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Seriesed Electrolytic Capacitor Charging Dilemma (lots of caps...)

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LAN_403

Bored Chemist

Sun Apr 15 2007, 06:30PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I'm certain that to pump nitrogen lasers you need very fast pulses and I'm pretty sure that holds for solid state ones too. It's to do with filling the upper state faster than it empties itself. There are a few references to tis on the various "how to make a laser" sites. Here's one that gives 2 msec as the cutoff.

It may vary with the type of crystal etc, but it's something to make sure of before building a lethal and expensive cap bank.

MikeT1982

Sun Apr 15 2007, 08:33PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Yeah man thats what I think Mr. Anderson was telling me. I just did a search on the fluroescence lifetime of Ruby and it said 3 milliseconds. So if a simple chunk of Ruby is flashed with a lamp with enough power to create that population inversion of electrons, when they drop back down to their preferred enrgy state, it will glow for about 3 milliseconds...if I am understanding the process right. Now I was told that my rod (my RUBY rod

) needs about 600 Joules of flash power to excite the atoms enough to lase. So I figured 3,900 into the flashlamp would put out close to that as a visible flash of light, like at least 3,000 according to what SAMS FAQ lasers suggests in flashlamp efficiency. He said what goes in most comes out of the flashtube in the form of light....UNLIKE the ruby rod where say I put 3,000 Joules in, it is like 1% efficient so I only get 30 Joules out. It's funny that I say only 30 joules, I gues thats a meaty amount, but my laser won't have a Q-switch so I think it will be over a much longer duration instead of microseconds. Oh yeah about the 10kJ flashlamp I don't have my digicam I loaned it to a buddy but its really not too impressive let me describe it. It has coils about 1/4 inch in diameter and 9 turns, it looks like you took 1/4 copper water pipe (but it is clear quartz glass) and wrapped it around a 1" bird perch dowel rod 9 times. Then in each end it has a tungsten? (i think) electrode that looks like a tall allen bolt. In total length it is about 6" with an outside diameter of about 1.5". What sucks (but is good) about mine is that it has a white teflon wrapping sort of built on reflector, but it doesn't look cool it looks like it is mummified. At least I don't need an external reflector (but will make one anyway, theres no way in heck I will flash it in the open and blind myself!!!! Mr. Anderson had a 20kJoule one and it was like a foot long!!! It looked quite like a coiled garden hose with a diameter of like 3 inches external, it put mine to shame. Then you got those linear ones for that military project oh boyyy

Oh about the electrolytics having hte can as ground, THANK YOU! I am glad I didn't go buy simple straight metal as buss bars this morning yet, back to the sketch pad LOL!

Eric

Sun Apr 15 2007, 08:37PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
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Posts: 116

The pulse required by a nitrogen laser is different than for flash pumped lasers. The flourescence lifetime for N2 is ~2 ns at 1 atm and is 2-3 ms for ruby rod. I think the electrolytics should work fine. Those have an internal RC constant of 0.15 ms (shorted discharge), so it should be possible to dump a good portion of the energy into a load in 1ms. Hard to say how long they'll last...

The question of what bleeder R to use is actually pretty complicated and depends on expected variations in C between your caps and how fast they'll be charging as well as internal leakage currents. You should be able to run some tests to verify that your bleeders are doing their job. Maybe get some throwaway DVMs like those $2 Harbor Freight jobbies and watch how things equalize on each capacitor during charging.

Are you q switching in your setup Mike? [edit]looks like you already answered the question[/edit]

MikeT1982

Sun Apr 15 2007, 09:48PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Hey Eric, nah for now the Q-switch is beyond me even though I really ought to be with using the size rod I am and all, but when things started to feel overwhelming I thought back to "What was my original goal?" and it is to create a decently sized (so freinds and people can visually see the parts and make sense of what is physically going on) OLD FASHIONEd Ruby Laser like Ted Maiman did back in 1960. Just a good old fashion ruby rod with integrated mirros....optics would fry my mind right now if I had to work with external mirros, I'm easily overwhellmed LOL!!! So the rod's integrated dielectric mirros save me so much work LOL. Oh yeah, one thing I forgot to mention before is if the fluorescence lifetime of Ruby is about 3 milliseconds, it seems from what I read all I need is a goal of a flashduration under that and it should lase.

Electroholic

Sun Apr 15 2007, 09:53PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

most lytic coil guns with less voltage and a large inductor can give pulses in the ms range, there should be no problem pumping ruby rod.

...

Sun Apr 15 2007, 11:36PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

just for reference guys, that 1ms number I quoted was just a random value I pulled out of my ass, I have no idea what length pulse you need.

BTW, do you have any more data on the tube or the rod? I might be able to give at least a rough estimate if I knew more about the rig.

hehe, my 1064th post. I should dust off my old yag laser and celebrate

MikeT1982

Mon Apr 16 2007, 04:59AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Yeah man fire that thing up! LOL!! I am unsure of the tube, even Dick didn't know too much about it, I have to get concrete measurements of its diameter and length, it is a Spaceglass tube that was made back in the 1990's LOL, and the box doesn't say anything on its specs, Mr. Anderson just knew it was a 10kJ max lamp. I'm gonna guess that that is it's explosion energy, I don't know for sure though, I hope it flashes with 3.9kJ. The rod is a Union Carbide like he has on the site of Anderson Lasers, it is 6.1" long and 3/8" in diameter, it was a surplus rod that he had refinished and coated years back. It's funny it's tiny compared to some of the stuff he had like this Ruby laser with a 8" by 1" rod, and then multiple similar sized amplifier rods, thats nuts!! The price on those huge rods were astronomical like multi thousands, I barely could afford (well couldn't but bought anyway) the 6 inch guy LOL. By the way I just got all 16 Nippon caps from the Electrostore unpacked just about 3 hours ago, I proceeded with heavyrubber gloves just incase they had a charge. I ran my 750 volt Fluke across each 400v cap and got readings up to 5 volts. I put a 120 volt lightbulb across them to drain them to zero and repacked them. After I get the screws for them tomorrow (they came without top screws as noted on the site) I plan to abandon the project for a week or two just for a break. One last thing though, about the Electrostore. I want to highly reccomend this guy, Daniel Greenhouse. First each capacitor was rolled into 2 feet of bubblewrap individually and taped at its ends, then they were all placed in a box lined both top bottom and sides with bubble wrap. More bubble wrap then a $5 role from Walmart has he used. Then he had fragile signs all over the outside of the UPS box and shipped it via UPS 3 Day select because he ran a single day late shipping it!! He emailed me apologizing for the 1 day delay (and I wasn't even aware of a delay and made no inquiries). From California to PA where I live that must have been a decent cost, so hands down I reccomend him. I mean he did this all at no extra charge.

Sulaiman

Mon Apr 16 2007, 05:39AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

When making an MMC of electrolytics I like to NOT put resistors across each row, instead I put:

TVS or ZENER diodes across each row to limit the voltage each row 'sees'
and put a diode across each row to prevent voltage reversal (discharging/inductive loads etc.)

If charging from an NST the zeners would need to be able to dissipate the heat generated by the charging current
which is less than you'd expect due to the magnetic shunts of the NST causing the NST output to be high-impedance
e.g. 15kV 30mA NST output impedance = 500 kOhm.

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