Voltage divider for high volt measurement

[EdStraker]

Trying to get a measurement from a 20kV Negative Ion Generator that is adjustable from 15kV- 20kV. My digital meter is 600v Max. and my analog is 10kV Max. Want to set it to 17k - 18kV

Was thinking the only solution was a Voltage divider. With that said, I need some advise on resistor type 1/2 Watt Carbon / Film? Ceramic power resistors? (I would imagine 1/4 Watt's are out) so I can get the the voltage down to a measurable range.

Or should I just not make the attempt? I figured if the was SAFELY possible, was thinking of making a divider bank similar to a selectable resistor bank.

The only specs I have for the unit is:
Input voltage: DC 12V
Output voltage: 15000V-20000V adjustable
Rated power: 5W

 

[MrAl]

Not an issue in this case though, as his analogue meter already has a higher input impedance (20Mohm on a 1000V range) than a digital one (10Mohm on all ranges). Something to bear in mind when calculating the resistor values. Perhaps it would be a good idea to make it for a 10Mohm digital meter, then add a 20Mohm resistor across the 20Mohm analogue meter to make it 10Mohms - making it more versatile.

He also needs to bear in mind the voltage ratings of the resistors, you often need to use higher wattage ones as they generally have higher voltage ratings.

Hello,

The amplifier idea is used to raise the impedance of the whole thing so that there can be a lower current draw from the source being measured. However, the calculated 50ua may not be too much of a load on the source being measured here. If it is not, then something like a 380M series resistor may be enough. If it has to be lowered, then we would need an amplifier unless we don't mind multiplying in our heads.
As it stands, the total resistance would be 400Megohms, and if we want that to read easily on a meter of 20M then we need to multiply the readings by 20, which would mean multiplying by 2 and then by 10.
So 10000v would measure as 500v, and multiplied by 2 and then by 10 would give us the 10kv. That's with a 50ua current draw.
If we use 780Megohms, we could multiply by 40 (by 2 by 2 and then by 10) to get the reading, unless again we want the meter to read exactly 10x. The current draw would then be 25ua.
To get to 10ua, we could use 1980Megohms and then multiply by 10 to get the actual reading.
If we don't use an amplifier, we would have to multiply by 200, which could be multiply by 2 and then by 100, which is not that hard to do really.

So it all depends what you want out of the meter. An almost direct reading meter or one where you have to multiply the reading in your head by some factor. Also if you want to lower the current drain on the voltage source being measured and you want a more direct reading.
If you don't care about multiplying in your head, then an amplifier is not needed.

I highly recommend anyone building one of these things that they look around the web and see what is being done already. Some of it involves protecting the meter and some protecting yourself from fatal shock.

 

[MrAl]

This kind of accuracy wasn't important, it was just to get an idea where the unit is set from the factory and kind of get it in the middle of the adjustable range. I was hoping for a quick and dirty (safe) method of doing it was all. I really didn't want to get into a construction project that in all probability will never have the need for ever again.

This was more of what I had in mind, but again outside my budget. BK Precision HV44A
View attachment 148885

Hello again,

I have to also highly recommend anyone building one of these things that they look around the web and see what is being done already. Some of it involves protecting the meter and some protecting yourself from fatal shock. High voltage is tricky because it can arc over in ways we do not normally see or even think about day to day.

Yeah these probes can be very expensive especially for one time use.
Maybe you can find a used one somewhere like on eBay (I don't use eBay though).

 

[danadak]

One must consider pollution, air, humidity, otherwise when discussing real
high V R's. How often should one clean these probes, R chains, and what chemical
to remove mankinds Air Pollutants covering these supposedly very high
R's.

Walking around summer or winter here in northeast one can hear arc leakage
even on glass insulators on HV power lines.

 

[Nigel Goodwin]

Hello,

The amplifier idea is used to raise the impedance of the whole thing so that there can be a lower current draw from the source being measured. However, the calculated 50ua may not be too much of a load on the source being measured here.

It's obviously not an issue, it's already more than high enough, with a typical analogue meter been twice as high as a digital one.

 

[Tony Stewart]

Cheap, dirty and safe is a long string of series resistors of 1GOhm and shunt R of 10k.