Help with HV divider

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James Henry
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Help with HV divider

Post by James Henry »

I am hooking up another digital meter to read the hv from the GE x-ray transformer, using 5 - 100 mega ohm = 500 mega ohm under oil, then to V IN on the meter,
then across the meter is a 1.5 k ohm resistor fixed then a 1 k ohm 10 turn pot, then back to the meter and ground.
The readings i am getting are 4X the amount it should be. In other words at 10 kv from the transformer it will read 40 kv on the meter.
I do not understand where i am going wrong. Brain fog.
Please help
James
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Rich Feldman
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Re: Help with HV divider

Post by Rich Feldman »

A schematic would help, even photo of sketch on a napkin.

Are you measuring AC voltage?
Are you confident that your meter in AC Volts mode is accurate, and has a symmetrical and resistive input impedance?
Have you tried the same attenuator on a lower voltage source, such as 120 V or 240 V or MOT or NST?
All models are wrong; some models are useful. -- George Box
James Henry
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Re: Help with HV divider

Post by James Henry »

Rich
I am measuring DC Full wave rectified 100 kv transformer pushes out -50 kv dc, When i use my multimeter with hv probe i get the correct reading.
But i want to use another digital meter to be permanent to free up my multimeter.
My problem must be in the voltage divider. The specs on this panel meter, input Impedence >10 m ohm.
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Rich Feldman
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Re: Help with HV divider

Post by Rich Feldman »

I see you're not a newbie. When can we see your vacuum chamber hemispheres?

In case your professional experience didn't get into high voltage components...
What is the voltage rating of your individual 100-megohm resistors?
https://www.mouser.com/datasheet/2/427/ ... 762971.pdf

Resistor voltage ratings are not generally about heating or external arcing-over, and are not affected by oil immersion.
They come from the physics of ohmic conduction in the resistive material.
Not long ago, I measured substantial voltage coefficient of resistance in old-fashioned carbon composition resistors.
IIRC, conductance was immediately 20% or 30% higher than low voltage value, before I got to 2000 V per resistor, and without enough power to get much of a temperature rise.

There is an axial-lead resistor product that John Futter likes to cite here, I think by Ohmite, that is not too expensive and is rated for around 10 kV per resistor.
HV rated R's are easy to find on ebay and at electronic component distributors.
DO not confuse them with high-ohm R's in glass tubes, meant to control nanoamps or picoamps at modest voltage levels.
All models are wrong; some models are useful. -- George Box
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Finn Hammer
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Re: Help with HV divider

Post by Finn Hammer »

James
With your potentiometer centered, you have a 500M/2K divider, that is a 250000:1 ratio, so by placing 50kV across it, you see 0.2V across the bottom resistor combination.
You may find it more handy to see something like 5 volts in that situation, and you will acheive this by replacing the bottom resistors with a 50k resistor.
If your multimeter really has >10M input resistance, then the effect of parallelling it with the 50k will not have any serious effect. One that you can correct with placing another 250 ohms in series with the 50K.
But in the end of your post, you refer to a panel meter, panel meters usually do not have that large input impedance, at least not any I am aware of, so:
What is it: multimeter or panel meter?

Pictures, please!

The resistors that Rich is mentioning are manufactured by Vishay, they are from the VR68 series, and they are great.
Following here is my present voltage divider: 30 pcs. 47M VR68 complemented at the bottom by 3 pcs. 470K 2W resistors, and (externally), a 22k resistor to compensate for the 1M input resistance of my _Panel_ meter!.

Voltage divider in the middle, dummy load to the left, voltage multiplier to the right.
Voltage divider in the middle, dummy load to the left, voltage multiplier to the right.

Hope this helps.

Cheers, Finn Hammer
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Rich Feldman
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Re: Help with HV divider

Post by Rich Feldman »

>> The specs on this panel meter, input Impedence >10 m ohm.

Easy to measure using another meter! I bet it's well above 10 milliohms. :-)

The tiny LED panel meter in my AC/DC high voltage meter has input impedance of about 115 K ohms.
Pictures and schematic as PNG attachments to this old post: viewtopic.php?f=11&t=12407&hilit=variac#p80678
Same kind of panel meter serves in a floating AC/DC milliammeter, reported much later in that thread.
All models are wrong; some models are useful. -- George Box
James Henry
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Re: Help with HV divider

Post by James Henry »

Thank You Both
The panel meter i am trying to connect is PM-1029A , And the 5-100 megaohm resistors are those vintage tiger stripe that i put under oil, i do not have a spec sheet.
And i am a newbie when it comes to resistance calculations. I am sorry that i have not put up the pictures of my Fusor yet. I have been very busy, Fixing Blown diodes,
Vacuum leaks, and a lot of other things.
Thank You for all the input.
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Finn Hammer
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Re: Help with HV divider

Post by Finn Hammer »

James,
Thanks to you too, the PM-1029A meter has features I have been missing, and I want some.
Cheers, Finn Hammer
James Henry
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Re: Help with HV divider

Post by James Henry »

Finn
The PM1028A is the LCD, and the 1029A is the LED need to use 9Vdc. I Have the LED for voltage and amps, easier to see.
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Richard Hull
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Re: Help with HV divider

Post by Richard Hull »

I have used Circuit Specialists PM-1029B since the 90's and the two I have used since 2004 on my fusor's volts and amps panel are still used. Great and easily readable. 5 volt common ground.
I bought a bundle of them back in the late 90's when they were only $10 each
https://www.circuitspecialists.com/ $23.00

I have added them to the list of special suppliers for electronics in the trading post Primary Sources

Richard Hull
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This dual PM-1029B panel is seen on fusor III 2001 and was moved to my current set up in 2004, where it still resides
This dual PM-1029B panel is seen on fusor III 2001 and was moved to my current set up in 2004, where it still resides
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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Finn Hammer
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Re: Help with HV divider

Post by Finn Hammer »

Common ground is fine for voltage and current, where you can tailor the output towards full scale. But with 65mA in the gound led, it will build up a measurement error at the end of a cable extension, for example to a remote operating station, where 1.5 ohm will shift the reading with 0.1 volts.
In the case that the metering part of PM-1029A is indeed floating, this makes it particularly attractive for monitoring a Baratron, and the analogue output from a 901P, because the high led display current will have its own ground return path, separate from the metering connections.
I would have expected the LCD version to draw less current, but the datasheet quotes 65mA for that as well.

Edit: my error, the LCD version indeed only draws 1mA, and has 100M input resistance, fabulous!

The high input resistance, the ability to read negative, and to place the decial dot where it is desired, places this panel meter i a class of its own, and separates it from the current offerings on Ebay.
Last edited by Finn Hammer on Thu Dec 09, 2021 2:58 am, edited 1 time in total.
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