Imagine their shock had you decided to speak up after they had discharged the capacitor through their hand.
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I provided Richard with a discharge probe that was designed to safely discharge high voltage capacitors. This was several months ago.
It was a string of resisters housed in a long piece of pvc, and further insulated with flexible tubing. There was a metal probe tip at one end and a long wire with an aligator tip at the other end.
The idea is that you connect the aligator clip toground on your circuit, you can then probe any capacitors to discharge them.
You could also string a few 10M resistors together and put that across the capacitor. That’d be a constant load and path for equalization between the plates when the system was off. You’ll need to get a resistor which is rated for HV or experimentally determine how many you need in series so the voltage drop across one doesn’t exceed the “creep” factor and arc across the resistor.
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Thank you. It’s come in useful several times since then.
Incidentally, PVC can catch fire when exposed to HV arcing.
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No thanks nescessary, just wanted to let @renkassa know that a system is already available to accomplish what he was suggesting.
Well, any plastic can catch fire if exposed to a hot enough arc. But if this is used properly, the only part of the probe that gets exposed to the actual arc is the metal tip, and then only briefly. The idea is to place the tip in actual contact with the capacitor plate and leave it there until the capacitor is discharged. You can determine when it is discharged by using that HV probe that I loaned you’ll with a multimeter, but usually a minute or three is enough.
I don’t remember what the resistance was on that probe I built, but it was designed to handle the power disappation when discharging a 1 uF capacitor charged to 25,000V, so it should have no problem with the energy levels your dealing with on your laser.
I think I measured it around 1.5 M-ohm.
Just for fun, how much current flows through a 50k-ohm resistance connected in series between ground and a fully-charged 15kV, 50 pF capacitor? What is the tau of this circuit?
Well, I will leave the calculation for total current flow and tau to someone who has more time today; however, the critical values are you have 300 ma of peak current, which isn’t really much of an issue; however, the real concern with a discharge probe is the power dissipation, which in your example would peak at 4,500 watts… a bit much for most commercially available resisters.
I figured you were working on a Tesla coil. It sure lit up the shop. 
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The neon sign transformer by itself isn’t too noisy, but when I connect it across the laser’s capacitor and spark gap, it gets really noisy!
So is the laser working?
No. Something happened while I was testing the electrical system, and it stopped discharging when the laser cavity is attached (it had been discharging across the laser cavity, but now it isn’t). It acts like something is shorting, which could be, as the circuit board capacity that I’m using is physically too small to accommodate the laser cavity. When I remove the laser cavity from the capacitor, the capacitor arcs across the dielectric and the spark gap; the capacitor is more than thirty times more powerful than necessary, and more powerful than we had anticipated when we specked the capacitor.
We haven’t attempted to evacuate the new laser cavity, yet, so we don’t know yet how it would perform. In theory, if the electrical system would discharge correctly across the laser cavity, the laser should function without drawing a vacuum on the tube.
We are right on the edge of getting the laser to work, but we need to correct several little things.
Meanwhile, every time I plug the capacitor to the transformer, everyone in the warehouse jumps. That thing is scary noisy! I had a prickling feeling when I stood next to it discharging last night.
Perfect sound and light show for any upcoming Halloween festivities!
You bet! It scares the daylights out of me!
The human body has somewhere in the neighborhood of 50k-ohm resistance (though, it can be as high as 100k-ohm or as low as 5k-ohm). The neon sign transformer we are using produces 15kV (I measured it, using the high voltage probe loaned by Walter; it’s 7.5 kV on each side) at 30 mA. The capacitor is about 50 pF, as determined by David.
tau = RC ~ 50k-ohm * 50 pF ~ 2.5 us
That means that about 25 microseconds (or, 10 tau) after I grabbed hold across the plates of the capacitor, I completely discharged it, experiencing a peak current of 300 mA and a momentary 4.5 kW of electrical power (for about 2.5 us). Not the smartest thing I’ve done, but, fortunately, probably not long enough to cause damage… I hope. Anyway, it didn’t hurt; it just shook me.
Replacing my body with a 1.5 M-ohm resistor gives tau of 75 us, meaning that less than a millisecond after using the discharge probe, the capacitors should be discharged.
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Ahhh, this discussion really takes me back to when at 14 I built this very style of Nitrogen Laser. Unfortunately, it’s so long ago that I don’t have any hints or recommendations.
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I strongly encourage that you maintain physical contact between the grounded discharge probe and the capacitor for at least a full minute, and longer if measurements indicate otherwise.
The estimates that the formulas you used are basically fine; however, there are a number of assumptions embedded in them. Most notably that the resistance will remain constant over the discharge period. The problem is that with real world components, particularly when you putting hundreds of watts of power through them they heat up. Resistance in components change with temperature.
It is better to be safe, and keep contact for at least a full minute.
BTW, the resistance through your body was likely MUCH higher then 50K, because the resistance between your body and earth ground is in series. And if you were wearing shoes, it should have been quite a bit higher. The latter is why standing in water and getting shocked is so much worse. You only have your bodies resistance to deal with at that point.
Most of the trouble we are having at this point stems from my decision to pull a Tim Allen for “More Power,” by making the laser cavity about a yard long. Jim Small had suggested in his “Scientific American” article that it should be possible to scale the nitrogen laser up to a meter long, resulting in megawatt beams, but the plans were for devices half that size or less. The problem with increasing the size is that the material used (plastics) aren’t dimensionally consistent at large sizes, and everything is more difficult to handle. Also, it’s difficult finding double-sided circuit board more than two feet long.
Agreed. Capacitors have the bad habit of “charging back up”, which is caused by:
Lesson: leave a resistor (100k, 1M, whatever) across those caps in storage or when you think you’ve discharged them already.
Unless I am gravely mistaken about the way lasers work (and I could be), you would need a 15,000V power supply capable of delivering in excess of 60 AMPS of current to provide a megawatt of power…
Which raises the question in the context of the author’s claim. Does increasing the size of the lazing chamber require an increase in power provided to the circuit? Simple common sense would seem to indicate it does, but I have no experience with lasers so I will leave that as a question for others.
On a side note, my basic practice when working from someone else plans for something I haven’t built before is to replicate them as closely as possible first, then (and only then) start tweaking the design.
It’s a pulsed laser, so we would only need pulses of high current, and, considering that the laser is less than 1% wall-plug efficient, we would need much more than 60 amps. The key to making a nitrogen laser (or other three-level laser) work is exciting the lasing medium to its upper laser level quickly; otherwise, population inversion wouldn’t be possible. For the nitrogen laser, the excitation needs to happen in less than about 20 ns (maybe much less, depending on the specific conditions). That’s why we can get away with using a pF capacitor as the current source. If we wished, we could use a 9V battery, instead of a neon power supply plugged into the wall outlet, but the charging time would be much longer (dropping the cycling rate from 120 Hz to 6 Hz).
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