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

 

[Tony Stewart]

Lexmark laser printers use 15 kV on the drums and use a sine wave stepup signal transformer to create this then rectify and regulate with a long string of 1/4W 10M~100M resistors back to an LM324 to regulate the output like AGC. You can string these inside a DIY plastic wand with a reinforced tip with a total R of 1 GOhm and make this suitable for measuring 50 kV using 1kV/mm for a clean wand with air at 3kV/mm. Surface contamination reduces the withstanding voltage.

Now you can get 1Gohm HV resistors at low cost in stock so you may use fewer in a string https://www.digikey.ca/en/products/detail/te-connectivity-passive-product/RGP0207CHK1G0/2389890

When we used these in the 80's outside a clean room to generate ions with needle inside a 1cm half cup, they created so much RF that it interfered with our magnetic servo writers inside the clean room (for HDD Mfg) , so I asked maintenance to remove it.

 

[EdStraker]

Thanks for the reply, what would this give me as a reduction factor? 10:1, 20:1, 50:1 ?
So I assume a standard 2 resistor VD is not a safe idea?

 

[Tony Stewart]

Not unless you have a long expensive resistor...

If you measure your DMM impedance it may be 1 Meg or 10 Meg then 1Gig is 100:1 ratio and < 1W dissipation (V^2/R)

 

[EdStraker]

the spec sheet says:
DCv = 20k ohm/V input resistance

 

[Tony Stewart]

then you choose max scale and use that V * 20k = Rin for the analog meter but use that to measure the DMM on 600 V scale so that 200V = 20 kV using 99:1 R ratios for 100:1 or whatever you prefer.

 

[EdStraker]

Wait, you lost me.....
use what voltage?

those specs were for my analog meter. Digital meter is 600V max / 10M impedance

 

[Tony Stewart]

Yes Whatever voltage scale your analog meter is multiple by 20k is it's input resistance while the DMM is 10M

I forget what Analog Vdc scales are used in Simpson meters and you didn't specify.. So if you used a series string and a shunt in parallel with meter, you have to compute the new ratio or tell me the voltage and thus the resistance to know what you need

 

[EdStraker]

Ok, so sticking with the 600v / 10M DMM........
The generator has a range of 15k - 20k output. Since I have no idea if its already set at Max or not. I need to protect the meter while dialing it down, or up as the case may be.