CO2 tube question

[Laser Ben]

How much was that tube and power supply? I want one!

[Hemlock Mike]

Ben --

The 15 watt tubes are $199 on the "bay".... You are on your own for power supply. A neon sign xfmr and some HV diodes, capacitor PLUS a ballast resistor and you get flame !!

Mike

[Hemlock Mike]

Measurements on my CO2 setup:

Power supply output (limited by a variac set for 75 VAC): 8500 VDC
Ballast resistors: 150 K Ohm - Volts across tube: 6200 VDC
Tube current: ~17 mA.

The tube is rated 7.8 to 9.3 KVDC at 12 - 15 mA.

I think the output voltage of this power supply is too high and can't be regulated this low.
SO - I think that a solid state speed control for a fan is suited to control this inductive load without danger of spikes and/or noise damaging the circuit...

What say, Oh knowledgable minds ???

Mike

[Laser Ben]

I'm interested, where exactly can I find one?

[The_Doctor]

eBay. As said before. CO2 Laser Tube. Should work. Put on the watch list, and check completed listings to see likely prices and past sources.

I guess these are the things Robin Bowden cooks his sausages with.

Mike, you might get away with that if the cooling is good, but try a bit more current limit resistance. The tube current is what matters, as with a laser diode current, the voltage will be right if the current is. Not sure how much though resistance to add though, maybe another 20K. If you add 100K, you will probably bring the current within the control range of that variac. If you can get solid carbon composite resistors, do it, film types (carbon OR metal) won't work, they'll arc over and burn. There are ceramic types that will also work, but they're rare and maybe expensive. Some kind of cermet or something.

[Hemlock Mike]

Yo Doctor --

My first Laser was a CO2 which I built 30 years ago !! What a monstrosity !! I still have it ............

I've increased the ballast resistance up to 150 K ohm and this ebay PS keeps pumping out more watts ... The feedback circuit seems to not work as the Vbe on the final driver transistor stays at 0.8 volts no matter what. It seems locked ON.

A variac is a big POS so I thought a 6 amp fan control would work because it's made for an inductive load. I'm trying to make this look good and not like a breadboard kludge. Added resistors simply allow the PS to output to the max and then have excess power burned off as heat in the ballast.

That's why I'm looking for a schematic for this 941S PS. It's max output is supposed to be 15 mA and I don't want toast in there.

Mike

[The_Doctor]

Point taken. That schematic will be needed (or some good reverse engineering), I didn't realise it was trying to maintain some control system, thought only the variac and the limit resistor were controlling current. That 0.8V could be a fault, or a value the control circuit is designed to maintain. If you can find out which without the schematic, you might get the rest sorted too. A post to alt.lasers is often good for rustling up schematics...

[Hemlock Mike]

Doctor --
Without reverse engineering, the PS appears to control a rectified power line voltage to a HV module. The final driver transistor seems to stay fully biased ON no matter what I do. This is a hard circuit to trouble shoot when the module is putting out over 12 KV !!!

I'm saying screw the current control and limit the input with the 6A fan control. If it can't pull it in, It can't put it out !!

How do I get to this alt.lasers ???

Mike

[The_Doctor]

If any part of that circuit is effectively a power converter, then even limiting the input might not be so easy, it might just try to draw more amps if the input volts are lowered..

alt.lasers is being mentioned in a new thread in the lounge... There's enough info there now to get you there quickly. I strongly suggest a newsreader over Google Groups, as it's the best way to learn it, but Google Groups is good for emergencies.

What you might get is an alternative idea for PSU.. I've read that people get results with neon transformers. Not sophisticated, but cheap, and easier to predict their behaviour. I guess your first CO2 might have used one?

[Hemlock Mike]

Doc --

My first CO2 was (is) 3' long and powered by a BIG neon sign Xfmr with a voltage doubler and variac. This "little" supply does limit OK with a variac. Will a light/fan dimmer/speed control work OK ?? I think it will because the input is AC line rectified to a capacitor -- A small transformer circuit provides LV to the circuit. The triac switching noise should filter out after a FW bridge and capacitor.

Mike