I spent all my money last year supporting my daughter's science fair project. Now, it's high time for laser fun. I refitted some parts in the power supply. When I powered on the power supply and current regulator using an old HeNe laser as a load, an interesting phenomenon was observed.
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So, Weebly notified me that they were concerned that I'd not updated my site in a while. Well, I felt that I had to take a break from the laser project in order to save up to buy an oscilloscope (looking at a 100MHz Rigol right now) and divert money to my daughter's science fair project this year.
As the science fair project is starting to ramp up, there will be some updates over there in the next couple of months. I recently received two new-old-stock Siemens Helium Neon lasers for further testing. The are roughly the same geometrically and electrically as the Mells Griot HeNe that met its premature end a few months ago. After the success of the small neon lamp, I expected all things to go well. Not so fast. Apparently the current was so low on the tiny neon lamp that the oscillation was sufficiently muted.
The pre-strike current flow certainly was stopped by the grounded grid. Good. Upon striking, my instruments showed 4 mA through the tube and somewhere under 2kV dropping across it. That was just as it should be. There was no flicker on and off as in the initial experiment. The main problem, however, came again from a high-pitched noise accompanied by the cooling fans' stoppage. The thought of the anode seal overheating and the oscillation destroying my FETs led me to shut down the experiment after 10 seconds or so. So, from whence comes this oscillation? If I had an oscilloscope (seems obvious, right?), I think I would probe (1) the op-amp output that is driving the MOSFET or (2) the FET drain. The suspected candidates include: (1) An unstoppered triode anode, grid, or cathode could cause the oscillation. (2) An academic paper I came across suggested that current regulator feedback chains should have a limited response to around a couple kilohertz in order to avoid "severe oscillations". I tried an integrator on the gate drive op-amp, but the same behavior was observed. The oscillation's frequency might interfere with the +24V PWM feedback loop or the DC fan controller; however, this behavior has been observed when the +24V is supplied by batteries. So the former seems unlikely. Could it be that in input voltage presents its own spikes and noise that get multiplied in the PSU? I'll try to cause this behavior again, then work on muting the power supply spikes, if found. Last night I finished reinstalling the power supply in its new half-rack home. The power entry is fused at 20A, the big Variac is connected, the wires safely routed around the HV supply; still, there is only one power receptacle that the regulator and whatever auxiliary items must share. It was just too inconvenient to somehow plug the regulator into an internal receptacle so near the HV poles of the supply. I'm going to make a sign reading something like, "Do not unplug this cord until supply has been powered down for 60 minutes."
I switched on the affair with my big toe, my fingers firmly inserted in my ears. No bang, boom, or fizz. I cranked up the Variac to 1000V, as shown on the input voltage meter. It looked perfect and I switched it off with my right index finger. I often get to talk to real engineers (after all, I'm related to several) and the question that keeps coming up in my imaginary conversations (since real engineers don't have these kind of conversations) is, "Why did you spend so much money on cosmetic bling?" And...I've got nothing. Still, it doesn't mean we hobbyists have to pay top dollar for that huge Variac knob or half-height rack. eBay has brought some great things my way -- some good, some bad, all cheap. OTOH, I've had custom self-centering steel washers literally made one-off. It feels arbitrary, but so far it's looking pretty good.
I just ordered several blanking panels for my project that will cover up most of the exposed openings on my power supply rack. It's pretty important to have a huge, grounded panel between my fingers and this power supply, so the order of the day was "effective, yet as cheap as possible, without having to dig through surplus." Ding! eBay. After rebuilding the current regulator innards, I calibrated the anode voltage and both laser drop voltage meters using the following method: (1) Attach the power supply to both the anode input and laser cathode inputs. (2) Adjust the input voltage to the 120:1 potential transformer to be 58.9V. This amounts to 10,000V output open circuit. I then adjusted the anode volts pot until the dial read 10kV and the laser drop volts pot until it read 0V (the laser drop voltage is the input voltage less the laser cathode voltage; thus, 10kV - 10kV = 0V).
Also, I mounted the 25A autotransformer to the custom mounting panel. It looks great and everything fits perfectly. The current regulator has been repaired, rewired, and all circuits appear to be functioning properly again. This required painstakingly replacing all parts associated with the triode socket, regulation hardware, MOSFETs, op-amp packages, and rewiring the ground paths. Also, the HV relays (i.e., the relay that switches on the HV power supply and maximum voltage relay for laser start) were installed and tested. I can now switch the big power supply on and off with the flip of a switch and momentarily switch the power supply to maximum voltage with the press of a button. Nifty!
I can't emphasize enough how helpful and supportive the 4hv.org community has been in general, and Proud Mary and Steve Conner, in particular. Steve Conner helped me make sense of triodes. Proud Mary stuck with me through my regulator self-destruction and repair. He also re-interpreted Posakony's paper to discover an oversight in the original design.
Please check out the Current Regulator section to see what is happening to fix it up, better protect it, and slightly change the underlying topology. Also, take a glance at the Optical Resonator section to see what direction I've decided to take the resonator. It's now longer, wider, and has manufacturers standing by to put it together as soon as I get the hundreds of dollars it'll take to do so. The damage caused to the regulator (blown power supply, fuse, and 2 op-amp packages) as a result of my negligence in testing taught me or reenforced some interesting lessons: (1) make sure you have a way to shut down your power supply without unplugging it; (2) don't do things when you have no idea how it might help; (3) put your ICs in removable sockets; and (4) HeNe lasers are more fragile than they look.
I've replaced all the parts that I know were damaged, less the laser tube. I'll probably use some basic gas discharge tubes for further tests, as they're $30 a piece new instead of $100 used. |
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September 2017
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