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hv probe
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m3vuv:
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.
tggzzz:

--- 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 ---

With anything like this the devil is in the details, and the devil can have a very nasty bite.

If you don't know enough to be able to calculate resistor values for this, then it is highly improbable that you have sufficient knowledge to get the other details correctly implemented.

Do yourself a favour, and buy an HV probe suitable for your meter.
m3vuv:
well,thanks for the advice,yet again waffle without answering the question that was asked,im using a string of 10 resisors of 1 megohm 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.
tggzzz:

--- 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 1 megohm in an old hv probe body for the main r1 and a 1meg resistor for r2,hope it should be a 10:1 ratio?,

--- End quote ---

QED.

You are wrong, and where EHT is involved you really shouldn't be guessing.

There is no way that I am going to be party to you doing something that might injure you. On my conscience, and all that.



--- Quote ---just waiting for some fiberglass to set b4 trying it.

--- End quote ---

What tests are you going to do to prove that is a safe construction?

BTW, if you manage to create a voltage divider with the desired ratio, then you should read and understand this bit of the upcoming TAoE X-Chapters book: https://www.dropbox.com/s/dli62uyuldr7z5q/1x.2_Resistors_DRAFT.pdf?dl=1

Sections 1x.2.3 and 1x.2.7 are directly relevant.
m3vuv:
well,its a 30kv probe body for starters,so insulation should be good for 2ky with ease,the resistors are mounted on a strip of copper clad board  with 4mm gaps between each pad where the resistors are soldered on to it,the resistor string is made up of 8x 1meg 2 watt resistors and 2 500k resistors againe 2 watt rating,had to use the 500k ones to get it bang on 10 meg ohm,its an old fluke hv probe,was duff s the 100g ohm resistor was open,the fiberglass is some p40 around the handle as it had a burn mark from a soldering iron,thats about 8mm thick,so far for a 1v meter reading i have to set my psu to 11.3 volts,i think r2 is slightly off as its the original  resistor.
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