Voltage divider with calibration - for measuring Geiger counter high voltage

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2 months 6 days ago - 2 months 6 days ago #6984 by Simomax
Some time ago I threw together (what I thought was) a 1000:1 voltage divider for measuring the HV on Geiger counters. This consisted of a Dale 1% 1G ohm resistor (R1) mounted all nice in a box with big chunky gold banana connectors and another quality 1% 1M ohm resistor (R2) connected to a banana socket and then a bunch of croc leads flying about. It worked, but wasn't right. I hadn't accounted for the multimeter's own internal resistance (known as a burden on a circuit). I'm pretty sure my multimeter is a 20M ohm resistance, or thereabouts, so that made R2 actually 950K ohm when the meter was in circuit. So a 1000:0.95 divider. The voltage shown on the multimeter was not that of the HV on the counter. I did some basic testing with and without the divider to confirm this.

As it is essentially a parallel resistor network, a solution was to add some resistance to R2. Increasing that will offset the meter's resistance. If the meter is 20M, then adding 50K to R2 should compensate for this. I decided to build a better divider on proto board that easily plugs in and unplugs from my meter.

 

I added a 100K trim pot and set it so when the 1M and pot were measured in series it read 1.05M. I calibrated it using 338VDC from inside a switch mode PSU and it worked really well. I first measured the voltage without the divider and made a note, then put the divider in circuit and whilst reading the voltage I turned the pot until the voltage was the same as without the divider, divided by 1000. It seems to be working very well. I have included the circuit below. *** I have just noticed that the R3 (trim pot) value in the schematic is 50K. It should read 100K. ***

 

 

 
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Last edit: 2 months 6 days ago by Simomax.

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2 months 5 days ago #6985 by nu3e
This is a nice test and measurement accessory.

There's no danger as-is because the 1G resistor will limit the current, but it would be better if you connect R3 pins 2 and 3. Potentiometer wipers can become intermittent, especially as they age. Then if the wiper loses contact with the resistive element, the voltage divider won't be left floating. This is common engineering practice, e.g., in audio volume control circuits,
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2 months 5 days ago #6986 by Simomax
That's a good idea nu3e. I have seen it done both ways in circuits but never really understood one way or another. I've never really thought about it in all honesty. It makes perfect sense. I shall make that change when I next have my iron hot.

Something I forgot to mention was that R3 could be swapped for a higher value. Say something along the lines of 500K. this would allow the divider to work with lower resistance meters. Using a value such as 500K should allow for meters with an internal resistance as low as 3M, and up to infinity.

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