High voltage measurement question.

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Boris Said
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High voltage measurement question.

Post by Boris Said »

I was looking at the FAQ's on high voltage measurement trying to understand it but I find some of it very confusing. I was comparing the circuit diagram and the picture of the actual circuit (Measuring of high voltage part 2 and Demo fusor wiring diagram). In the actual circuit I did not see a ground. Also, the demo fusor wiring diagram included no potentiometer but the real life photo of the circuit does. Thank you, in advance for all answers. Also, any pictures of your high voltage measuring circuits would be great.
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Richard Hull
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Re: High voltage measurement question.

Post by Richard Hull »

The potentiometer is a small one used to hone in on the low value range resistor in order to trim it to make the meter accurate. In general, you need a good HV meter to adjust this potentiometer to great precision. If you do not have a good high voltage meter already to use to calibrate, you are just going to have to rely on the accuracy of the two scaling resistor without a potentiometer.

Example: A certain meter needed to read 10,000 volts requires a two resistor divder. One of the resistors is a special100megohm resistor capable of handling the high voltage. The second series resistor in the divider to ground might be a 10k ohm value. The meter is placed across this resistor. The math says this is correct, but with a 10% 100 megohm resistor and a 5% 10k ohm resistor, there is likely to be some ratio variability.

If there is already a 10kv meter on hand that is accurate, The smart thing to do is get a 7.5k ohm 5% resistor and place a 5K ohm potentiometer in series with it in place of the 10K ohm resistor. Hooking up the accurate meter across our homemade meter and applying 5kv, we can trim the potentiometer to make the homemade meter read exactly the same as the know accurate meter. 5,000 volts excatly....

With just the two resistors as orginally spec'd with no pot, we might read 5210 volts instead of 5,000. Not accurate, but good enough when you are reading 20,000 or more volts.

Again, the pot is of no value unless you have a known accurate high voltage meter to trim your homemade meter with. I have a precision meter to 1% up to 30kv and use it to trim all of my high voltage meters with a potentiometer. I used to manufacture such accurate meter systems and offer them for sale here, but gave it up.

You are on your own.........

Richard Hull
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Boris Said
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Re: High voltage measurement question.

Post by Boris Said »

Thanks for the reply, you really cleared everything up. Also, do you know anywhere where I can get an HV power supply for under $150(if they sell that cheap)
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Dennis P Brown
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Re: High voltage measurement question.

Post by Dennis P Brown »

One must use ebay or similar sights and be patient; also, be very careful about what you buy - many power supplies are just a part of a system and without all the components, are worthless. Big x-ray power supplies are often NOT turnkey systems and need extensive electronics knowledge (i.e. ability to build extra parts to get operational.) Getting an operating 25 kV or so and 10+ma power supply for under $150 will be a challenge. Best of luck.

If all you want is high voltage source, a good neon sign transformer - 7 kV or so supplying a few tens of ma - are obtainable for well under $150. Good only for demo fusors.
Boris Said
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Re: High voltage measurement question.

Post by Boris Said »

Thanks for the reply about the HV power supply's, I think I'll go without the potentiometer. Do you know where I can obtain the 10Kohm resistor and does it need to be high voltage rated(I'm assuming it doesn't as the first resistor would lower the voltage).
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Rich Feldman
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Re: High voltage measurement question.

Post by Rich Feldman »

Why is this discussion still in Fusor Power instead of New User Chat Area?
All models are wrong; some models are useful. -- George Box
Boris Said
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Re: High voltage measurement question.

Post by Boris Said »

In the future, I will post simple questions in the new user chat area.
Boris Said
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Re: High voltage measurement question.

Post by Boris Said »

Since I am only creating a demo fusor would you recommend that I use a 1Kohm resistor in stead so I can get more accurate voltage readings? It
prestonbarrows
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Re: High voltage measurement question.

Post by prestonbarrows »

Boris Said wrote:Since I am only creating a demo fusor would you recommend that I use a 1Kohm resistor in stead so I can get more accurate voltage readings? It
A resistor divider follows this equation
Image

Basically what you are talking about is error propagation. The value of each resistor has some uncertainty in it. One labeled "1000 ohm" may actually be anywhere with a range of say 995-1005 ohms. It is easy to buy small value resistors that are very close to their labeled value, <1%, for R2. The high value resistor, R1 in the megaohm range, that you need for a high voltage divider typically has lower precision in the say 5-10% range. It won't benefit you to you know the value of one resistor very well and the other very poorly; the larger uncertainty will basically dominate here.

So, either you find a very high precision resistor and pay extra for it, or you measure the actual resistance directly with a multimeter and adjust your scaling factor with the above equation. In this case you don't really care about the accuracy, just the precision. The problem here is that most common multimeters can not measure the very high resistance R1.

The other way around this is to apply a well known high voltage, Vin, and measure the resulting low voltage, Vout. This allows you to calibrate the resistors and work out what the value for R1 is since you know all the other terms in the equation. However, to do this directly you need a second meter which can accurately and precisely measure the high voltage. You can get around this by using two steps and use a higher value R2 (with a well known resistance) and lower Vin temporarily during the calibration to make things easier to measure with basic tools. Then you can calculate the actual value of R1 using the test jig before you put it back in your final HV divider; the resistance value will stay constant over time neglecting any thermal effects.

In a nut shell, you need precise and accurate values for three of the four variables in the equation to calculate a precise and accurate result. Or, you just propagate the uncertainties and figure out what the uncertainty in the output measurement is and live with an accurate but imprecise reading.
Accuracy-and-precision-picture.gif
Boris Said
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Re: High voltage measurement question.

Post by Boris Said »

Thanks for this reply, it really helped me figure out how to make it more accurate and precise WITHOUT a costly supply. What I meant when I that it would more accurate to use a smaller resistor was different, I was thinking about the placement of the decimal place. If a 1K resistor was used instead of a 10K resistor the decimal point would be xx.xx instead of xxx.x. I think it would be more accurate to call this precision.
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