How to measure High-voltage? (above 30kV, maybe 50kV) [With common Multimeter]

[Distelzombie]

Hi

(Yes, it's the beginner section. But please don't tell me to be safe and scrap this project.)

I finished this thing a few years ago. A flyback transformer from an old CRT-TV and a Halogen-lamp driver. Few milli-amps of course.
Yesterday I was experimenting with it again and thought: "Hm. Would be nice to know how good it really is."
I obviously don't have a meter that measures such high-voltages, or the amps attached to it. So, could you please help me out?
I wish to measure Volts and Amps - but! With almost no money involved! As less as possible.

It's kind of a research and experimentation project. No real use for that thing.

EDIT: Ups forgot to say that I got the 30-50kV from measuring the distance the arc can reach maximum. Roughly.

[HighVoltage]

You can build yourself a capacitive divider with a 10,000:1 ratio and measure the output on a scope.

[Tomorokoshi]

Measure the voltage across a resistor divider, similar to how it works with the feedback node of a switching power supply.

If it can output on the order of a few milliamps, then let's say at 60kV the divider will flow 1/10 mA, or 10 uA.

The total resistance would be:
60000 / 10E-6 = 6 Gigaohms.

With three of these in series for the upper resistor:
https://www.digikey.com/product-detail/en/ohmite/SM108032007FE/SM108FE-2000M-ND/824216

A lower sense resistor of 600K would give a multimeter reading of 1V per 10kV. However, there will be an error due to the shunt resistance of the multimeter. If it's 10M, then, then the lower resistor would be 600K = 10M || R, or R = 634K to 649K.

[PartialDischarge]

6GOhm is too much for a resistive divider and will only measure strict DC, as any slight frequency component will couple to the output capacitively and false the reading by a huge amount. To give you an idea at 50Hz a 1000:1 uncompensated resistive divider with a 6G upper resistor will divide by around 100:1

[Distelzombie]

Do resistors not break down with such high voltages because the path around it would have less resistance?
And I guess the capacitors for the divider would have to have a rating for 100kV or more to be safe, right?
I actually build such a capacitor one once. With twenty A4 sheets of 0.5 to 1mm thick plastic and aluminium foil. (I'm not sure how thick exactly. I lost my caliper. They're something for inkjet printer) I rolled it around a plastic cylinder. It worked great until I enhanced the thing with the halogen-lamp driver. Then it broke down.
Hm... I could use a coax cable as capacitor. Those used for cable TV. But that's inefficiently expensive.

I'll take a look around for cheap ones.
BTW, I only have a common multimeter atm. It is rated for 600V max. I was hoping to do that with it alone.
(Edit)Woa, are they expensive! Cheapest are the 50kV ceramic caps. For around 50 bucks. That's not the way to go for me. (Edit)
Ah frequency, yes. Since the Halogen driver works at around 400kHz I'd guess the flyback does also. But the longest arcs sound more like 10 to 50 times a second.

[HighVoltage]

You can use Teflon (PTE) as an insulator to build your high voltage capacitor and encapsulate everything with PU (polyurethane).
Keep the high voltage divider capacitance low, like 1 to 3 pF max.
 

[Gregg]

It may be cheaper than replacing a toasted meter to purchase a HV probe, unless you like to fix meters like Joe. 
https://www.alliedelec.com/b-k-precision-pr-28a/70146130/

[Distelzombie]

It may be cheaper than replacing a toasted meter to purchase a HV probe, unless you like to fix meters like Joe. 
https://www.alliedelec.com/b-k-precision-pr-28a/70146130/

No, it costs more than my meter. XD

HighVoltage, I guess I have to be precise with‫‭‭‭‭‭‭‭‭‬ the capacitance, do I not? I‏ mean I‫‭‭‭‭‭‭‭‭‬ cannot cal‏ibra‏te my b‫‭‭‭‭‭‭‭‭‬uild before using it to m‫‭‭‭‭‭‭‭‭‬easure something.

[HighVoltage]

HighVoltage, I guess I have to be precise with‫‭‭‭‭‭‭‭‭‬ the capacitance, do I not? I‏ mean I‫‭‭‭‭‭‭‭‭‬ cannot cal‏ibra‏te my b‫‭‭‭‭‭‭‭‭‬uild before using it to m‫‭‭‭‭‭‭‭‭‬easure something.

It depends, how much accuracy you need.
Once you have it finish, you can send it to me and I calibrate / measure it for you.

[Zero999]

6GOhm is too much for a resistive divider and will only measure strict DC, as any slight frequency component will couple to the output capacitively and false the reading by a huge amount. To give you an idea at 50Hz a 1000:1 uncompensated resistive divider with a 6G upper resistor will divide by around 100:1

That might not matter in this case, since it's quite common for CRT flyback transformers to contain a built-in capacitive voltage multiplier, which will output a DC voltage.

[Distelzombie]

I don't think that is applicable in my configuration. I'm not using the pins of the transformer. (Except ground) I should have said that earlier.

Here's a picture ...

[tecman]

look on ebay for "electrostatic voltmeter".  You can find several in the 30KV to 50 KV for under $ 200.  They are "capacitive" devices and except for the initial contact they provide a no-load way of reading.

paul

[Distelzombie]

look on ebay for "electrostatic voltmeter".  You can find several in the 30KV to 50 KV for under $ 200.  They are "capacitive" devices and except for the initial contact they provide a no-load way of reading.

paul

...

I wish to measure Volts and Amps - but! With almost no money involved! As less as possible.