mesureing current at high voltage

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byron addams
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mesureing current at high voltage

Post by byron addams »

what is the best way to measure current at high voltage?
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Richard Hull
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Re: mesureing current at high voltage

Post by Richard Hull »

Always through the earth power connection. (the lowest potential end.)

High voltage current measurement is possible at the high end but is to forever be looked at as filled with major issues. Sometimes, in certain situations, it is unavoidable due to any number of system design issues that force it to be the case.

In the case of the simple fusor, it is the obvious as safest choice to measure fusor power current at the earthed end of the power supply connection.

This plainly seen in all the FAQ examples of power schematics.

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Rich Feldman
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Re: mesureing current at high voltage

Post by Rich Feldman »

I've had some success with a floating milliammeter in a metal case with round corners.
Here measuring anode current of a color television CRT; that's an application where it isn't easy to insert meter in low-side return path.
Sparky HV circuits are notorious for damaging high or low side meters that aren't designed with protection in mind.
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Richard Hull
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Re: mesureing current at high voltage

Post by Richard Hull »

It is a nightmare to measure current to a fusor at the HV input terminal. I attach two photos of how the Farnsworth team did this.

The process used remotely, telescoped views of the analog current meters. The first image is the ideal and shows the "pit" fusor that I called the warp core. The meter is fully enclosed within the HV terminal. Like the ice pail of Faraday's experiment, the meter and all connections are within an electrostatic shielded Ball that might be at 100,000 Volts potential. (There is zero ES field within the Sphere). A simple monocular telescope could be used to read the meter.

Such terrible ways to read current as this might have been forced due to issues with feeding the ion guns to avoid undue insulation complications that made HV terminal current measurement the way to go.

The second picture puzzles me. This is the cave fusor. The huge HV aluminum terminal allows for electrostatic shielding, however, the meter Movements are outside of the shield!!!! Knowing what I know of analog meters, having the HV relative to ground or the environment with exposed movements would seem to ask for electrostatic differentials to play havoc with the charge that would have ES forces screw with the readings. I do know there is still a bit of shielding offered near an ES shield and perhaps the charge differential is minimal. Let's hope this was the case here. I would never do it this way! I would, like the pit fusor cut viewing holes to view the meters well buried within the terminal. Again, I do not like this second photo arrangement.

Inside a metallic sphere a new and isolated electrical universe exists. This little universe, inside, has no idea of any electrostatic potential in our world or the entire universe we live in. This sphere could, itself, be charged to millions of volts relative to the universe we live in but there is zero charge within. No being inside the sphere could perform any test to show that a charge existed within his closed electrical universe!

Finally, in the simple fusor always measure the HV current near the earthed leg connection to the fusor shell.

Richard Hull
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Well done current metering within the ice-pail where ES field is zero
Well done current metering within the ice-pail where ES field is zero
I question this setup.  I assumed it worked.  However this could have been a proud, un run photo made just after this setup was made and one or both meters blew up or had ES force issues acting against or overpowering the EM action of the movements within the meters.  If so, they dropped back to a pit type metering.
I question this setup. I assumed it worked. However this could have been a proud, un run photo made just after this setup was made and one or both meters blew up or had ES force issues acting against or overpowering the EM action of the movements within the meters. If so, they dropped back to a pit type metering.
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: mesureing current at high voltage

Post by Frank Sanns »

Depending on the type of power supply you are using, chances are you can measure input current to your transformer or multiplier stack and get a good indication of the current of the HV output.

Short your HV through your milliamp meter and measure peak current with full input power. Stand clear of all HV leads for safety.

Then turn power off and short your HV terminal with a jumper to ground. Put your milliamp meter on the transformer input and run the supply up to full power and get that reading. This will help you find the efficiency of your supply.

You will now have the maximum currents on the low and HV sides. On the input side you have the maximum power Watts= Volts x Amps.

It will be the same for the output but now you only need the voltage to find the output current. The efficiency of most supplies will be between 30% and 90% if just a transformer. To find the efficiency and linearity you will have to use your floating milliamp meter up from hundreds of volts up to a couple to few thousand volts using ballast resistors. Graph that and you will have a calibration curve so you will only have to measure input current and output voltage. From that you will know the output current by just knowing the Voltage output.

