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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Sounds good. If the resistance value is accurate (120e6) and the current reading is accurate (600e-6) then the average voltage is I * R = 72 kV. If you want to directly measure average voltage on your x-ray tube, this contraption must go in parallel with the tube. You implied that resistor and meter will go in series with the tube. Then if the voltage source is fixed, the tube operating point will be along a line between 72 kV at no current, and no voltage at 600 uA. Measurable by the meter, and adjustable by filament temperature.
If you are making electrical measurements around a HV-energized x-ray tube, an economical safety element (along with shielding and distance) is an on/off switch for the high voltage source. A plain old power strip with a rocker switch served Henry and me last weekend, in an ad-hoc x-ray lab session. You should be able to take a reading with power on for 1 second.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
600 uA is a lot of current to drain just for measurement purposes. A series string of precision resistors would not arc so easily and would drain less current.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
I know this isnt the best way for measuring voltage, as its intended purpose is to determine how much current the tube is drawing during operation. I was just hoping to kill two birds with one stone.
Once I put the tube in, one end of the meter will be grounded while the other end will connect to the cathode/filament of the tube. The 120Mohm resistor is on the anode end of things.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
BTW, you should never rely on the internal resistance of the multimeter as the bottom half of the HV divider.
If the internal meter resistance ever "opened" up, you would have full voltage on your meter which can lead to safety issues, or at the least destroy your meter.
If you are doing this, then you need to float and put your meter where no one can ever touch it.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
hmm,
i did have an idea to use a shielded internally powered I2C A-D, at the end of an optical fibre. I believe this is how they measure line voltages in the field as it is very reliable.
its feasible to use multicolour LEDs as the signal lines, as each will only respond to its counterpart.
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Measuring HV with mA meter?
