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### Topic: Measuring High Voltages with a low voltage meter (Read 1 time)previous topic - next topic

#### Joseph3502

##### May 20, 2018, 02:51 amLast Edit: May 20, 2018, 02:52 am by 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?

#### ted

#1
##### May 20, 2018, 03:17 am
Should work, use high resistor values so current is low, perhaps 1mA. Calculate also the power.
google - high voltage resistor divider

#### Southpark

#2
##### May 20, 2018, 05:11 amLast Edit: May 20, 2018, 05:19 am by Southpark
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.

#### Paul_KD7HB

#3
##### May 20, 2018, 05:38 am
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

#### JohnRob

#4
##### May 20, 2018, 05:46 am
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.

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

#5
Try here.

#### ReverseEMF

#6
##### May 20, 2018, 07:23 amLast Edit: May 20, 2018, 07:29 am by ReverseEMF
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!

"It's a big galaxy, Mr. Scott"

Please DON'T Private Message to me, what should be part of the Public Conversation -- especially if it's to correct a mistake, or contradict a statement!  Let it ALL hang out!!

#### ReverseEMF

#7
##### May 20, 2018, 07:27 am
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.
"It's a big galaxy, Mr. Scott"

Please DON'T Private Message to me, what should be part of the Public Conversation -- especially if it's to correct a mistake, or contradict a statement!  Let it ALL hang out!!

#### gwareloth

#8
##### May 20, 2018, 07:47 am
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.

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?

#### srnet

#9
##### May 20, 2018, 07:55 am
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 ?
\$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

#### gwareloth

#10
##### May 20, 2018, 08:04 am
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?

#### Wawa

#11
##### May 20, 2018, 08:57 am
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..

#### ChrisTenone

#12
##### May 20, 2018, 09:02 amLast Edit: May 20, 2018, 09:05 am by ChrisTenone Reason: fix the %&#! picture
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:

Atmosphere carries combustion vapors to places where they will do good instead of harm - Mike Faraday's 'History of a Candle': https://www.youtube.com/watch?v=6W0MHZ4jb4A

Whoops

#### ted

#13
##### May 20, 2018, 12:47 pm
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.

#### ted

#14
##### May 20, 2018, 12:54 pmLast Edit: May 20, 2018, 12:55 pm by ted
I agree with post #12, but not necessary - sphere

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