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full wave CW multiplier won't work very well

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Mat

Sun Nov 02 2008, 02:23PM

Registered Member #1795 Joined: Sun Nov 02 2008, 02:05PM
Location:
Posts: 8

Hi everybody,

As a home project, I am building a Co2 laser. Laser is finished and when I first tried to test it did not work (breakdown voltage not high enough).
So I've assembled a full wave CW multiplier seen in the picture below:

My NST is 2x5.5kV 100mA and as you can see it is a 3 stage CW multiplier.
To test the CWm, I've tried to get sparks but the longest ones are only 1 or max 1.5cm and I think this should be more (assuming 1kV/mm).
When I make the spark gap much wider I can hear a zooming noise and when I put the CWm in mineral oil the oil starts to move slowly.
Caps are 1000pF 15kV each and diodes are 20kV 0.1A each.
Each time I take 2 capacitors in series and in the middle 2 in series parallel with 2 in series (so the ones in the middle have twice the capacitance).

For your information: this is the way I did it:

On the two AC inputs I've connected each side of my 2x5.5kV NST.
The spark gap is between the GND and the DC output.

I really don't know why it is not working properly.
Do I need to use bigger capacitors?
I've seen similar CW that do work(eg. Cw2

).
Or should I use doorknob capacitors?

So if someone has any suggestions for improving please let me know cause I really want to get my laser going.
Thank you,
Mathias

Mat

Sun Nov 02 2008, 02:57PM

Registered Member #1795 Joined: Sun Nov 02 2008, 02:05PM
Location:
Posts: 8

I've tried some tests again.
Without resistor, i can get a spark of about 1cm but when I put a resistor in series with the DC+ wire, the spark gap decreases.
So could it be that I don't have enough current (so I need bigger capacitors)?
I think the zooming noise comes from the spak gap electrodes: when I put out the light I see purple light on them, like small sparks on the electrodes themselves.

Shaun

Sun Nov 02 2008, 03:14PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Ideally you'd be using a higher frequency drive to power a small multiplier like that. With 60Hz, you won't be able to get much power at all through those capacitors. So yes, bigger capacitors will help.

A good high-voltage high-frequency source would be an AC flyback with a good drive circuit. That should give you 10kV or so at 10-15kHz, which should be great for your multiplier setup. Just make sure your diodes are fast enough.

But can we have some pics of the laser? That could help us help you a lot more.

Mat

Sun Nov 02 2008, 03:49PM

Registered Member #1795 Joined: Sun Nov 02 2008, 02:05PM
Location:
Posts: 8

Here are some pictures of the laser:

The crack in the borosilicate glass you see here is fixed. Ie filled it with epoxy so breakdown won't occure here. As you can see there is a lot of evaporated water on the side of the tube but this is not a problem. During laser operation this is filled with water.
This is how it works briefly:
The outer tube is purely for cooling. Especially the cathode gets hot (electorde bombardment) and needs to be cooled.
The inner tube is called the resonantor (with the outer tube surrounding it for cooling).
2 electrodes are put in at each side of the electrode an gas (mixture of Co2, He and N2) is running through the resonantor at low pressure (vacuum at one side).
Behind each electorde is a special mirror (One fully reflective, the other 75% reflective.

I hope this explains a bit how it works.
Maybe this drawing will make it clearer
http://members.misty.com/don/co21asm.gif

About the capacitors:
The NST gets 50hz 220V input (I live in Europe) so this 50hZ is not enough to get a large spark.
I would like to feed the CW by my NST because I think the flyback won't give enough power (I need about 800W). So if I use caps that are big enough it should work?
The caps used here seem very good: http://users.tm.net/lapointe/Cockcroft_Walton.htm
and are also fed from an NST, but they are expensive.
Will these caps also work?:[http://cgi.ebay.com/12-x-30kV2000pF-High-Voltage-capacitor-HV-HAM-audio_W0QQitemZ360093277824QQcmdZViewItem?hash=item360093277824&_trkparms=72%3A1205|39%3A1|66%3A2|65%3A12|240%3A1318&_trksid=p3286.c0.m14
The laser needs a higher voltage to start @ a low current, but when the laser starts I need about 30-40mA @15kV (when I rectify the 11kV by the CWmultiplier I get 11kV*rootsquare(2)=15.4kV or am I wrong here.

Any help on solving my problem further?
Already thanks to shaun.
Regards

uzzors2k

Sun Nov 02 2008, 04:14PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The input from the NST is only 5kV, so 15kV caps will be enough. What you need is more capacitance as NSTs only run at 50/60 Hz. The time between peaks is so long that 1nF caps will have discharged long ago if supplying 800W, or any other moderate amount of power. For a mains driven CW at 800W, caps in the order of 100nF (at least) are what you need.

Mat

Sun Nov 02 2008, 05:19PM

Registered Member #1795 Joined: Sun Nov 02 2008, 02:05PM
Location:
Posts: 8

So the cheapest would be to go for a flyback, but I'll never get sufficient power out of that or can I?
I see everybody talking about a drive circuit for the flyback but can't you just take that out of the TV where you got the flyback from?

On the site I've shown (

they use 2500 picofarad, 30 kVDC, ceramic capacitors (doorknob) and these do give large sparks.
I'm probably missing something but these doorknob capacitors are much bigger than my cermaic disk capacitors but still they are only double the voltage and 2.5times the capacitance.

@uzzors:
thanks for the info. I understand now why my capacitors are too small, but I still don't understand why those doorknob capacitors from the mentioned site work much better.

Mat

Mon Nov 03 2008, 03:16PM

Registered Member #1795 Joined: Sun Nov 02 2008, 02:05PM
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Posts: 8

Nobody who knows if doorknob capacitors would make a differance?

teslacoolguy

Mon Nov 03 2008, 04:04PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Mat wrote ...

So the cheapest would be to go for a flyback, but I'll never get sufficient power out of that or can I?
I see everybody talking about a drive circuit for the flyback but can't you just take that out of the TV where you got the flyback from?

On the site I've shown (

why do you say you cant get much power out of a flyback? A zvs driver can pump 700-800 watts through a flyback before it starts overheating.

Mat

Mon Nov 03 2008, 04:13PM

Registered Member #1795 Joined: Sun Nov 02 2008, 02:05PM
Location:
Posts: 8

Indeed I've seen some video's on youtube that even show 1000W flybacks but I still prefer the NST.
If the capacitors for the NST get too expensive I might try it with a flyback but first I'll take my chances on the NST.
Still don't know why the doorknobs do seem to work for other people with NST.

teslacoolguy wrote ...

why do you say you cant get much power out of a flyback? A zvs driver can pump 700-800 watts through a flyback before it starts overheating.

uzzors2k

Mon Nov 03 2008, 09:41PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

You can try them, but I don't see how they will differ noticeably from the ceramic caps in performance. A 10kHz+ AC source is what you need to do this within a reasonable budget. Mates made a CW with mainly homemade components, and an ignition coil. See here.

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