High Voltage Multiplier Issue

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Dennis P Brown
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High Voltage Multiplier Issue

Post by Dennis P Brown »

Decided to float my High Voltage Multiplier (VM) just to see what that might do and I was extremely shocked by the results (fortuitously, just the expression.)

In my previous post, I had the NST with one side grounded (including the VM's hot side output in the stack) which resulted in the system yielding + 100 kV or so at 100% full load on the variac.

With the ground of the VM and NST removed I got a full 50 kV on my HV meter and the variac was just at 20% load.

That isn't at all what I expected - I am a bit confused. That implies that the stack can reach close to + 250 kV at full load. The VM stack itself is still a half wave VM so while the frequency is now doubled(?) that shouldn't give me more than double voltage (for that mater, I wouldn't even expect a true doubling either since impedance issues should lower the voltage multiplication factor further, I would expect.)

I am confused and would appreciate any clarification on this aspect of voltage multipliers.
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Re: High Voltage Multiplier Issue

Post by John Futter »

A quick circuit of what you are doing /have done would be useful
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

Here is the basic circuit - mine has eight stages and uses a center tap grounded x-former.
JEPEGHalfwave.jpg
JEPEGHalfwave.jpg (11.23 KiB) Viewed 6986 times
With the ground shown as in this circuit, I get about 100 kV; with that ground removed (but the center tap still grounded) I get 180+ kV
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Re: High Voltage Multiplier Issue

Post by John Futter »

Dennis
please draw both options showing the transformer winding connections
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Re: High Voltage Multiplier Issue

Post by Nnnnnnn »

Dennis, from the drawing and your short explanation I think you are getting exactly what I would expect.
If I understand correctly this is your circuit:
VM.jpg
In the above case the ground and therefore the centertap is connected to D1 and C2 (your drawing). The centertap is our 0 (reference), while your top winding oscillates from v_pp to -v_pp relative to this. The lower winding is exactly pi out of phase with respect to the upper winding. Therefore relative to ground the upper winding oscillates from -v_pp to v_pp. You can easily see that the voltage between the top and bottom windings oscillates between 2*v_pp to -2*v_pp.

In the first case you are doubling/tripling etc. the voltage of the top winding relative to ground or your centertap.

In the second case you are doubling/tripling etc. the voltage of the top winding relative to the bottom winding, which is two times more than the voltage relative to ground. Your measurement results seem to confirm this.

That or I got your explanation wrong.
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Re: High Voltage Multiplier Issue

Post by Rich Feldman »

This was all composed before I saw John's and Niels's posts, but I'm going to submit it without more editing.

Dennis, there's still plenty of ambiguity, IMHO.
Let's guess that the upper side of voltage source in your schematic is one hot terminal of the NST.
Is the lower side of voltage source the NST ground or the NST other hot terminal?
Either way, that leaves one NST terminal whose connection needs to be stated for the before and after cases.

And how many diodes and capacitors constitute one stage?

You might find it easier to draw your actual complete circuit than to find an accurate representation online, or explain it adequately with words.

For nerd-to-nerd communication, plain text can present the connectivity easily & unambiguously, e.g.
"sort of like SPICE" style:
VI node1 node0
C1 node1 node2
C3 node2 node4
C2 node0 node3
C4 node3 node5
D1 node0 node2
D2 node2 node3
D3 node3 node4
D4 node4 node5
Rvoltmeter node5 node0

or "netlist style"
node0 VI.n D1.a C2.1 Rvoltmeter.n
node1 VI.p C1.1
node2 C1.2 D1.k D2.a C3.1
node3 C2.2 D2.k D3.a C4.1
node4 C3.2 D3.k D4.a
node5 C4.2 D4.k Rvoltmeter.p
All models are wrong; some models are useful. -- George Box
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Dennis P Brown
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

Here the stack is floating but the NST center tap is grounded.
NST1.pdf
(10.49 KiB) Downloaded 419 times

