of the error) was that I had installed a 5 G-ohm resistor on the out put of the stack to reduce the current output of the VM system. I forgot that I in doing this I also create a voltage divider for my high voltage probe; this resulted in my VM meter displaying, for my thinking, an "incorrect" max voltage for that variac setting I was using. Of course the value read was correct but only for that point - on the top of the multiplier column but below that output resistor where a far higher voltage was occurring that I wasn't aware.

I bring this up for others who, like me, are not as experienced with HV systems. While the output of my stack is invariant for steady state conditions (both the base of the resistor and its top thru it have identical equilibrium voltage values) a test meter isn't such a system - the current drains and the voltage I read at the top of the resistor is no longer the normal, non-current steady valve (but the base point of that resistor doesn't drop anywhere near this amount. Forgot this important point until I did a test on both sides of the VM output resistor.)

I will need an exposed calibration test point on the stack that does not include my high giga-ohm resistor output point on the top of the stack when I calibrate the system relative to the input variac.

So, exceeding 20 kV per multiplier unit in the stack will burn out my smaller top caps; I need to be aware of this voltage drop whenever I directly measure voltage from the top of the VM output resistor.

So, with RMS and using ten stacks, my max reading on the variac should not exceed about 45% full scale - that should provide approximately 200 kV on the stack (and that would be dangerously at the limits of my weakest componets; however, leakage current off the system should make this max voltage somewhat lower.)

User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Sun Oct 22, 2017 1:55 pm

One additional point - this might be a time to ground the output side of the VM stack to cut the voltages in half; then I could more closely reach near the full scale of the variac without worry of blowing my stack. I guess I will do this and run a test and see what max voltage this creates; that is, after I fill the stack with oil - no more dry runs.

I never suspected that such a simple lower current 60 Hz neon sign x-former with a GFI could provide such high voltages as a driver for a simple half wave VM (over 200 kV!) Past experience didn't lead me to believe this was at all possible - using doorknob caps was, apparently, a waste (besides creating a very dangerous voltage source.) Hopefully, the smaller caps and the 5 Giga-ohm resistor changes that situation - I will soon see when I install an micro-amp meter (at some future date.)

User avatar
Dennis P Brown
Posts: 1424
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: High Voltage Multiplier Issue

Post by Dennis P Brown » Sun Oct 22, 2017 4:39 pm

Ran the voltage multiplier using one NST output terminal grounded, as well as one stack side and didn't get the half voltage from the stack I had at first expected - that is, 100 kV.

Rather, I obtained a quarter - a mere 50 kV with full scale from the variac; I should have realized that this is what one should expect with that specific (classic) circuit (again, my lack of experience using VM's and no recent calculation of performance/outputs using the correct ground configuration that previously, I had not properly understood - see up stream in this thread.)

The issue, in a nut shell, is that I not only reduced the x-former's applied output voltage to the stack by half - I also, if I recall the operation of VM's correctly - halved the applied frequency the stack experiences. These two factors reduced the output to a small 50 kV (this also explains my failures some years ago that caused me to discount the std calculations and my efforts in this project and caused me to abandon this approach (in hindsight, luckily I did or I would have most likely missed out on the 32 kV, 50 ma simple x-former I still have the use of ... for now. Still trying to get them to sell it to me ... but that isn't germane to to this thread .)

So, I have returned the VM's NST x-former to the full 15 kV applied and will be careful not to drive the VM stack past 45% of the variac setting - I want it to operate over 150 kV but not exceed 200 kV.

Since this will accelerate (repel) deuterons, this is a positive VM system.

While a lot of this could have been predicted by reviewing the literature, frankly, I had far more fun doing the tests (experiments.) Old hands will smile at my previous assumptions but having the system available (and not fully convinced the literature explains details as clearly as one needs to modify circuits) I must admit I prefer this method when I have a system to test - since I am using components that aren't easy to get spec's for, and since my system is a woefully low 60 Hz (again, I'd like to see that stove heater system built as a VM...), and previous calculation's I had done really didn't even closely match my earlier results, this was, for me a good methodology (now I see why in electronics course, experiment with real devices, can really clarify what the theory often leaves out.)

What I have done this thread is not, necessarily, what one should follow when building your own (lol. I have one especially valuable device for this type of work: an analog HV probe good to +50 kV.) But using this approach did starkly clarify issues relative to the system and mod's one can make and what then results - invaluable.

Besides, one thing Richard points out to some people on this forum from time to time is:

"If you think something can work, then build it and prove it."

That is, paper projects are exactly that - paper. Some years ago, I took ideas from paper (theory) and failed (through I did build some VM's using these and other components - my error, at the time, was not questioning the basic designs (the reasons for them) and then, not testing different design approaches.)

Getting back and doing real experiments by building and trying different configurations has enabled me, also with help from people here, to figure out the issues/problems relative to these extremely high voltage VM's.

The final result is that I have succeed in building a successful +200 kV power supply suitable for my deuterium accelerator.

Learning the in's and out's of these HV systems is something I will have long after I sell or disassemble this VM.

User avatar
Richard Hull
Site Admin
Posts: 10626
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: High Voltage Multiplier Issue

Post by Richard Hull » Sun Oct 22, 2017 6:40 pm

At 60 hz I fear your system will collapse under any form of real accelerator load to a rather pitiful voltage. In VM systems operated at 60hz it is all about the value of your capacitors in each stage. .1ufd is none to big for filtering. Door knobs of 2000pf will buckle. Adding more stages demand much higher capacitance. to sustain any form of accelertor or fusor load current.

However, for electrostatic experiments a VM system need not have large caps to get the true VM multiplication factor provided the load is in the hundred microamp range. at 100kv just bringing the leads out to air will have such huge corona loads, (50-100ua), that a voltage multiplier with door knobs will start to buckle below the stage multiplication expected.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Post Reply