Typing on my phone so excuse the choppiness of this explanation.
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We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: mesureing current at high voltage

Post by Richard Hull »

As the original posting asked about current. Just do as every living entity has done. measure current in the ground, (earthed leg), of the power supply.
No need to short of anything, which I would never advise. Try shorting anything and see what happens. Just be ready to replace your diodes or ballast, transformer, drive circuitry or any number of components and of course, your milliammeter.

Just do what the attached drawing shows........

Richard Hull
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Fusor metering plus shunt.jpg
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Fusion is the energy of the future....and it always will be
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Re: mesureing current at high voltage

Post by byron addams »

I was thinking of something like this I want to make the HV supply a self-contained unit with all of the controls and monitoring built-in so that I can use it for other stuff such as x-ray tubes and stuff. Will this work?
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Re: mesureing current at high voltage

Post by Frank Sanns »

Richard,

I know of exactly zero commercial power supplies that measure the current coming back through the case or a lead. Commercial units measure current is by shunt (voltage drop across divider) or find power going into, through, or after a stage(s).

What I propose would not work if a multi kilowatt pole pig where feeding some 1 watt rectifiers but that would be poorly designed home brew setup.


Byron,

The setup that you have will not work. The ground next to the ammeter will short circuit before the meter. I also do not know what kind of supply that you are using and you don't want to tie one end of an open ended power supply to ground. The most general fix is to not ground the ammeter at all (DANGEROUS) but to just leave it in connected as you have it but omit that external ground. If your supply has the + grounded internally or to the case, then the meter will be grounded by the existing circuit and the meter will work.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: mesureing current at high voltage

Post by byron addams »

I am going to make my own power supply from some flyback transformers. The output of the supply would be isolated from the ground until after the ammeter as shown in the diagram.
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Re: mesureing current at high voltage

Post by Rich Feldman »

I think what Byron drew will work. Got no problem with the GND connection as drawn, or if it were on the other side of A meter as in Richard's plan. It needs to be somewhere in HV loop, to close the circuit for V meter.

Familiar caveats apply:
* The "A" meter must be robust, for example using copper wire shunt as shown in Richard's drawing. Should be able to withstand the effect of unplanned sparks and arcs at the fusor.
* Open circuit at the "A" meter, by accident or a connection failure, creates hazards of shock and fire.
* HVPS in drawing has only two terminals. Unless it includes batteries or an engine, there are electric power input connections (not shown) and internal details of HV galvanic isolation which Byron gets to deal with.
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Re: mesureing current at high voltage

Post by Frank Sanns »

I am not buying any of this. You can't meter to ground with an ammeter. Worse yet, the drawing as posted makes the + supply tied to ground. While tying to ground may or may not be a problem for the supply, the configuration drawn ties both the + to ground AND the other side of the ammeter to ground. There will be no current measured between ground and ground from both side of the meter. Won't work. Current will read zero no matter what the actual power in the fusor is.

Back to the metering through the ground. There are multiple grounds and not just one. Grounding the shell through the meter shunt sounds like a good idea but what about the metal vacuum connections that go to ground through the pump? What of the gas feed that is metal to the chamber and to ground (hopefully for the operator)? Even if all plastic or glass is used, gas at low pressure is very conductive and will bleed current into the vacuum system.

The voltmeter is fine. It will work as drawn.

The only way to know your power to the grid with any certainty is to measure the current directly or indirectly on the grid HV feed side.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: mesureing current at high voltage

Post by Matt_Gibson »

Frank - Would suspending a small (lightweight) analog meter above the high voltage feed through terminal work?

So, hv from supply to ballast resistor, ballast resistor to ammeter, ammeter to high voltage feed through terminal?

If the current shunt fails, I guess you just lose your ammeter this way, no?

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Re: mesureing current at high voltage

Post by Richard Hull »

All current is the same throughout a "closed circuit". First year electronics and part of Kirchhoff's laws. An ammeter in the hot negative lead going to the grid will read the same in the ground connection to the fusor.
Power supply, ground referenced to Grid, through gas load to ground. A simple series circuit. Power supply ground referenced....hot lead to grid. Resistive gas load that the supply powers and heats. Shell is grounded. It makes no matter how many other grounds there are in the electronics, pumps, gas tank, etc. They have their own little world and power supplies hooked to earth ground/electrical ground just like the fusor.

Power to the fusor goes supply to the load to ground. Break this simple series chain at any point and the current will read the same. Fusor ground current doesn't know anything about the hundreds of other ground currents as it is isolated from all the other sources of power to those other ground points. Ground is ground, a common return from many power supplies.

