HV AC Current Measurement

[alan sailer]

I've got a measurement requirement that I want to solve as simply as possible. I have an old Geissler (?)
tube that I want to power up. The tube is supposed to run at ~1mA. I just bought a small neon tube
power supply 5kV @ 30mA. The tube is supposed to start at 5kV and run at ~1kV. I hooked it up and
it lights up nicely but I want to make sure I don't burn it out.

The tube is small and it seems reasonable that it is rated at about a watt.

The power supply is running at 30KHz. My first thought was to use a cheap DVM but I'm pretty
sure that they don't give good results at higher frequencies. I have a diff probe but it's rated to
about 1kV, so no go. A series resistor current monitor seems like a good direction but I don't have
much real experience in probing stuff that is sitting at high voltage relative to ground. I also don't like
killing equipment figuring out what works.

Any advice?

[klugesmith]

How about a primitive Rogowski coil with electrostatic shielding? Frequency is known, so you don't need to electronically integrate the RC output.

Or you could put any little neon-sign element or fluorescent tube (even from curly CFL) in series with the Geissler tube, so carrying the same current, and measure brightness of the metering lamp with a lightmeter or something.

How about, in series with the Geissler tube, a bridge rectifier made from fast diodes.   Feed the DC output to an analog milliammeter or little battery-powered panel meter.    Make whole thing compact & enclosed some kind of Faraday cage, so the floating AC milliammeter can ride along at whatever HV HF is present on the output of neon sign power supply.    Voltage on both sides of power supply probably does not exceed a few hundred volts when lamps are lit.   Rectifier diodes never experience more than a few volts, and never experience more current than the lamps.

[edit] The paragraph above is like what you suggested with current monitoring resistor.   Rather than worry about instrument performance with high frequency, and with HF HV common mode voltage: do the HF rectification yourself, with rectifier before the current sense resistor.  I did that & had whole thing inside an Altoids box, with case tied to one end of the current shunt resistor and panel meter.   The AC inputs are always within a few volts of the case, so no special insulation except between case and the workbench.

How bout a plain little NE-2 glow lamp in series?  You can determine its brightness curve, and maximum current that doesn't scare you, using 120 V 60 Hz -- a frequency at which you trust your good DMM in AC mA mode.

Finally, you can limit the current using a HV capacitor in series.   But if the capacitor is too small & drops too much voltage, the power supply might think the tube didn't strike, and shut down after a few tries.

Just remembered: might some Geissler tubes light differently with positive, negative, and AC current?  That's the case with some decorative  lamps with phosphor-painted metal figures inside.

[davekni]

I've got a measurement requirement that I want to solve as simply as possible.

Simple depends on what parts you have laying around.  Presuming your goal is just to detect a 1mA threshold (above or below) rather than a full linear current measurement, I'd suggest something somewhat similar to Klugesmith's analog meter idea:
Make a bridge rectifier of fast signal diodes (1N914 or 1N4148 or BAT54 or ...).  AC inputs in series with lamp current.  DC output goes to a small filter cap (0.1uF or higher), a resistor, and an LED, all three in parallel.  Most newer LEDs are visible at 50uA.  Resistor and LED forward voltage define the average current where LED starts to glow (average being 0.9 * RMS current) .  If a more precise threshold is needed, add a zener (6.2V or higher to get good low-current performance) in series with the LED.  I've done this with known antenna ends to measure Tesla coil fields.
If you happen to have any NiZn ferrite cores around such as 4S2 material, you could make a current transformer.  At room temperature, such material is a good insulator.  Windings on opposite sides of core.  Wind the output side (for scoping) with ground end on outside for shielding.  Or, make a symmetric center-tapped secondary.  That way capacitive coupling from primary will cancel.

Finally, you can limit the current using a HV capacitor in series.   But if the capacitor is too small & drops too much voltage, the power supply might think the tube didn't strike, and shut down after a few tries.

Yes, limiting current is likely to be difficult.  Supply is likely to shut down at only 1mA load.  Any parallel load to keep the supply happy is likely to keep starting voltage too low.  Good luck experimenting!  I'm guessing you'll need to make your own supply.

[klugesmith]

Hey, Dave's DIY transformer idea suggests another approach.

Neon sign PS feeds primary of a transformer, whose secondary feeds your lamp with increased voltage and reduced current.
The neon sign PS probably doesn't care if loaded output voltage is 1000, 100, or 10 volts --- normal steady state operation with luminous tube is not much different from shorted output.

Even though you only need to transform 1 watt of power, you need to mind your volts per turn (proportional to frequency and core area) and HV insulation.   Probably not an after-dinner project.

Alan, if you can wait a few days I can send you a little 60-Hz 6.5-mA luminous tube transformer. Picture & I-V characteristic here:
https://fusor.net/board/viewtopic.php?p=80709#p80709
If its natural current limit is still too high, when primary voltage is just enough to light up your Geißler tube, you can add resistive or 60 Hz inductive ballast on primary side.

[MRMILSTAR]

Have a look at my posting describing my CW voltage multiplier. In that posting I explained how I measured the 28 Khz HV input to the multiplier stack (about 15 KV peak) using fast HV diodes, a smoothing capacitor, and a COTS HV DC voltmeter. Here is the link.

https://highvoltageforum.net/index.php?topic=1524.0

[alan sailer]

Thanks for all the replies. I will absorb ideas and see what I have that might fit.

Klugesmith,

I'd be interested in the transformer as another path. I'm assuming it's a standard 60Hz unit which would change the measurement landscape. Let me know what you would need for price/postage etc.

I am in no hurry, I'm currently recovering from eye surgery so am moving more deliberately than usual in the project arena.

Cheers.

[klugesmith]

The meters collection includes this specimen, with 500 uA 250 mV full scale, suitable to use with high frequency low-V, low-I rectifier as Dave and I have discussed.

Alan, I wish you good luck with the eye(s).
Will dig up your email from our discussion of pulse x-ray generators a couple years ago.
Remembered that my miniature "NST" got put into my "kolovolts" power supply, which was abandoned when found to be unsuitable for biasing a radiation detector tube (due to ripple & lack of regulation).   Its I-V curve is not a whole lot different from a regular 60 Hz NST operated with reduced primary voltage. With either 60 Hz transformer, you can turn down the short-circuit current as much as you like by putting ballast in series with primary.

[klugesmith]

Of course that meter, like Dave's indicator LED, would need a resistive shunt.   With a silicon diode as part of the shunt circuit, the meter's useful range could be extended to a couple decades of current-under-test.  The range compression at high current would be sensitive to temperature of the shunt diode.

High frequency rectifier diodes introduce no significant error in this thread's titular application, if they are plenty fast and have insignificant reverse leakage current.