Comments on: Creating 1Tera-Ohm divider at 100kV

By: George Kontopidis

George Kontopidis — Wed, 10 Oct 2012 02:54:18 +0000

Your are absolutely right for DC. One of my reasons to make this divider was to capture the discharge voltage from a fully charged TV tube. The discharge rate was expected to be 1000 Volts per nanosecond and I needed a “balanced divider” with high frequency response. At the end, my experiment did not work, despite my FET probes of the oscilloscope — the bandwidth was too small for any level of HF measurement. Well… next time…
Regards

By: Howard A. Landman

Howard A. Landman — Tue, 09 Oct 2012 14:51:49 +0000

I don’t understand why parasitic capacitances would be a problem for DC measurements. Under DC conditions, capacitors don’t source or sink any net current, they just stabilize the voltages that are already there. To first order, you can just ignore them completely.

For AC, of course, they would be a serious problem.

By: George Kontopidis

George Kontopidis — Wed, 04 Jul 2012 02:02:38 +0000

I recommend to buy from Mouser the

MOX94021007 (1GOhm),
MOX94021006 (100MOhm),
MOX94021005 (10MOhm),

all are rated 45kV of 1% tolerance. They are about $20 a piece.
The MOX750 is 10GOhms, also 1% for 7.5kV for about $10. To make a TeraOhm, you will need to string 100 of them.
The MOX1125 is 100GOhms, 1%, 7.5kV for about $40. To make a TeraOhm, you will need to string 10 of them.
(I also sent to you a private email on this matter)

By: s s khichi

s s khichi — Tue, 03 Jul 2012 09:03:01 +0000

we want to purchase decade of 1M /1G/1T range from, 1 G ohm to 10 T ohm standard resitors woking at 1000/5000 volts for calibration of high resitance meters capable of measuring upto 10 Tera ohm at 5000 V……..pl contact us

By: Winfield Hill

Winfield Hill — Tue, 14 Feb 2012 15:43:24 +0000

(Copy of Win’s comments from his email)

1G and 10G resistors are fairly easy to come by. But you can buy
1T-ohm resistors from Ohmite (was Victoreen), e.g., their Maxi-Mox
thick film MOX-5-12 or MOX-5-13 types, rated at 50kV. These are
five inches long. Expensive, special order, and a long wait.

Mouser has 10 pieces of a standard 3″ long, 1G-ohm, 10-watt,
45kV resistor on back-order, MOX94021007FTE, only $19 each.
They stock the MOX-750231007, a modest 1G part, and the 10G
MOX-750231008, both 1″ long, but only rated at 5kV. Sigh.

Ohmite also makes ¼” 3kV chip resistors to 50G, but Mouser only
stocks low-voltage 10G versions in 0805 and 1206. Expensive.

The highest-value low-cost SMT types I’ve found with distributor
stock are the Vishay Dale 200M and 470M parts, in 0603 and 0805,
very cheap but only rated to 75 and 100 volts. Mouser will order
KOA Spear HV732ATTD476J parts, 47M in 0805, rated at 400V.
100 of those would make a 5G 40kV wand resistor for about $20.
At that rate the MOX94021007FTE doesn’t look so bad at $19,
allready assembled! if you can live with 1G instead of 5G. Hey,
maybe you prefer 1G for faster discharge?

Have you explored electrometers? These can measure high voltage
with capacitive rather than resistive dividers. I have an old Keithley
probe for doing this. You can make your own with the appropriate
electrodes and shields, and fA-sensitivity op-amps, like the LMC6041.
I’m hoping to find time to write about that when we do chapter 4x.

Speaking of high-value resistors, to 10^12 ohms = 1T, one way I’ve
found to get them is in old Keithley electrometers, cheap on eBay.