Star grounding is common to reduce noise in grounded high current pulsed systems or where sensitive instrumentation can create or respond to "ground bounce". This is another issue and will not generally be found in fusor systems, but a common precaution taken using a star grounding effort. By starring all grounds in a "system", the system has a common ground point that is more or less invariant regardless of the fuse box power ground to the driven ground rod.

Richard Hull
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Re: mesureing current at high voltage

Post by Frank Sanns »

Maybe I am being overly rigorous with all of this but here is why the argument.

There are at least three return parallel circuit paths to ground. The fusor ground, R1 in the picture. Gas feed line ground R2. and Vacuum gas/connection line R3. There are then three paths the current can take. The sum of all three does equal the feed but individually they do not. It could be that the resistance and impedance is essentially zero but there is no guarantee that any of of these three legs is passing all of the current that is fed in. Putting the meter in any one of the R1, R2, or R3 legs does not guaranty the total circuit current.

The only sure fire place for the meter is at a point closest to the power supply and before any branch points.

Maybe I am missing something here so I shall not beat a dead horse.


Matt,
Yes, suspending the ammeter in the line above the fusor is fine. That is what Farnsworth was doing with the fusor in the photo earlier in this thread. The two important factors for doing that is safety. Be sure the meter is well insulated and away from anything that can touch it and cause electrocution or harm. It is much safer to have it at the HV feedthrough than at the power supply for that very reason. The other factor is corona. At 10kv or 20kv sharp edges of meters and connections is not a huge issue. Go any higher though and ionization of the air will carry all of your power away. That is also why the Farnsworth device had a conductive round dome over the feed through and business end of the meters atop the fusor.
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Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: mesureing current at high voltage

Post by byron addams »

I was thinking of doing it that way because as Richard said it would be better to measure the current through the earth and it would be a positive earth as that is what is needed to propel the deuterons towards each other. The chamber of the fusor would need to be grounded because the pumps and valves are grounded. And I would like to build the ammeter into the power supply so I thought of putting the ammeter between the earth and positive out of the power supply. So it will still measure the current because the current would have to go through the ammeter to complete the circuit but still be at ground potential.
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Re: mesureing current at high voltage

Post by Richard Hull »

Frank there is only one resistor in the circuit here. There are no other resistors in a star ground as all other instrument are isolated from house driven ground wires via their internal power transformers be they 60Hz or 20kHz switchers. Instrument grounds are not in the current path. All home built stuff needs to keep this is mind to avoid ground fault currents. In a star system the fusor body is isolated from ground currents as they do not normally exist with real instrumentation. Only a terrible fault within a motor or instrument will flow to the star ground point and flow only to the home driven ground and not into the fusor or its isolated ground circuit.

Metal TC gauge bodies are not connected to their electronics. Their metal bodies are grounded by attachment to the fusor shell (safe), as are all metal lines and components in the vacuum system.

From all manufactured items, under no circumstances does any electrical energy or power ever flow through the green ground connection

I attach a diagram of home ground wiring and the fusor on a star ground system.
I also attach franks image with explanation.

When choosing a power supply or for information on power supplies for a fusor reference this FAQ

viewtopic.php?f=29&t=13492

Richard Hull
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Parallel circuit frank.jpg
Current path ground return.jpg
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Fusion is the energy of the future....and it always will be
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Re: mesureing current at high voltage

Post by Dennis P Brown »

I also use a star ground and have three separate power plugs with ground pins. My fusor has ground wires on two sections to be certain.

As for current measurement, I have a full wave diode bridge for the HV output of my transformer and use that system; that is, the diode bridge has a two connection points from the transformer. It then has a negative output and a 'positive' output. Of course, the positive node is usually the ground point. I use that node and run a wire to my ammeter and a wire from that to ground. If the meter fails, the bridge floats but that does not affect my fusor ground or endanger me in any manner.

As for the high voltage measurement, I simply read the output on the variac (0 - 125 V) using a digital meter that was calibrated via my high voltage probe. That is, I use a digital meter that is scaled (using a pot) to read from 0 to 35.0 for the full range of the variac (0 -125 V). This entire system I had measured using the HV probe (0 - 35 kV) with a plasma and is my actual KV readings from the transformer under normal plasma load. Yes, during extremely heavy or near short conditions the voltage readout by this meter is wrong but those conditions are not steady state and not of any interest for my purposes. Not an ideal system but very functional and fail safe for fusor grounding.
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