Here is the VM with the stack grounded; also, the NST is center tap grounded
NST2.pdf
(10.95 KiB) Downloaded 437 times
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Re: High Voltage Multiplier Issue

Post by Nnnnnnn »

Dennis, if the grounds in NST2 are the same (e.g. earth ground), then it is what I said above. To me it does look like that is what you are doing. In that case you just shorted the lower half of the transformer together (lower winding and centertap @ same potential), so it is the same as just connecting your VM to the centertap and the upper winding.
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

Not sure I follow your last post; in my one drawing my NST x-former center tap is grounded and the voltage multiplier stack is also grounded - of course, these are common grounds. However, in my other, the Voltage multiplier stack is floating while the center tap is still grounded. The ground wire from the center tap does NOT connect to the VM stack. I just can't easily show that so I use standard circuit drawing crossing two wires without a dot so they are shown not to be connected - hope that clarify's that issue in my drawing.

In one of your drawings, the diagram shows the NST x-former center tap connected to the VM stack and one NST terminal floating - a very different situation, I think. One I have not done at all.

For me, the issue isn't so much the double voltage (the NST's output is 15 kV when both terminals are used since this, I believe , references the two terminals to the center tap ground. Which the VM then see's.)

But what bothers me is why are all VM's I see in books always designed so as to have the output stack of the VM grounded? Mine works far better with the stack floating. Is it because the NST is center tapped? I am rather not clear on the two methods nor why one is preferred.
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Re: High Voltage Multiplier Issue

Post by Nnnnnnn »

Dennis, yep that is what I thought. The drawing where I left the lower terminal floating is different from your drawing where you ground both the center tap and the lower terminal however it does generate the same output. In that case your lower terminal is connected to ground via the VM. Your centertap is also connected to ground. So the centertap and lower terminal are in fact connected through the ground. Which in turn means that effectively your VM is connected to the centertap (like in my drawing except that the lower terminal would also be grounded). So I know you didn't directly connect the centertap to the VM but this is what effectively has happened due to the common grounds.

To answer your question about VMs in textbooks: a VM output does not have to be referenced to ground. I have found that text books often use the ground symbol to symbolize a reference point not necessarily earth ground. When you "float" your VM (disconnect the lower termibal from ground) you are referencing the output to the lower terminal of your transformer instead of earth ground.

My HV supply uses a transformer and a 4 stage VM. At first it was isolated. That is to say it was not referenced to ground. The VM output was simply referenced to one of the transformers terminals.
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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.
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Re: High Voltage Multiplier Issue

Post by Nnnnnnn »

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.
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Re: High Voltage Multiplier Issue

Post by John Futter »

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
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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.
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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 12:59 pm, edited 2 times in total.
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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.)
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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.)
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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

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.
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Re: High Voltage Multiplier Issue

Post by Richard Hull »

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.

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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

Yes, in all likelihood you are correct relative to the 60 Hz, which is bad news for my final goal; still, will try and run a current measurement using the stainless steel accelerator globe as my current sink - likely, it appears, my VM will not to be enough. Still, nice creating the high voltage - oh, well.
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Re: High Voltage Multiplier Issue

Post by Rex Allers »

Dennis,

Not trying to sound too negative, but sometimes your posts, especially electronics-related, leave me scratching my head.

I haven't followed all the details in this thread, maybe there was a more detail later, but I wonder if your NST2.pdf diagram is accurate? The two ground points would short out half of the transformer secondary. That can't be good.

But I'm writing mainly to discuss details of your HV measurement method.

In one post you wrote,
"... my high voltage probe, which directly reads kV on a display scale, is based on a Giga-ohm resistor."

Did you mean one resistor?

I think you wanted to have a supply in the 100 KV output vicinity. Most HV measurement resistive dividers I have seen for voltages above 10 KV use multiple resistors in series with each resistor being of a 100 - 300 Mohm value (all the same value but different designs could choose a particular value somewhere in that range).

