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HeNe Power Supply issue

Speedy78

Speedy78

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I got bored and decided to remove my Melles Griot tube from its metal case. All went well from what I can see but now I'm having a weird issue with my power supply. Not really sure what to think other than I damaged something. Before removing the tube everything worked fine, now I have this weird buzzing sound from the PS. The tube glows but is flickering. Video below is my issue. If you turn your speakers up you can hear it. Was silent before. I fear I have damaged something :(

 





LtKernelPanic

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My first thought was no ballast resistor but it's clearly visible in the video. You did attach the leads to the same ends of the tube as they were in the case correct? One of my HeNe PSUs made a funny noise (more of a high pitched whine actually) because it wasn't getting enough current. Turns out it needed 2.1A and my DC supply could only put out 1.95A. A 5A works perfectly with it.
 
Speedy78

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This is the same supply that was running it before. Positive was on the side with no output and the negative was on the output side but was using the case to get power up there. I have since soldered a wire in place of the case but other than that its all the same. From what I know it takes more to fire the laser than to maintain it right? I found a little set screw thingy on the inside of the PS. If I turn it all the way clockwise i get shorter clicks almost. Not a buzz anymore and more like a pulse than flickering. All the way ccw i get a higher pitched buzz. Not sure if any of that meand anything but I can take a video if you think it will help.
 
Sigurthr

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Did you change the amount of wire between the ballast resistor and the tube's anode? This is the behavior a tube exhibits when there is insufficient ballast resistance, which can be caused by the resistor being too far from the anode or a damaged resistor.

As for the pot on the psu, they generally control current, and not voltage. Video would be nice. If you have a low resistance resistor (<10Ohms) place it between the cathode and cathode wire and measure the voltage across it with a DVM or DMM, this will tell us how much current is going through the tube and will help set the pot correctly. We will need to know the exact resistance of the resistor (measure it, don't use a marking).
 
Speedy78

Speedy78

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There may be 2-3mm mising. I had to salvage what I could out of the potting material but for the most part its the same length. Unless that 2-3 mm is enough to cause this. Resistor measured on my DMM at 35k even. Thats whats printed on the resistor as well. My lowest resistor currently is 330 advertised. Unless I can use this and you do some crazy math I wont be able to provide that answer.
 
Sigurthr

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35k is awfully low for a ballast resistor. You wouldn't happen to have something in the 50-75k range would ya? If ya did, then swap it out with the 35k one and see what happens. 68k is usually standard. Shortening the distance between tube and resistor causes no harm, so that shouldn't be an issue.

Heh, you'd need at least three of those 330R resistors to attempt it. Anything higher than 100ohms or so will start to significantly impede actual current flow.
 
Speedy78

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Well I have 4 left. I can throw all of them on there. Not sure what that equals in resistance but that's all I have to work with.



Edit: From my goggling I have determined it to be 82.5ohms with 4 330s in parallel. Also no crazy high resistors.
 
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Sigurthr

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82.5ohms should work, little high but shouldn't drop too much voltage in relation to normal running voltage.

Parallel up those resistors and put them in series with the cathode wire to the tube cathode. Measure the voltage across the paralleled resistors, it should be around 0.4125V.

I assume you mean 68k when referring to "crazy high", lol, my definition of crazy high is about 35gigaohms (35,000,000,000 ohms). I only stock up to 1Gohm generally =P. Got any 10K resistors?
 
Speedy78

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Yes, that is way higher than anything I could ever need or imagine needing. How many ohms is the human body from head to toe? haha Stand by for that reading.
 
Sigurthr

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Haha, it depends greatly on person.

Dry skin thumb to pinky I am 3.5MOhms. One hand to the other I am 3.14MOhms (larger current path cross section, lower resistance).

Most of the body's resistance is in the upper epidermis, once dielectric breakdown occurs of that layer on one or both interfaces the resistance drops dramatically.
 
Speedy78

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You know to much.

When I can get a reading on my DMM and not feel the electricity puling through my fingers holding the leads its like .179. Tube goes crazy if I even get a single lead close to the resistors.



Here's my set up. Let me know If I'm doing something wrong.

.3ohms DMM leads
81.8ohms measured 4 resisters in parallel
.179-.181V

A lot more crazy buzzing and popping while measuring and the feeling of the high voltage. haha
 
Sigurthr

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LOL, you have the test resistors on the wrong side.

Put the paralleled resistors on the CATHODE (negative) side, not the ANODE (ballast - positive) side.

The HeNe tube acts as a load and drops the majority of the voltage across it so the cathode side only sees a low voltage proportional to the resistance of the cathode lead. By increasing this resistance we can measure the voltage developed across it and determine the current flowing through the tube. With the test resistor on the anode the tube cannot drop the majority of the voltage going to the test resistor as it is "before" the tube in the circuit, so you wind up electrifying the entire test setup with high voltage and invalidate any readings.

Edit: I lot lots of things that many would consider to be "knowing too much". Skin dielectric properties are not that exotic, lol. Human body RF absorbency curves for common transmission frequencies and the frequencies and intensities that will cook the human cornea is more appropriate for the aforementioned category.
 
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Speedy78

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Whatever, magic in magic out = light. <- My laser/ electronics knowledge.

.404V 4 330ohm resistor in parallel on the negative side(Im familiar with that word haha)

Side convo: How does a DMM measure resistance? Does it send out high voltage and measure what was left?
 
Sigurthr

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I = V / R
I = .404 / 81.8
I = 4.9mA

Looks like current is set properly. 4-6mA is common for most red HeNes.

No idea why the PSU is flickering at this point. See if the voltage measurement across the test resistor changes when you change the pot position. Try to increase the voltage slightly.

DMMs use low voltage and form a divider network and then work out the unknown resistance by using the above formula and comparing the measured voltage.
 
Speedy78

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With the pot all the way CCW (faster flicker) I'm getting .333V. With it all the way CW(slow pulse) I'm getting .454V. If its putting out enough current then whats the issue? Could I have damaged the tube taking it out? Looks perfect to me.
 
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