Another HeNe power supply question

[LtKernelPanic]

I feel almost dumb for asking and reading the HeNe PS section over at Sam's made my brain start to run out my ear so I thought I'd ask the knowledgeable folks here.

I'm looking at a tube that from what I can tell takes about 1750VDC @ 5ma. My supplies are either way under or over that. I found one that can do 1600VDC @ 5ma and was wonder if that'd be close enough or would that damage the tube? I also found one that's a perfect match from MI but I really don't want to pay 4x what the tube costs for one.

 

[daguin]

I feel almost dumb for asking and reading the HeNe PS section over at Sam's made my brain start to run out my ear so I thought I'd ask the knowledgeable folks here.

I'm looking at a tube that from what I can tell takes about 1750VDC @ 5ma. My supplies are either way under or over that. I found one that can do 1600VDC @ 5ma and was wonder if that'd be close enough or would that damage the tube? I also found one that's a perfect match from MI but I really don't want to pay 4x what the tube costs for one.

:crackup: Welcome to the world of used gas lasers.

Every "new" laser is an experiment (and usually requires lots of patience)

Try the lower powered PSU (or wait around and keep looking for what you need/want).

Used gas lasers are NOT the hobby for people who are n a hurry

Peace,
dave

 

[LtKernelPanic]

Heh no kidding. I decided against it because I'd rather get one of those green bare tubes since I know I have a PSU that will work since I have the same tube already in a head. I just need the right hookups.

Edit:

One related question. I've seen male aldens with only a single HV cable coming out of it. I assume these only have a single wire in them so how would you wire up a tube? I see there's a guy selling them with dual leads which I'll be ordering later today but I'm kinda curious since I usually only see connectors with the single cable coming from them.

 

[james1095]

Most laser heads have a single cable, it's actually a piece of coax with the anode (+) wire in the middle and ground is the shield.

You can't hurt the tube with an undersized power supply as long as it's enough that the discharge is stable and not flickering, but you can overheat a power supply trying to drive a tube that is too high. That said, there is usually quite a bit of compliance range beyond what is on the label. A PSU rated to do 1600V will likely produce 1750V just fine.

 

[adamsium]

One related question. I've seen male aldens with only a single HV cable coming out of it. I assume these only have a single wire in them so how would you wire up a tube? I see there's a guy selling them with dual leads which I'll be ordering later today but I'm kinda curious since I usually only see connectors with the single cable coming from them.

Where did you find the Alden connectors with dual leads? I've mostly seen the ones with coax, also. Coax looks neater, but I think dual, separate leads would be much easier to hook up.

 

[james1095]

They're typically used in OEM equipment containing bare tubes rather than heads. I've seen them in old supermarket barcode scanners that used an ordinary brick PSU. I know Sam mentioned to me at some point that he had a few so you might ask him.

The coax isn't too hard to work with as long as you don't have to splice it. Connect the center pin directly to your ballast resistor, and then solder your ground wire to the shield and run that to the cathode of the tube. The shield and cathode are at ground potential so they don't need to be HV wire. Do insulate the connections around the ballast resistor though, I like to use several layers of heatshrink and sometimes a bit of silicone.

 

[LtKernelPanic]

Where did you find the Alden connectors with dual leads? I've mostly seen the ones with coax, also. Coax looks neater, but I think dual, separate leads would be much easier to hook up.

Found them on flebay a couple days ago. I ordered 2 of them.

HeNe Laser Power Supply Plug, New | eBay

Now if I could just find some of those plastic tube mounts like Sam included with the kit I bought from him I'd be happy.

On a related note does anyone know if using a 2 and 3 watt 75kO resistor in series for the 5W total would be a problem? I'm placing an order with digikey tomorrow and they don't sell 5W ones. I've done that with other projects fine but I didn't know if that'd work for ballast. I'd rather not pay 8 bucks for one resistor from Mouser or MI unless I have to.


Edit: The male plug I linked to seems to use the same HV wire like the one I got from Sam instead of coax. I had to solder a broken lead on mine and while the insulator around the conductor was like what's around coax there was no shield between it and the jacket like coax and the stranded conductor was made of about a dozen fairly stiff wires. They must have been some sort of plated copper because they weren't the standard copper color but I had no problems soldering them so they weren't aluminum.

 

[james1095]

You can certainly use multiple resistors in series, it's often done in commercially made lasers but they have to be the same impedance for the dissipation to be split evenly. If you connect two 75k resistors in series, you will get a 150k resistor with a power rating equal to 2x the lowest in the string. The load will split evenly between the two so they will both have the same dissipation regardless of the fact that one of them can tolerate more.

