# How to measure high voltage

#### Obmar

##### New Member
Help would be welcome from anyone having knowledge of high voltages.

I have made a nice little electrostatic generator that works well and generates very high voltage static charge (~100 to 200 KV) giving decent 8 cm spark (and also shock if you are careless) by few rotation of handle. The current is rather small (not more than few micoamps).

My next aim is to measure this voltage (and if possible current) I am generating. Meters obviously do not help and I can't spend for costly HV measuring equipment. Is there a simpler method? I don't want very accurate mesurement but only rough idea of range so I can do some calculations and also understand how the machine works in different conditions.

#### RODALCO

##### Well-Known Member
HV measurement

Voltage measurement:
Best is probably to make a chain of 10 Mhm: standard E 12 resistors in series to do HV measurement.
Say 20 x 10 Mhm: 1 Watt.
Do your measurement over the last resistor at the negative or neutral point.

If you measure 20 kV then 19 kV is over 19 resistors and 1 kV can be measured with a high impedance DMM and will read 1000 Volts.
The perfectionist will work out the impedance of the meter and match an exact resistor with it. (for the last measuring resistor)

If you want to measure 100 kV you may need to go to a chain of 100 x 10Mhm: resistors unless you can access high value Mhm: resistors and make a shorter chain.

Current measurement: Put your mA or µA meter in the negative or neutral side of your supply.

edited for typo's

#### SYE

##### Member
Hi,
if you do make a resistor chain make sure your resistors are rated for the correct voltage, most carbon resistors are only rated for 200V, metal film can be as high as 1.6KV

#### CheapSlider

##### Member
Obmar said:
.......

My next aim is to measure this voltage (and if possible current) I am generating. Meters obviously do not help and I can't spend for costly HV measuring equipment. Is there a simpler method? I don't want very accurate mesurement but only rough idea of range so I can do some calculations and also understand how the machine works in different conditions.
All you need is a ruler!
You can get a rough idea from the spark length

#### Hero999

##### Banned
And make sure it's a plastic ruler, not a metal ruler.

#### Blatman Bond

##### New Member
Not plastic, but vacuum glass ruler.
Using resistor need more current, it will drop the source. 1mA 200KV is 200W.
Might be a vacuum glass mosfet (not mosfet but triode). I only hoping there any 200KV mosfet sold .

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#### Obmar

##### New Member
Length of the spark does not help- it depends upon so many factors- mainly humidity.

I tried resistors before (not 100 not only 3! I know it is stupid!). The sprk jumped straight through these.

Electroscope is a good idea, only if I know how to make it- and more important- how to calibrate it. Any suggestions?

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#### RODALCO

##### Well-Known Member
1 Watt Resistors

Hi Obmar,

The reason i specified 1 watt resistors is that they can handle about 1 kV each.
A ¼ Watt resistor will have a flashover at about 200 Volts.

So to measure a very high voltage the resistors need to be soldered in a chain, preferrably spaced apart to avoid flashovers and 1 Watt or more to have enough body length of each resistor.
For a voltage measurement with a DMM a very small current is needed so the power rating of 1 Watt should be adequate to provide you with a test current of < 1 mA.

We use 10 kV testers at work which use a series resisitor of 80 Mhm: consisting out of 24 x 3.3 Mhm: 1 watt R's in series. to drive a volt meter 0 - 10 kV dc.

#### Blatman Bond

##### New Member
Might be just store some coloumb to 200KV capacitor and then measure it.
How ever to measure, the current is no more a problem. Beware, 200KV in capacitor is lethal. I have some 2KV strobe capacitor with "lethal" label.

#### Hero999

##### Banned
That wouldn't be any good since the capacitor will leak charge and will still be discharged through the meter's impedance.

#### Obmar

##### New Member
RODALCO said:
Hi Obmar,

The reason i specified 1 watt resistors is that they can handle about 1 kV each.
A ¼ Watt resistor will have a flashover at about 200 Volts.

So to measure a very high voltage the resistors need to be soldered in a chain, preferrably spaced apart to avoid flashovers and 1 Watt or more to have enough body length of each resistor.
For a voltage measurement with a DMM a very small current is needed so the power rating of 1 Watt should be adequate to provide you with a test current of < 1 mA.

We use 10 kV testers at work which use a series resisitor of 80 Mhm: consisting out of 24 x 3.3 Mhm: 1 watt R's in series. to drive a volt meter 0 - 10 kV dc.
Yes that looks like the simplest way. I would try it. Thanks a lot.

Only one question and possible problem: I may have voltage far beyond 100 KV. (somewhere I read that 5 mm spark needs 16350 volts. So if relationship is linear, with 8 cm spark, I may be generating 261,600 volts.) Will it still work, or I should plan for 250 resistors. (rather a tall task, but worth trying.)

#### Styx

##### Active Member
careully comes to mind

#### RODALCO

##### Well-Known Member
Res. chain

Obmar,

You are only loading your HV circuit a little more, and the output voltage may be reduced a bit more.

If you can get access to 22 Mhm: or higher value resistors you can reduce the size of your R. chain.

I find 10 Mhm: the only common value easy obtainable in 1 watt at most electronic shops.

Regards
Raymond

#### Obmar

##### New Member
Rodalco,

Thanks a lot Raymond. I will do exactly that. When I get it done I shall let you and all know the results.

Thanks once again and regards.

#### things

##### New Member
1mm of spark is actually about 1.1kv