MightyOhm.com

Jeff,
I just started getting SPAM email sent to the email address I used to register here. It is a unique address, used ONLY to register on these forums.
William

I'm doing more testing but I really don't think the swapped resistors cause any problems. They might cause the HV supply to vary more with battery voltage, but on the flip side they make the circuit more tolerant of low battery voltages. I'm going to play with the circuit a bit more but since every...

Jeff, are these back in stock yet?
Thanks,
William

I picked up an Anton 6107 on fleaBay for around $52 with shipping. Modified the kit, reversing the terminals so it can poke out on the left side. Replaced the anode resistor with 1 M ohm, and the VR with a 20 ohm pot so I could get it up to 700 V. Seems to work quite well. With an AM241 source (from...

If my calculations are right, for models 117 and 289 (the two that I have) the lowest range is 10.6 M and the others are 10 M ohm.

William

The Mega has a 3.3 V supply which is rated at 50 ma. If you don't have anything else using it, that's more than enough to run the geiger counter. Why not just use that?

William

Aha! Found this on Fluke's user forums: the HV probe manual states that: "* The input impedance of Autoranging Fluke handheld digital multimeters varies as a function of range. The only range that deviates significantly from 10 MW is the 3V (Models 21, 23, 25, 27, 70, 73, 75, 77) or 4V (Models ...

Interesting. Put both meters into manual range. On the lowest range, I get 4.0 V. On any of the higher ranges, I get 3.75 V. So there IS a change in input impedance in the lowest range, but if I do the math right, it's a 6% increase, not a decrease. Interesting that both meters exhibit the same beha...

I just went over it again. Checked Q1, Q2, D1, D2, L1, C3 is the smallest of the caps. Except for R5/R6 it looks just like the image on Flickr. I have two Fluke meters, a 117 and a 289. Specs for both give 10M Ohm input impedance for DC V. I will try putting the 289 in manual range and see if that m...

Yes, fresh alkaline. Tried 2 different sets to be sure. Wouldn't low battery make the voltage lower? Original 15 ohm r8 measured right at 14.9.