High Voltage Current Measurement Methodology

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.
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Dennis P Brown
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High Voltage Current Measurement Methodology

Post by Dennis P Brown »

While I certainly didn't get this design from the FAQ, it follows the logic. But not being versed at all in real world electronics I realized my logic could be far off. So, I am asking if this circuit does in fact provide true current measurement for my fusor?

In the drawing of the circuit, the circle that is mark with the "MA" in the drawing is the location of the milli-amp meter - one side is 'input' and the other goes to system ground. This side of the bridge is the normally 'positive output which I use for grounding (and metering.)

The fusor is on the negative side of the diode bridge and is supplied through the 50 K-ohm resister. The transformer (x-former) outputs are located on the other diagonals of the bridge.

In the actual picture, the diode bridge is assembled as drawn, the two 25 k-ohm resisters are under oil at the top, and the x-former is to the right.
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Richard Hull
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Re: High Voltage Current Measurement Methodology

Post by Richard Hull »

All is fine....If...
1 An ohmic measurement from the core to each leg of the hot lead output of the transformer checks as an open on the 1 megohm range. Also the transfomer must have no center tap. In short, the HV winding must be floating and fully insulated from ground and the core.

2. The ma meter in the circle is not a simple DVM set to a suitable ma range of say 50ma or a simple analog meter 0-50ma. These are not allowed from a safety stand point, long term.
The ma circle should be a wire wound .1,.2,.5 or 1 ohm resistor going to ground with a DVM measuring a suitable low voltage across the resistor that is an analog of the ma reading. (1 ohm resistor to ground in place of a simple meter will read .050 volts at 50ma with the DVM set to read 200mv or .2v across the resistor.) Diagrammed in the FAQ
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Rich Feldman
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Re: High Voltage Current Measurement Methodology

Post by Rich Feldman »

Nice picture.

As Richard said, the HV transformer winding needs to have full HV insulation at both ends of the winding.
(or you could ground the inside connection and use half wave rectifier with 1 diode).

I also agree that a simple mA meter by itself is risky.
Normal moving coils, or 50 mA shunts inside digital or analog meters, are delicate & could be blown up by a current spike in HV circuit.
Then fusor current would get through by hook or by crook, inside or around the "meter", with many kV to push it.

As Richard said, a substantial and robust shunt resistor is highly recommended.
That doesn't rule out a pretty analog meter in parallel, if you prefer that style.
Not hard to find moving coil meters with full scale voltage less than 200 mV, e.g. for use with:
snip.png
Meters with low voltage drop can also be protected with parallel diodes, gas tubes, etc, so HV current has a safe way through.
All models are wrong; some models are useful. -- George Box
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Dennis P Brown
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Re: High Voltage Current Measurement Methodology

Post by Dennis P Brown »

Hello and thank you both - Richard, and Rich - for answering my question. After looking through the FAQ's and extensive googling this question I was surprised to find that this is not something that one can find an answer; again, demonstrates how valuable this forum is to users. One can just ask the question and get help! As simple as this question is on an electronics knowledge scale and how basic the circuit - really just a diode bridge and transformer with a simple load - trying to find out how to properly measure the current wasn't something I could locate any information. Even for low voltage systems (through that issue is more obvious to even a layperson.)

As for my transformer and if it is centered tapped, I did what you, Richard, suggested. I ohmed the transformer's hot wire relative to the core using the Meg scale setting. This showed that the x-former isn't centered taped because the 'hot' lead measures "open" on the that scale relative to the core. To be certain I tested against both the core as well as the common ground, too (I also checked that the core was in fact, properly connected to my main ground since I was doing this measurement - and it is.)

As a minor side issue, I took the time to measure all the other parameters for the x-former wiring: the primary inputs (110 v AC input wires) read 0.5 ohm relative to each other as it would appear they should. The output high voltage leads read 2.3 k-Ohm against each other (this I have no idea if it is correct). Both sets read open against each other as well as to ground (as they should since the x-former isn't shorted). So all aspects appear both logical and I believe appropriate for the x-former. Bottom-line, I believe this means that the meter is an accurate measure of the load the fusor is experiencing.

As for the milli-amp gauge, that unit is a standard analog mechanical meter (0-50 ma); that is, wire wound. I will certainly (in the near future) add a shunt* to my meter back panel connections; through it isn't a significant safety issue if it fails for my particular setup. If the meter failed that would either ground that side of the bridge (leading to no meter movement) or simply 'float' the bridge (meter somehow remains 'open'.) But in no case would that cause a safety issue for the fusor - the chamber has its own permeant ground system (double redundant.) My bridge is safely insulated from any possible ground sources (it is mounted on a thick plexiglass board that is, in turn, mounted on a wood /plastic coated tray) so the failure would not endanger the x-former nor the operator. Such a failure would, of course, be an undesirable situation but unlike a floating fusor (which is extraordinarily dangerous), this would simply cause a loss of current reading which would flag that the system has, in some manner, failed.

* I guess I can install a 50 watt, ceramic 30 k-ohm resister. I have one on hand. I assume that will do or should it be in the mega-ohm range?


I will add a minor aside: my high voltage cable has shielding. But because I am near the cable's "failure" voltage (rated to handle only 35 kV DC), I float that ground shield (had a failure once due to the shielding being grounded.) The down side of doing this (floating) is that it creates a problem I didn't think about - the ground shield mess and internal conductor act as a very low value capacitor. So after I shut the x-former down, I have to short the HV section or I'll get a surprise 'shock' if I touch any of the HV sections soon after turning off the power (yes, I found out the hard way. I will say that I do follow careful safety protocols when servicing the system after any run. I also designed the HV panel so it requires two 'on' switch's be selected before it can become 'hot', and I always unplug the unit before doing any service. Just didn't realize that there is a cap in the system ... can't think of everything, I guess.)
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Richard Hull
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Re: High Voltage Current Measurement Methodology

Post by Richard Hull »

I assume you measured the low side of the secondary near the core and did not find continuity to the core (open circuit) if this low side is conductive to the core or ground, the bridge will not work. This wasw actually the critical measurement. (low side to core or ground)

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
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Dennis P Brown
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Re: High Voltage Current Measurement Methodology

Post by Dennis P Brown »

I checked both wires from the secondary (for the high voltage side) against the core and both showed "open" on the the Meg scale (or any scale). The ohmic readings (non-zero) I made where relative to the specific windings only not relative to the core or ground. Sorry about adding those remarks in the post.
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