If you connect three 27k 2W resistors in series the dissipation will be spread equally and you will get 81k with a 6W rating. This is plenty close and 27k is a common value that should be readily available. Just remember that all elements of a series circuit will always have exactly the same current through them, so Ohms law V=I*R can be used to calculate the voltage drop across any one element. Once you have the voltage drop you can multiply that by the current to get the power. Or you can cheat and just use a calculator Ohm's Law Calculators

 

[adamsium]

Found them on flebay a couple days ago. I ordered 2 of them.

HeNe Laser Power Supply Plug, New | eBay

Now if I could just find some of those plastic tube mounts like Sam included with the kit I bought from him I'd be happy.

On a related note does anyone know if using a 2 and 3 watt 75kO resistor in series for the 5W total would be a problem? I'm placing an order with digikey tomorrow and they don't sell 5W ones. I've done that with other projects fine but I didn't know if that'd work for ballast. I'd rather not pay 8 bucks for one resistor from Mouser or MI unless I have to.


Edit: The male plug I linked to seems to use the same HV wire like the one I got from Sam instead of coax. I had to solder a broken lead on mine and while the insulator around the conductor was like what's around coax there was no shield between it and the jacket like coax and the stranded conductor was made of about a dozen fairly stiff wires. They must have been some sort of plated copper because they weren't the standard copper color but I had no problems soldering them so they weren't aluminum.

Thanks... those cables look great. Much easier to work with than coax, I'm certain.

Also, as for the tube mounts you're looking for, I'm guessing you mean the ones that go on each end of the tube and have integral electrical connectors...? They're the ones I've seen in the kits from Sam, anyway (I have one on the way in the mail ). Anyway, I don't know where you'd get those, but, I have seen these cradle mounts on the Meredith Instruments site:

Meredith Instruments : Tube Cradle Mounts [TCM-1] - $3.00 - The source for laser surplus

So, if you're mostly just after something to actually support the tube, these may be quite useful. I've also ordered one of the bare green tubes from Meredith and I'm getting some of these mounts for it - can't wait for it to arrive!. They also have similar cradle mounts available for heads.

Cheers,

Adam

 

[james1095]

The wires are probably either tin or tin plated copper. The HV insulation is soft so they often sleeve it with color coded PVC insulation.

Those mounts with contacts in them that slip over the ends of the tube are used in some of the barcode scanners that those small Uniphase tubes were made for. They are most likely a custom made part produced specifically for the scanner.

Lots of things can be used as cradle mounts. A technique I've used is to take a hole saw or spade bit about the same diameter as your laser tube and bore a hole through a piece of plastic or wood that's a half inch or so thick. Trim it up to size, then cut it in half right across the hole and you will end up with two little cradle blocks. If you don't have the right drill, cut some rectangles of material and saw out a V in one side. Nylon zip ties or a dab of silicone caulk work well to hold the tube in place. If you want super quick and easy, put a blob of silicone near each end on the side of the laser tube and simply set it on a piece of plastic and mount that in your enclosure.

 

[LtKernelPanic]

James-

Thanks for the replies on the resistors and mounts. The ones Sam includes are square that have a hinged side so you can open it and slide the tube in. They don't look like they were pulled from somthing but I could be (and likely am) wrong. You also reminded me why I shouldn't do electronics projects after midnight. I somehow managed to forget that resistors in series sum to both their total resistance and rated wattage.

I may pick up a set or two of those tube mounts that adamsium linked to from MI and I may just grab one of the 3W 75kOhm resistors from them even though I hate paying 3 bucks for one resistor. I also see they sell some clips that attach to the tube's electrode that you can solder your ballast or leads directly to so you don't have to wrap bare wire around the electrodes. Not sure which size I'll need though. Hmm. Wonder if they could just sell me the tube for $100 with the rest of my stuff so I don't have to go through fleabay .

 

[james1095]

If they're the ones I'm thinking of, they came from barcode scanners, I have several sets myself along with the polygon mirrors and some other parts from the scanners. I had the tubes too traded Sam the small Uniphase tubes I had for some different tubes.

There's nothing wrong with using 2-3 separate resistors in series for the ballast. Actually there are some advantages to it. The voltage is less across each resistor so you don't have to worry as much about exceeding the voltage rating of the resistor, and it can be more compact since a string of resistors in heatshrink can be flexible. It's common for laser heads to be made with 2-3 separate resistors in series. The clips are handy, though sometimes you can find fuse clips that are close enough to work with some tubes.

 

[icecruncher]

Hmm. Wonder if they could just sell me the tube for $100 with the rest of my stuff so I don't have to go through fleabay .

Yes almost. The one I bought came with the clips for the tube and the correct ballast resistor. It, however did not come with any mounts. But there were some good suggestion above for quick and dirty mounts.