HeNe Help... Part 2

[hwang21]

Alright... successfully got it bare tubed and a nice PSU for it's PSU and everything

Now the problem is... if I run the HeNe for more than the few minutes the output slowly fades to nothing. I noted that the anode side ballast resistor (68k 1%, doesn't give a wattage rating but it's quite a bit thicker than the cathode resistor) gets *extremely* hot shortly after powering on the HeNe. Strangly enough, the cathode side also has a 10k resistor, 1% and 4W rating :thinking:

Anode resistor: DALE RS-5 68K Ohm 1% CI2028
Cathode resistor: DALE RS-2 10K Ohm 1% 4W M0044 Mexico

I'm thinking that the 68k resistor may need replacing? I accidentally hit the surface of the 68k resistor with a flat blade screwdriver a few times, I can see white underneath the black exterior coating of the resistor. I'm not sure if this ruined the resistor or not, but given the fact that it's melting the electrical tape I put around its leads... the 10k resistor stays pretty cool, as far as I can tell. Certainly not hot to the touch, at least.

Any ideas where I can get a big wattage 68k resistor, if this is indeed the problem? I saw that Meredith Instruments has a 3W 68k resistor, but only 3W? Is that enough? Thanks

 

[ultimatekaiser]

well, most of the input going into the HeNe is wasted, little of it is used, so the ballast usually does get pretty toasty. if you nicked it though it is possible you damaged it. I use a single 75K on my P-177 tube on the anode end within 3 inches of the tube. if its damaged or too far away it doesn't do its job properly. it needs a very steady input or it will not function correctly. If the resistance isn't steady then you run into problems with the voltage and current ramping up, which causes problems.

It sounds like thermal runaway of sorts, which is causing grossly high amounts of current to run through the tube, which slowly kills the output as it gets too much. normally it stabilizes and reaches a thermal equilibrium so you get a very tiny/modest up and down change in output (not detectible by eye) as it does so. if it gets too much current, then it slowly just falls steadily as the tube peaks and then overheats, much like a diode.

that or for some reason its not getting enough gain when it warms up, such as the mirrors are misaligned/damaged as the tube heats up...but that seems unlikely at this point.

EDIT: yes a 3 Watt 68K resistor should be plenty if it runs @ 6.5mA which I believe is what your tube operates on. (since you have a 10K on the other end for stability)

 

[bloompyle]

543.5 has your lowest gain. Mirrors coated for this wavelength are in the 99.8-99.98 range.

It takes a peculiar gas fill, and good mirrors to get any resonance here.

As far as your best, that would be 640. 632.8 actually has a fairly low gain, the reason they use it for the standard is beyond me.

3.39um is also very high, I'd say second highest. Mirrors for this wavelength are sometimes as low as 50%.

Orange, and yellow are also low gain lines. Not as bad as 543.5, but still low.

 

[hwang21]

Ok so should I just buy a replacement resistor from Meredith instruments? I think this 68k resistor is a 5 watt resistor but I'm not sure. Thanks

 

[pschlosser]

MI is kind of expensive. I bought nice ones from digikey.com

 

[hwang21]

Is 5% tolerance good enough? My current one states 1% tolerance...

I guess this one is the one we're talking about? Invalid Request

 

[ultimatekaiser]

It's probably be fine, but 1% would be preferable, especially on larger tubes.

 

[hwang21]

Is there any place I can get 5W or higher resistors?

 

[ultimatekaiser]

You shouldn't need a 5W with the 10K ballast still on the front. A 68K 3W MO ballast is just fine or you could up it to 75K 5W, but you will increase resistance, thus requiring more voltage. The ones from mi or that you were linked to are both fine.

 

[hwang21]

I just don't really understand why this guy is heating up so much. Inside the tube the resistor was stuck in some kind of rubbery stuff in the endcap, thus me using a screwdriver to pry all of the stuff out, accidentally hitting the resistor a couple times in the process. So I'm just not sure whether the resistor was heatsinked or something with that rubbery stuff or if it's damaged due to the screwdriver, or if it is just supposed to be hot to the touch. Hot enough to be alarming...

 

[ultimatekaiser]

There's always a lot of waste heat from hene lasers. They're not very efficient and ion lasers are even worse

 

[hwang21]

Yes yes haha but the resistor is heating up to a pretty hot temperature in seconds... Seems like a short or something

 

[ultimatekaiser]

Yes it sounds like it's damaged. Just replace it and go from there. One thing at a time my friend :beer:

 

[hwang21]

Haha thanks :beer: I happened to be messing around with a bluray player at school today, and there happened to be some pretty thick resistors on the board. The biggest one... a turquoise ish axial resistor, probably 3W or 5W. The bands... blue gray orange gold DMM read 67.5k

Guess I won't be needing to buy myself a resistor now