Measuring current on a non-floating PS - a question

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Dennis P Brown
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Measuring current on a non-floating PS - a question

Post by Dennis P Brown »

A quick question relative to current measurement. My positive supply cannot be floated and then grounded by a wire to measure current used by the fusor. As such, can the current be directly measured from my fusor's "grounding wire" (i.e. return current wire)? That is, can a voltage meter with a 10 ohm resistor across it as per your circuit in the HV FAQ still be used on this fusor return wire or will the voltage still be too high? I was thinking that the fusor voltage will not be an issue since this wire goes directly to ground resulting (?) in the cathode maintaining a near "zero" potential ... maybe?
Including some pics of the wiring for a positive supply. In this configuration, the anode is supplied by a moderate gauge wire running to ground while the high voltage led carries the +30 kV into (from) the fusor.
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Fusor with Positive HV feed configuration
Fusor with Positive HV feed configuration
Close up of fusor HV and ground wiring
Close up of fusor HV and ground wiring
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Richard Hull
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Re: Measuring current on a non-floating PS - a question

Post by Richard Hull »

Much depends on how your supply is internally wired. For safety, a Positive HV supply usually has its minus or negative terminal grounded.
measure the ohmic resistance between ground and your negative high voltage lead. Report back. If zero you can't use that supply at all for a fusor.

A totally isolated HV supply is a very, very rare bird, indeed!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
prestonbarrows
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Re: Measuring current on a non-floating PS - a question

Post by prestonbarrows »

As Richard mentioned, pretty much every HV supply is going to tie one output directly to earth ground. This is not strictly required by physics, but greatly simplifies engineering in the real world.

Once you get 'absolute' voltages with respect to earth that are above 1kV or so, you have to start paying attention to the physical shape and location of all your conductors. Any sharp points begin throwing off corona and any conductors too close to each other across too high a potential difference begin to arc.

It is a safe assumption that any commercial supply will have its case tied to earth ground for safety. It is much easier to physically lay out a circuit if you only have to allow for high voltage standoff distances and curved geometries for one half of the output. High voltage engineering is very non-linear. Tying one output terminal to earth ground means you can easily route it wherever you like in relation to the chassis and also gives you a place for simple current telemetry at logic level voltages.

Having both ends of the output untethered to ground is technically possible but means that you have to build in the full voltage standoff to twice as many components and account for at least twice the physical space. It also means you can't assume your telemetry signals are anywhere near ground logic levels, which requires the addition of complex isolation circuits before feeding them into your control circuitry.



To address your original question, any time a current flows through a resistance there will be a measurable voltage across the resistance by Ohm's law. In practice, the difficulty is that the voltage at either end of that resistance relative to ground may be huge. In the worst, sloppy, case you can always resort to floating a DMM to read the current-driven-voltage across a resistor anywhere in a HV circuit; just remember not to reach out and adjust the meter range while the circuit is energized...
A better solution is a solid pair of HV dividing probes that can be attached across the current-driven-voltage resistor and bring the signal safely down to logic levels at ground.
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