I looked at several HV resistors I have around here. The highest voltage rating for one resistor was 30 KV and that resistor was about 3 inches long (7.6 cm). Comparing specs for a lot of HV rated resistors it seems that a reasonable rule of thumb for length vs. voltage rating is 1 inch per 10 KV. I've never seen any single resistor that was rated for 100 KV.

Some years ago I bought (on ebay) a HV divider board that was a part from some custom supply design that was making 100 KV or maybe a little more. This divider is a circuit board that has 10 200Mohm, 20KV rated, resistors in series. Each resistor is about 2" long. They are arranged in a slightly zig-zag pattern to make the board a little shorter than straight line but the board is still about 20 inches long.

So the 10 20KV-rated resistors would give the board a max 200 KV rating. I think in its original application it was immersed in oil too. I have used it several times in air to measure supplies up to around 60 KV.

This divider also has 500pF 20KV-rated capacitors in parallel with each resistor. I have seen this same general design in the voltage sensing dividers in Glassman supplies. That is, low-hundred Mohm resistors connected in series with HV caps in parallel with each.

I looked for a reference describing why dividers have the caps in parallel with each resistor but didn't find anything to quote. I'd guess it is to improve the sensing response to fast transients on the HV.

So, I wonder if your divider really has a practical voltage rating suitable for measuring around 100 KV?

I don't think I heard any discussion of issues I would expect at these voltages like corona discharge, either.


A thought on checking accuracy of HV divider:
-------------------------------------------
If you have a good DVM that you trust, most will measure up to 600 or 1000 V. They will usually also measure down to millivolts, maybe less.

If you can get a supply that will make a voltage in the 500 to 1 KV range you can use that to check your divider. I'll skip details but maybe a 12V out mains transformer, reversed and fed by a variac and then a diode and capacitor to make it DC. A nice commercial supply would be best but the preceding might work if you only put tiny load on it and don't go above 1KV out (500 V would be safer).

Use the meter on its high scale to set a nice high input voltage, say 500, 600, 900, 1000, depending on the upper range. Then measure the output voltage of the divider on the lowest appropriate meter range.

If you can live with a divide ratio of 10000 or less (that would be 10 V out for 100 KV measured) then if you put 500 V across the divider, you should see 50 mV out.

I hope that idea makes sense. In my experience, just doing the math for the resistance on the low (output) end of the divider might be quite a bit off from actual measurements.
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Re: High Voltage Multiplier Issue

Post by Richard Hull »

Assembling a good, reliable HV divider relies on experience with such voltages. One must come to realize that corona is a real issue above 40kv. At 100kv + Under oil, one needs to be concerned with the oil, if it is not extremely pure, can become part of a parallel divider network. measuring above 150kv will usually go back to air in a potted ladder ring divider with large field control torroids acting as equi-potential rings.

Simple bread boarding of chained resistors will be most dubious at 75kv without a calibrated reference to check one's work against allowing him to fine tune the effort.

We like to think of a high voltage supply as an electro-dynamic instrument but at the highest of voltages, accurate measurement of voltages and even currents become a battle with electrostatics.

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Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

Rex, my high voltage probe is a commercial unit; someone or a group with high voltage experience/knowledge designed/built these to operate accurately if one is careful in their use - more on that later in this post.

This HV probe is and must be a simple divider system (since it is analog); all I am referring to in the unit relative to resistors is the first (primary) resistor which is (as I recall) a 1.00 Giga-ohm monster (has a 1% accuracy.) It is a bit over seven inches long and a single piece - special made for the probe, I assume. I never paid the slightest attention to its built in meter (which, I assume, provided the remaining resistance that defines the system - I also assume the meter is a micro-amp meter that is calibrated so its read-off scale can be marked in "voltages", which is not what the meter is really reading, of course, but the current/voltage relationship does directly follow.) This unit only reads up to 50 kV and sorry if my posts didn't make that clear.

