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Question Concerning using Franceformer to charge Cap Bank

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MikeT1982

Sun Apr 01 2007, 12:46AM

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

Hi guys I'm new here, the reason I came here is because I need an answer from personal experience. I am designing a power supply for a Ruby Laser project and here is my question. If I choose a 6kV 30mA Franceformer NST and brdige rectify it, how high of a voltage cap bank do I really need, and how high will the voltage get if I were to just turn it on and let it run? What voltage will it level off at? Some say multiply by 1.4, which gives me 8,400 volts.....so according to that I'd really need a 9kV or so cap bank.... Anyone who has messed with cap charging from rectified NST's or just knows please help me out. Thanks a ton guys!!

ragnar

Sun Apr 01 2007, 02:24AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

When rectifying a sinewave AC waveform, the peak voltage will be the square root of two times the RMS voltage.

In your case, 6kv * sqrt(2), which gives you close to 8,485VDC. I really recommend that you leave plenty of overhead -- a 9kV cap equates to 5% overhead, which isn't much.

If you can afford to, leave 50% overhead, try, say a 12kV cap.

Same goes for the rectifier -- with silicon devices, you may well want to leave 100% overhead on your rectifier.

MikeT1982

Sun Apr 01 2007, 06:12AM

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

Thanks a million. So knowing this, I believe I may go for a 4kV Franceformer instead, as it will charge my cap bank to about 5656.8 volts and my Xenon strobe tube should prefer between 5 and 6kV. My only dilema now is getting a small panel mount analog style meter that can read up to 6 or 10kV....do you guys have any suggestions? I's love to be able to watch my cap bank charge and level off, and if I could mount it in power switch/key and power light indicator end of my laser's housing it would look awesome!

Mike

...

Sun Apr 01 2007, 06:36AM

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

A few things:
1. It is often desirable to set the charging voltage higher than you need, and have an active shutoff that kills the power when you have enough voltage. I am unsure exactly ho NSTs work, but I don't think a 5kv nst puts out the full 20ma at 4.9kv, it will be a small fraction of the rated current--so the caps will charge very slowly near the end.
2. You can easily make a 6kv panel meter. Basically, you get an ammeter that is designed for a very low current, and put a large series resistor in series with it, using ohms law to calculate the resistor.. For example, if you get a meter than reads 0-0.5ma, and you want it to read 0-5kv you would add a 10meg resistor in series with the meter, and hook that across the cap bank (for obvious reasons, the resistor goes to the high voltage site, the meter to the low voltage side)
3. Care to tell a little more about said laser? I have played with my share of yag lasers (both flashlamp/diode pumped) and 5kv on the tube seems like way too much; unless you have a monster of a laser... Perhaps create a thread in the emag radiation forum?

MikeT1982

Sun Apr 01 2007, 07:26AM

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

Hey thanks a ton again!! Well I have always been fascinated with lasers since I was a child, and used to be thought of as weird because I would constantly take laser books out of the elementary school library LOL. Out of all of the laser designs, the one that totally infatuated me is the original Ruby Laser with a Helical Xenon flashtube surrounding a Ruby Rod. I realized though at the time being just over 8 or 10 years old that there was a snowballs chance in hell that I could afford to have one or even the parts. When in Middleschool a favorite science teacher of mine gave me a Scientific supply catalog that had Metrologic Helium Neon lasers in it. Now keep in mind, this was before the craze of diode laser pointers came out, if at the time, I had seen a diode laser pointer in a Mini-market gas station I literally would have passed out, as the concept of a Laser seemed so incredible and hard to obtain! So I saved up for nearly 2 years and purchased the smallest Metrologic HeNe laser and witnessed my very first laser beam on the wall of my room. This guy brought me into contact with laser light and I enjoyed it for about 5 years untill I sold it for cash when I came of the driving age to get into car audio systems LOL!! Well here I am now about 15 years later from me realizing I could not afford a ruby laser and just got done with college and have a full time job. So here goes....I decided I want to live out that childhood dream. Last week I made a 4 and a half hour trip from my homestate of PA to Ohio to meet Dick Anderson, owner of Anderson Lasers and to take a tour of his place. My GOD this guy is AWESOME and very generous, even though I got there late he took me on a tour of the whole place and fired up a few lasers including a Big YAG to let me witness Pulsed Solid State Lasing for the first time in my life!!! What an epirience, I am actually returning next week to see more!!! Well when I was there I left with the heart of my dream project, a 6.1" x 3/8" Union Carbide Ruby Rod with AR coatings and integrated Dielectirc mirrored ends AND a brand new Spaceglass 10 Kilojoule Helical Xenon flashlamp with 9 helix turns. I spent nearly an entire paycheck but Dick was very generous and helped me out. Now owning the heart, I am in the planing stages for the rest of this project. I plan to use a 4kV France NST bridge rectified to charge 16 series Nippon 400v 3,900uF electrolytic capacitors to provide me with just about 6 kilojoules of input flash energy to my flash tube. Dick said give her about 5 kV and it should be happy as the Rod has a threshold of 600 Joules. I can't recall my exact figures as I am at work but I will have a few hundres farads and about 5656 volts to give me about 5-6kJ pulses. I have no idea what to expect as far as power output I really hope it can burn something, maybe alot! All I know is I am very excited!! I plan to house the entire thing in a chassis measuring 8x8 by 48" or so.

mod edit combined double posts

Above I said farads I meant a few hundred Microfards. LOL! Oh by the way I just found a surplus site with a 10kv Panel mount meter man!

The model is MTR 7531-10kV I wonder if that thing is self contained and you can just put 10kV right to its contacts?? Impressive if thats the case. Very expensive though $85, I think I may do your way of using an ammeter!

Mike

Steve Conner

Sun Apr 01 2007, 08:27AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

wrote ...
it will charge my cap bank to about 5656.8 volts

Are you sure it might not be nearer 5656.9?

BTW no double posting please! Also, that meter needs an external resistor, if you read the surplus sales page carefully.

MikeT1982

Sun Apr 01 2007, 08:41AM

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

LOL! Thanks man, i'm an idiot I didn't see the edit button lol. Sorry about the double post, I just went back and better read the forum rules.

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