The double-sided copper-clad circuit board can hold and has held a charge for several days after being charged by the neon sign transformer. Do not touch both sides of the capacitor plates. Especially twice in one day.
That will be all.
Good night.
Did your hair get curled?
My back between my shoulder blades twitched.
I think we need a grounding rod to, perhaps, ground it after use and discharge the capacitor.
Ah, capacitors - just waiting for their chance to kill you.
In a previous career I was once tasked with replicating the layout for a circuit board we lost the prints for and were finally near exhaustion on the previous 5-year buy. It was a fairly simple affair with perhaps 2 dozen thru-hole components (relaty, transistors, coils, resistors, capacitors, etc). The sample I was given was fully assembled and had been pulled from stock. I set about getting the dimensions with a steel ruler - it had 0.01" graduations, but I would be doing well to be accurate to within 0.05".
As luck would have it, the last measurement I took was the pins for the biggest cap on the board - probably a 100V 500uF job - and was rather surprised when it arced, gouging out the second contact point on the ruler, rendering it somewhat useless as a precision instrument for that distance. Not wanting to further antagonize the capacitor and further plasma-cut my ruler, I fudged that measurement then went out into the shop and had one of the metal shop guys precision-shear the first inch off the ruler and put it back on the boss’s desk…
No idea how long that PCB had been sitting inventory - it’s equally likely to have been pulled from the test fixture as to have been the last one from the bottom of the bin.
But that was nothing compared to the time I grounded the lawnmower’s ignition system. Having the entire side of your body suddenly convulse involuntarily is quite exciting. Suspect I would have been sitting there with my fist grasping that wire with maximum force until the mower ran out of gas if it hadn’t stalled within a few seconds.
The summer of 1980, we had a new window AC unit that stopped working just as that heat wave hit. Sears was sending out total idiots to work on units. They started to change out the fan without discharging the run capacitor (they had to have the wiring diagram out ) I caught it and made them discharge it. They were a little shocked that a woman knew about capacitors!
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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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