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Topic: Measuring High Voltages with a low voltage meter (Read 882 times) previous topic - next topic

TomGeorge

Hi,
Can I suggest you forget about making an EHT probe and buy one, it will be properly insulated, use the appropriate resistors and will be calibrated.

If you google    EHT Probe          you will find DIY and proper probes.

If you want to make an output meter for your power supply, then using HV resistors in a series array will work, BUT make sure your insulation and physical protection is adequate.

How do you know at the moment that your powersupply puts out 20kV?

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

Joseph3502

About there. It uses a flyback transformer and a dimmable 277 volt ballast.

ChrisTenone

I agree with post #12, but not necessary - sphere

For pointy electrodes, the multiplier is different.
I'm not disparaging the differently abled. I'm stating the fact that thirty years of junior college has made me mentally retarded.

ted

so multiplier for spherical will not work with pointed electrodes ?
the more the electrodes are sharp, the smaller the distance is needed to start the arc.

ChrisTenone

If the electrodes are pointed, the spark will be longer.
I'm not disparaging the differently abled. I'm stating the fact that thirty years of junior college has made me mentally retarded.

JohnRob

I can't talk directly to HV spark length but when working with ESD humidity played a large part on the discharge distance.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

ted

 Read again post #1, second line - images

Paul_KD7HB

so multiplier for spherical will not work with pointed electrodes ?
the more the electrodes are sharp, the smaller the distance is needed to start the arc.
It all comes down to the ionization of the air surrounding the charged metal. The critical measurement is the area of the charged material. A point has very little area, but the voltage is the same as for a sphere or a point. So the air or gas will ionize at a lower voltage because the surface is tiny.

In the 1950's a group at Linfield College discovered they could create X-rays between two sharp pointed electrodes  sealed in a vacuum tube with a rather low DC voltage applied. The points were sharpened to a single metal molecule at the point.  Google "field emission".

Paul

DVDdoug

#23
May 20, 2018, 09:01 pm Last Edit: May 20, 2018, 09:04 pm by DVDdoug
You can buy a High Voltage Probe.   They have a built-in voltage driver and they are big & goofy looking (big to prevent arcing).    And, like any "serious" measurement equipment, they are expensive.

You could probably find a cheap used one because every TV repair shop used to have one to measure the voltage on a CRT tube.

Quote
Using multiple lower voltage resistors is like using multiple diodes in series to block a voltage that exceeds the individual diodes.   If the voltage is not shared equally one is likely to experience over voltage and fail.
If the resistors have equal resistance the voltage will be divided equally.   That's Ohm's Law...  The voltage is proportional to the resistance...  That's how a voltage divider works.   (And that assumes no arcing.  ;) )

Wawa

You can buy a High Voltage Probe.   They have a built-in voltage driver...
No, they don't have a voltage divider.
It's just a single 990Megohm (3*330Meg) resistor.

Read post#11 again.
Leo..

TomGeorge

No, they don't have a voltage divider.
It's just a single 990Megohm (3*330Meg) resistor.

Read post#11 again.
Leo..
I don't think so.
I'll check the Fluke EHT Probe at work today.
Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

Wawa

Try measuring a 9volt battery through a 10Meg resistor.
Tell me what voltage you read.
Leo..

Southpark

Also, for the OP..... it would be wise to have all this sort of thing automated...... so basically have EVERYTHING de-energised when you're working on it. And then....... once you're ready to take measurements..... then walk away..... eg..... like a few metres or more away.

TomGeorge

#28
May 21, 2018, 03:23 am Last Edit: May 21, 2018, 03:26 am by TomGeorge
Hi,
I could only get to our Fluke 6kV probe and its manual shows the circuit.
I measured the resistances of it and they agree.

I have also added the manual for the 6kV and the 40kV units


The manuals also give equations for compensation for odd input impedance meters.

Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

TomGeorge

Hi,

The 75k resistor actually measures  76.0k.

So with 10M from DMM.

10M // 76.0k = (10,000*76.0)/(10,000+76.0) = 75.4k

Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

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