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26th April 2011 21:17

Alan Yates wrote...


Indeed good idea. I've been doing similar to measure very tiny currents (pA) for ion chamber experiments and semiconductor leakages. Very high input impedance followers are required to buffer the capacitance into digital measuring equipment - I've been using JFETs, MOSFETs or some of the more amazing op-amps available now days with fA bias currents!

JFETs are very easy to use, you can even discharge the capacitance by pulling the source below the gate and forward-biasing the diode. This works well with a microcontroller pin which can be tri-stated and read with an ADC until a certain voltage is reached, then driven low to discharge the gate. You can even use just a digital IO pin and the logic transition voltage as the set-point. I've used this without a follower at all to make a sensitive light meter using nothing more than an Atmel tiny13V, a photodiode and a resistor. Some software in the MCU counts the charge-up time and logs it over RS-232 back to the PC. I was watching the decay rate of glow-in-the-dark phosphors using this an a photodiode.

Using MOSFETs required a reed-relay to do the reset - which worked but didn't offer much better leakage than JFETs. The simplicity of the JFET circuits for now has won out. :)

Using a NE-2 is a great idea for high voltages. You could read the flash using a phototransistor. I have observed the breakdown voltage of NE-2 bulbs is quite sensitive to ambient light - yellowish or shorter anyway, red LEDs do not seem to make any difference - no doubt the photoelectric work function of the electrode material at work? The light sensitivity could be avoided by keeping the NE-2 in the dark. As you say it needs to be kept clean, and feed-throughs into the dark box would need to be low-leakage too. Actually I've heard some NE-2s are leaky already with radioactive isotopes in the electrodes or gas to set their breakdown voltage. The ones I got in quantity recently do not appear to be radioactive to any appreciable degree.



8th March 2011 19:44

Jeff Furman wrote...

Alan, you might be able to estimate the charging current by connecting the neon bulb directly to the terminal(coke can) and ground. each time the bulb flashes, it discharges the terminal from the bulb's breakdown voltage (maybe 60 to 90 V) until the terminal voltage drops to the bulb's extinction voltage (20 to 50V lower than the breakdown voltage.) (Putting the bulb in an ordinary relaxation oscillator circuit allows measuring your bulb's characteristics with a scope.) Your estimate of the coke can's capacitance to ground, the delta V of the bulb, and the bulb's flashing rate is enough information to measure the charging current at this low terminal voltage. A polystyrene (very low leakage) capacitor in parallel with the bulb can be used to measure the can's capacitance using the formula for parallel capacitors (this added capacitor value must be known.) It reduces the flashing rate (inversely proportional to the total capacitance) assuming the charging current remains constant. The ratio of the flashing rates before and after adding the capacitor and the capacitor's marked value are all that's needed for the terminal capacitance calculation assuming delta V of the bulb and the charging current are constant.

The neon bulb with bare leads needs to be clean enough to avoid a "low" resistance leakage path across it. This might prevent building up sufficient voltage to breakdown the bulb. Of course, a leaky bulb still works when you draw a spark to it through the insulating (until the spark) air, so the terminal voltage can increase without leakage from the bulb.

25th January 2009 23:17

Alan Yates wrote...


Thanks, that would be most appreciated.

I once enquired locally about getting 500 mm diameter hemispheres spun from Aluminium (about 28 pF capacitance - about 200 mJ at 120 kV). It wasn't too expensive, but I never went ahead with it.

I figure some of that big elastic band thing that people use for aerobics would make a pretty good belt. I have the bearings in the junkbox and could use the coronatron supplies I have in the junkbox as a charging source. PVC is a fairly good insulator for the pylon. The rest if just a matter of metal and wood work and finding a suitable motor, probably from an old fan. It is really just the hemispheres that are the part that stopped me scaling up from this little toy one.



25th January 2009 22:44

Mike wrote...

Hi Alan

I have some old plans for building van de graff generators, In the the next week or so I'll scan and email them to you.