The reason I have discussed 100 kV (which I never directly measured) at all is I am extrapolating the voltage multiplier's (potential) performance once I get full power off the variac - that is, if the variac is providing a power usage of 50% of its rating and my voltage multiplier is reading 50 kV (and has show extremely linear response to that point) I assume the max the VM will reach when the variac is dialed up to 100% of its rated output is about 100 kV (not really true but should be close.) All this is done/measured completely UNDER oil - otherwise, corona issues (as Richard points out) would be extremely serious (leading to very false readings.) All values above 50 kV are extrapolations from the variac's scale. Real world measurements could be wildly off above my 50 kV readings once I get over this limit but if I am under oil, I am reasonably certain my extrapolations are not too badly off. I am not building a manned aircraft here so feel this is ok to assume.

As for possible shorting in the diagrams I posted relative to the transformers and voltage multiplier circuits - these circuit drawings are taken directly from the design of professional EE people. I do not claim to have knowledge of EE so can't defend those drawings (they are not mine.) From a logic stand-point, I too was EXTREMELY confused by those designs (relative to ground location and circuit performance) but I admit, after working with them and getting explanations, I feel they do the job but one must be careful about application - also, wire resistance/induction with a x-former core when an applied AC (60 Hz) excitation is used does not behave as a simple short as one might expect with these rather simplistic diagrams (for example: a NST 's high voltage secondary is, in fact, center grounded to the case and it works fine (but one gets out-of- phase +/- 7.5 kV, if I recall correctly); if one where to apply DC excitation to one of these circuits, then yes, short circuit big time in the x-formers to ground.)

For me, those ground points drove me to distraction since they had a huge effect upon final voltage performance of the VM and I had trouble understanding this effect.

I feel I do now, mostly, understand these circuits a good bit better (relative to grounding; the basic VM circuit is not at issue for me) but discovered that performance issues relative to where grounds are located for given transformer type is an issue one has to be very careful about - hence, my extensive posts here for others so they don't have to do what I've done. That is why I did these experiments and posted so extensively - I have seen that the literature is sorely lacking in addressing these very issues - that is, explaining why a given ground is located where it is and what this causes for the VM circuit is not explicit in most treatments on VM stacks.

I apologize for my own initial confusion relative to these issues since I, too, was "scratching my head" wondering why these circuits worked (i.e. gave a specific voltage value) for a given x-former/ground configuration. This made my total thread very confusing, I think; again, sorry but this is a place to learn and ask questions so someone like me, can figure out the why's of a given experiment. In summation, I decided to plow through various iterations, post these results with questions and then get to the "bottom" of the problem. Not necessarily how this should have been done but what occurred.

Aside: digital voltmeters are not something I have any knowledge of and as a consequence, can't comment upon. How caps/resistors and so forth are used to get accurate results is, for me, a black box (lol.) Analog meters (using simple voltage dividers) are easy to understand and I often make my own (for custom voltage ranges) besides using the high end one I have.
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Dennis P Brown
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Real name: Dennis Brown

Re: High Voltage Multiplier Issue

Post by Dennis P Brown »

Back to my original goal (exciter for the deuteron gun.) In that retrospect, John Futter is likely correct and I should (have) build (built) a full wave voltage multiplier to start.

In any case, I will convert my current ten stage (mixed caps) half-wave system back into the original seven stages (all door knob caps. Gives me the max current for my acceptable voltage.) Then measure the actual current at max voltage and see what this yields. If marginally within the range required, a full wave VM would be the next step since this will double the current making my application more likely to succeed. If this current VM exceeds what I would likely need, then a full test would then make sense using my accelerator tube.

However, if the seven stage half wave creates too low of a current, then best to abandon the effort.

Regardless, of these outcomes still glad I did this to see what making a 200 kV system involves - even if the current is not sufficient.
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