# Measuring High Voltages with a low voltage meter

Hello! I have a 0-20kVDC variable power supply that I have just recently built. It works great other than I do not know what voltage I am putting out to test things. I do not have a 0-20kVDC meter, but I do have a 0-20VDC meter. I was thinking about using a simple voltage divider to use the 0-20VDC meter to measure 0-20kVDC. Would this work? If so, how would I calculate the resistor values?

Should work, use high resistor values so current is low, perhaps 1mA. Calculate also the power.
google - high voltage resistor divider

Joseph3502:
Hello! I have a 0-20kVDC variable power supply that I have just recently built. It works great other than I do not know what voltage I am putting out to test things. I do not have a 0-20kVDC meter, but I do have a 0-20VDC meter. I was thinking about using a simple voltage divider to use the 0-20VDC meter to measure 0-20kVDC. Would this work? If so, how would I calculate the resistor values?

R1 on top, and R2 on bottom. Mid-point of R1 and R2 is the output.
20kV ... to R1 .... to R2 .... to GND.

Want 20V DC max at the divider's output. So the 20000 V needs to be scaled (by the divider) down by a factor of 1000.

20V = 20000 * R2/(R1+R2).

R1+R2 = (20000/20) * R2.

Choose R2 to be something.... eg. 100000 ohm.

R1 could then be something like 100 Mega-ohm resistor ..... like a 100 Mega-ohm 25kV; 5 watt resistor.

I'm thinking that extreme care of our health should be taken when working with such high voltage. This is to avoid becoming a statistic.

ted:
Should work, use high resistor values so current is low, perhaps 1mA. Calculate also the power.
google - high voltage resistor divider

You will need to use multiple lower value resistors because resistors have a maximum voltage rating of a few hundred volts. And keep them in a row if possible, so there is no arcing between resistors.

Paul

Having been there myself, I would recommend high voltage resistors designed for such voltages.

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.

Try here.

Since this has only been mildly mentioned, I feel I should emphasize the importance of proper handling of such high voltages! Especially if this supply can deliver currents of 50mA or higher -- BUT, any high voltage should be respected and high voltage protocol should ALWAYS be followed!!

If you don't know how to handle high voltages, or how to conduct yourself around high voltages -- in other words, if you don't know how to be safe around high voltages, then make sure you learn how BEFORE YOU MESS WITH SUCH HIGH VOLTAGES!

ted:
Should work, use high resistor values so current is low, perhaps 1mA. Calculate also the power.
google - high voltage resistor divider

Right, Ted, and what if this 20kV supply can't handle 1mA (not unreasonable since that would be 20 Watts)? You're certainly making a lot of assumptions.

Joseph3502:
Hello! I have a 0-20kVDC variable power supply that I have just recently built.

20kV can jump 1mm or so across a spark plug gap under a petrol engine's compression; I shudder to think how far it can leap in free air.

Joseph3502:
I am fairly new to arduino at age 15.

I say this with all due respect, but it's a serious question: Do your parents know you are playing with lethal voltages?

Joseph3502:
Hello! I have a 0-20kVDC variable power supply that I have just recently built. It works great other than I do not know what voltage I am putting out to test things.

Then the obvious question must be, how can you know 'It works great' when you are actually unable to measure the output voltage ?

srnet:
Then the obvious question must be, how can you know 'It works great' when you are actually unable to measure the output voltage ?

I was wondering that too: perhaps it's making nice sparks but hasn't caught fire yet?

High voltages are usually measured with a voltage divider with the meter itself as the 'bottom' resistor.

A common DMM has a 10Megohm 'resistance'.
Adding a 90Megohm resistor in series with the red lead changes a 200volt meter into a 2000 volt meter.
You need a 990Megohm resistor to change a 200volt DMM into a 20kV meter.

90Megohm can be made with nine 10Megohm resistors in series inside a piece of heatshrink,
but 990Meg with common resistors is not practical/safe.

20kV should only be measured with "high voltage probe for a DMM" (Google it, pictures).
Leo..

I usually measure HV by how wide a sphere to sphere air gap is. Use spherical electrodes much larger than the gap - I have a pair of 1 foot hollow metal balls. Under these circumstances, and a 40% relative humidity, you can expect 30KV DC to jump a centimeter of air.

Here's nearly a megavolt giving it up for my hand held spectroscope:

ReverseEMF:
Right, Ted, and what if this 20kV supply can’t handle 1mA (not unreasonable since that would be 20 Watts)? You’re certainly making a lot of assumptions.

Red post #1 again, I didn’t make calculations I suggested to make them by OP.

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

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...

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

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

For pointy electrodes, the multiplier is different.

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.

If the electrodes are pointed, the spark will be longer.