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hv probe
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vk6zgo:

--- Quote from: m3vuv on October 30, 2019, 02:53:11 pm ---well,thanks for the advice,yet again waffle without answering the question that was asked,im using a string of 10 resisors of 1megohm in an old hv probe body for the main r1 and a 1meg resistor for r2,hope it should be a 10:1 ratio?,just waiting for some fiberglass to set b4 trying it.

--- End quote ---
Read, & inwardly digest:-
https://en.m.wikipedia.org/wiki/Voltage_divider

What you have is an 11:1 ratio, so for an ideal DMM with infinite input resistance, you will read, (for 2kV),  181.182 volts.

But wait! there's more!

You don't have an ideal meter, so your DMM looks like 10 Megohms in parallel with R2, reducing the resistance of that part of the voltage divider to 0.9091Megohms.

The voltage divider is now a 10.9091: 0.9091 ratio, so 2kV will read as 166.668 volts.
That is 0.0833 times the voltage you are probing, fitting well with the figure of 0.833 volts tggzzz predicted for a 10 volt source.

There are two ways to adress this problem;-
(1) Determine the correct values required for R1 & R2 to give you a 10:1 ratio.

(2) Just go with what you've got, & apply a correction factor to the result, determined by your tests on an accurate voltage source.
m3vuv:

--- Quote from: sourcecharge on October 31, 2019, 06:30:33 am ---
--- Quote from: m3vuv on October 30, 2019, 10:27:44 am ---Hi,ive a fluke 85v and want to build a high voltage probe for it,the input inpeadance is 10Mohm on the 1000v dc scale,i want to measure the grid bias on a scope,ie circa -1900v,whats the best way of doing it,looking at building a 100:1 divider anyone know what value resistors to use,also would a 10:1 divider be better,or would that load the dut to much,cheers Paul m3vuv.

--- End quote ---

Why would you want to build a probe for that low of voltage?

They readily make probes for about 100 bucks that go up to 2000v at a 100:1 ratio for oscopes and you can use an adaptor for the banana plug to CNC connection.

That way, you can use it for both an oscope (if you have one or are planning on getting one) and your Fluke 75v dimm.

Here's an example:
https://www.digikey.com/product-detail/en/b-k-precision/PR2000B/BKPR2000B-ND/3783011

PR2000B
Datasheet
https://bkpmedia.s3.amazonaws.com/downloads/manuals/en-us/PR250B_manual.pdf
Digi-Key Part Number    BKPR2000B-ND    
Manufacturer    B&K Precision    
Manufacturer Part Number    PR2000B    
Description    OSCOPE PROBE X100 200MHZ 100M    
Manufacturer Standard Lead Time    8 Weeks    
Detailed Description    100:1 Voltage Probe 200MHz 48.000" (1219.20mm) Black and Red Oscilloscope Probe Test Lead 100M Input Resistance 2000V

This one has a 200 Mhz bandwidth!
 :popcorn:

--- End quote ---
Reason :A,im out of work and cant afford to buy one. Reason B,i already have a 1000:1 Probe,was rated at 30kv but the hv resistor is faulty,im basicaly building a new guts to fit inside the body of it!.
sourcecharge:
Get a job, da na nah na, da na nah na nah, get a job...

That sucks, hope you find some luck soon.
m3vuv:
Well,looks like i have sorted it as near as  dam it,swapped and experimented with resistor r2,after a few trys i finished up with r2 at 1.235mohm,on the 240v ac mains  measured without the probe i read 241.8v on my meter ac,with the probe attached i get a reading of 239.7v ac,measuring the anode voltage of my yaseau fl2100z i read 245v,it should read 240 in an ideal world ie x10,its within 50v at 2.4kv,thats near enough for what i need,73 m3vuv.
ArthurDent:
Well the old saying "consider the source" applies here as well. There is a very good reason why HV probes designed to measure CRT voltages use divider resistors 1000 time greater than what you have chosen. The testing you have done so far is meaningless in this application. You should follow the advice that other posters have given you and wait until you understand what you are trying to do.
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