Anyone got some info?

[Doc]

^^^^See Steve's post^^^^^

Hi Steve, thanks for the help

I am wondering, what do I need the 200V for? Does the igniter require 200V tube voltage open circuit before it will self trigger or something?

Cheers,
Dan

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But you need 175-200V open circuit to form the cathode emission spot when the glow transitions to arc and drops to ~90V. Don't sell your self short on starting voltage.

Steve

!

[Things]

Is that not the igniters job? Sorry, this is my first gas laser, so I'm still learning a heap.

[RedlumX]

Even if you may be able to start the tube with less than 200V, you still need to have a voltage overhead in order to sustain the 90V or so at 10A. But this means that while the tube is not yet ignited, the idle voltage will jump up to 130V, say (read somewhere about rms and peak voltages with respect to rectification). So you can't get away with a 100V cap.

Moreover, if the specs say the tube needs 70-90V or so, it does not mean that you have the choice. The tube chooses its own voltage and this does not depend on the current like an ohmic resistor. It means that the tube voltage lies somewhere between 70 and 90V when running at the specified current. Lets assume 90V at 10A, for simplicity. Then if you tune down to 5A (with a fraction of the power), then it may take 85V. (Numbers estimated for illustration only).

[Things]

Hm, so does that mean I can run the tube right from rectified 240VAC, as long as I keep the current below 10A, and it'll only draw it's max rated voltage, or doesn't it work like that? It would be nice if I didn't have to use the MOT, but I think I have to.

[RedlumX]

No, if you run this thing from rectified 240V you'd need to kill like (240-100)=140V times 10A = 1.4KW with the linear regulator..... believe me that you don't want to do that (I did that initially for a tube that runs 160V which is slightly better but it was a nightmare).

BTW I got caps and transistors lying around since ages suitable for your purposes, I could send them for free (except shipping) if you are interested (salvaged used items but OK).

[Things]

Ahh OK, I get what Steve was on about now. Can't believe I totally missed that aspect.

Ok well, to fix that, I really just need to rate everything in the PSU a bit higher I guess.

[Things]

Looks like I'm gonna have to drop the MOT on this one, I simply cannot fit enough windings in it to give me 100VAC

After a lot of discussion, seems the best way to go would be to spend an extra $100 and buy 2 50-0-50 500VA toroidal transformers locally, which is going to mean a bit more saving before I can fire this tube up. Oh well ...


Dan

[Things]

Hmm well, it seems using a toroidal transformer or 2 is gonna be too costly. I am thinking about maybe getting 2 MOT's, and putting about 50 windings on each, and putting them in series.
That is kind of a last resort though, as I can't think of any cheaper or easier ways to get 100VAC from 240VAC without wasting 1.4kW heat in the passbank.

Anyone got some idea's?

Cheers,
Dan

[mixedgas]

Your transformerless options are 1. Fet Switcher. 2, SCR into inductive filter, 2. SCR cap charger 3. IGBT switcher 4. IGBT cap charger.

5. Massive series resistor. MSRs are hard enough/hot enough at 110, I would not want to try it at 250. MSR values would be about 10-12 ohms at 250V for a 105V tube based on my experience.




Coherent keeps a max of 8 volts across the passbank in the I60 and I70 psus using a SCR bank. If the passbank gets greater then 8 volts across it the scrs stop jumping charge into the caps at the next zero crossing.

This does it for 220 3P tubes, its a commercial SCR firing board and a passbank, its what I know is on line.

http://www.laserfx.com/Backstage.Las...uts/pythan.pdf

The coherent schematic is a LOT simpler...

If I had to do it, well, 450 volt surplus computer grade caps are cheap. Charge em to 200V, start the tube, then look at the voltage across your passbank. Zero crossing trigger optoisolators are cheap, You need some "L" or "R" between the FET or SCR and the caps. Soft charging the caps through 30-60 ohms prevents the massive surge into uncharged caps, and a relay shorts out the precharge circuit when its done. If your trimming the voltage across the passbank, you need far fewer passbank transistors. Coherent runs the I70 to 30 amps off 4 2n6259s using the scrs.

Second choice would be the fet chopper. I know what the values are for the inductor, and you need 4 pieces of 24N50 or better fets.

One other way is to start a ramp circuit at each zero crossing, run the ramp into a comparator against the current feedback signal, then gate a scr into a inductor in series with the tube, this is used in yag lamp supplies where they are not so concerned about ripple in the lamp.

What do you get into a cap if you just half wave rectify the UK line? ( I know, ripple... but...)

Steve

[RedlumX]

about maybe getting 2 MOT's, and putting about 50 windings on each, and putting them in series.

That will do. Just be sure to have a few different taps on the secondary in order to fine tune the voltage, in steps of 5 or 10V. The transistors I will send to you are pretty robust, though, so that's not essential, it is just to keep the cooling requirements down. At any rate you'll need a huge heat sink, ideally a fan cooled tunnel. I got one around but I guess shipping to AU woulnd't be worthwhile.

I also include the power resistors for the circuit shown about, I had built it myself and it worked fine. They need to be mounted on the heatsink as well. Count easily 100W dissipation just for the resistors.