On Wed, 30 Oct 2002, Joe wrote:
> Hi Pat,
>
> I'm assuming that your laser is helium-neon laser. If so, then if
> it's 23 years old it almost certainly has what they refer to as a
> soft seal between the electrodes and glass tube. The problem with the
> soft sealed lasers is that the helium atoms are small enough that
> they slowly leak out of the seal. When they do, the mixture ratio
> changes and the ignition and operating voltage goes up until the
> laser will no longer operates. That's what it sounds like is
> happening with your laser. Your's sounds like it's it border line in
> that the PSU firing voltage can make it fire but the operating
> voltage isn't high enough to keep it operating.
The laser is a Spectra Physics Stabilite model 24B 35mW HeNe laser, and
the power supply is a Spectra Physics Laser Exciter model 255.
That seems accurate. I've rigged up a 'line conditioner' to give me a
cheap pseudo-variac to change the input voltage to the laser power supply
(the supply is a simple transformer -> rectifier -> capacitor type with a
current regulator stuck on it, here's a PDF I found:
http://www.repairfaq.org/sam/255apsch.pdf
Does anyone know what the specs are on that transformer (eg rated output
voltage), and how far I can (fairly-safely) run it above its rated input
voltage? Specifically, it'd be nice to know an upper limit so I don't
explode the HV caps or diodes.
> Further you can't just use a resistor as a dummy load. The laser
> tube is similar to neon and floresent lights in that it has a
> negative resistance. That is it has a certain amount of resistance
> until it fires and then the resistance decreases dramaticly. You
> have to use a ballast with such devices or else the current will
> increase drasticly and almost instantly burn out the device. The
> ignition voltage will be on the order of 20,000 volts but the
> operating voltage will be roughly 12,000 to 14,000 volts.
By looking at the schematic I linked above, it looks to run at a lower
voltage than that, but it's hard to tell without a HV meter or knowing
what the tranformer is outputting.
> Be careful using a scope around these things. It probably isn't made
> to handle nearly the kind of voltages involved in the laser. You
> need a good high voltage probe for checking these things. You can
> use one made for TV repair. They're fairly easy to find and are
> reasonably priced.
I wasn't going to scope the HV side, and since there is no LV side of the
power supply, I'm just not going to bother looking at it with a scope.
Besides, it's a simple enough circuit to troubleshoot with a HV meter.
> If you send me the brand and model tube I may be able to tell you if
> it's a soft sealed tube and other details.
>
> Joe
>
> At 04:38 PM 10/30/02 -0500, you wrote:
> >I know this is a bit OT, becuase it's not really computer related, but the
> >hardware is at least 23 years old, so that's my excuse for posting to the
> >list...
> >
> >I've got a 35mW HeNe Laser + Power supply that I picked up today for next
> >to nothing. It seems to be having some problems - the laser (somtimes)
> >blinks a few times when I first turn it on, and then stops. From the
> >sound of the power supply it's either a loose connection (which I doubt
> >after opening it up and taking a good look around) or there's a problem
> >with the power supply.
> >
> >Now, I've never really worked on a HV power supply before, and I'm trying
> >to be careful when I play with things. First off, does anyone have a
> >general idea of what sized dummy load ("resisitor") I should try haning
> >off of the HV output to properly load it? I noticed that the laser tube
> >has 3x27kohm resistors in series, would a couple watt approx 81kohm
> >resistor be a good idea?
> >
> >Also, does anyone have an idea for a failure mode to look for? I've got
> >an O'scope, dmm, and various other tools at my disposal, but no
> >'authentic' HV test/mesurement gear.
> >
> >I'm just looking for general guidelines.
> >
> >Thanks!
Pat
--
Purdue Universtiy ITAP/RCS
Information Technology at Purdue
Research Computing and Storage
http://www-rcd.cc.purdue.edu
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>
Received on Wed Nov 06 2002 - 21:04:40 GMT
