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Oscillations Redux

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sngecko

Wed Apr 18 2012, 09:28PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Some of you will remember my gas laser/discharge power supply, current regulator postings in the past, which led to a nice, stable neon lamp glowing merrily.

Today I 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.

The pre-strike current flow certainly was stopped by the grounded grid, thanks to Proud Mary. Upon striking, my instrumentation 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.

When I first ran SPICE simulations, I saw that the TL084 JFET-input op-amp model provided by TI would oscillate very quickly around the set current value. When I used the similar Linear models (LT1001), the output was solid. The regulator is using TL084s. I thought the simulation oscillation was a fluke, and I don't want to buy LTs unless I know that the problem is related to my application of the TL084s.

Also, what is the link between the oscillation and the fan stoppage. The fans all run off of a +24V SMPS that is not connected at all to the drive/instrumentation circuits (they run off of a linear +/- 15V PSU), except through the main input and chassis ground... I think. I should also mention (in relation to a post by Forty earlier) that the power supply is referenced to earth ground. So, all the power supplies, the regulation circuitry below, and the HV power supply share a connection to chassis ground and, by extension, earth ground.

Here's the circuit,

Proud Mary

Wed Apr 18 2012, 11:27PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

What is the source of the 5V/14.5A filament supply?

sngecko

Wed Apr 18 2012, 11:31PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Each filament has its own 5V, 20A transformer. The center tapped secondary is not connected. A simple relay switches on the transformer.

I forgot to mention above that there are two such circuits, but I can only use one at a time, as I have only one triode.

Proud Mary

Thu Apr 19 2012, 12:22AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I won't comment on the SMPS, and its failing fan, as there are many others here who know them far better than I ever will.

But as for the 3-500Z - triodes are always looking for an opportunity to oscillate, and will find feedback paths beyond the limited imaginings of circuit simulators unless we take definite steps to stop them.

I would start off by sticking some chokes in the anode, grid and cathode lines. The self-resonant frequencies of these chokes should all be different and not harmonically related. If you are unable to actually measure the self-resonant frequency, then choose values such as 2.2, 4.7, and 6.8 mH. These different values will stop the triode from implementing any Miller effect or tuned-anode-tuned-grid oscillatory stunts that it may have been planning.

Then I think you should decouple the line between the laser cathode and the triode anode. For example, wire a 4K7 wirewound from the laser cathode to the triode anode choke, and bypass the junction between resistor and choke directly to Earth/chassis via a suitably robust capacitor of 100nF. This will also make it harder for the laser - a gas diode - to get up to any negative resistance oscillator hi-jinx which it may have been planning with the triode.

Make sure that you use a common earth point for the connections around the valve socket, or you will have trillions of electrons running round in little loops all over your chassis. Each loop will have its own magnetic field, which will try to induce currents in whatever conductors will listen to them.

sngecko

Thu Apr 19 2012, 12:57AM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

How would I measure the self-resonance of this arrangement, if I had an o-scope? it would depend on the laser, right? If I knew this self-resonant frequency, how could I best attenuate it? I may look into buying a little 2MHz USB oscilloscope.

I certainly will look into the chokes on the triode, but I'm concerned about putting a resistor in the main current line (i.e., between the laser cathode and the triode anode). The circuit could be passing 80mA and I was going for efficiency with the whole active regulator thing.

Proud Mary

Thu Apr 19 2012, 01:22AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The problem with trying to establish the self-resonant frequency of your whole arrangement is that it is a dynamic circuit, and even very small changes in parameters might caught instability.

Outside of the new wave of thermionic valve audio designers, the use of chokes as I have described is a common prophylactic measure in power valve design - especially where directly heated valves are used. You assume there will be instability unless you take steps to prevent it.

If you are not ready to add the series resistor I have proposed, then as second best, I would suggest bypassing the laser cathode to Earth via a capacitor of 100nF - 500nF so that unwanted blips and spikes that might trigger oscillation are sent directly to Earth. Clearly, this capacitor will add some lag into the circuit's response, so don't make the capacitor larger than you have to.

sngecko

Thu Apr 19 2012, 01:28AM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Sounds like a plan. Before I rip this thing apart, two things occur to me. (1) It might work just fine with a huge CO2 laser head on there, like the much smaller neon lamp, and (2) it would be pretty informative to just see the noise in the line...

Also, any idea why the +24 V power supply fails as soon as the discharge lamp starts, but then comes back on when it's extinguished? That's weird to me.

Proud Mary

Thu Apr 19 2012, 09:30AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

sngecko wrote ...

Sounds like a plan. Before I rip this thing apart, two things occur to me. (1) It might work just fine with a huge CO2 laser head on there, like the much smaller neon lamp, and (2) it would be pretty informative to just see the noise in the line...

Also, any idea why the +24 V power supply fails as soon as the discharge lamp starts, but then comes back on when it's extinguished? That's weird to me.

When you say the 24 V (SMPS) supply 'fails', I take it you mean it trips. Is this a response to an over-current condition, or an anomalous condition caused by high voltage or unwanted circuit artifcats going back down the line? Stick a high voltage diode in the 24V line to block anything that may be trying to go the wrong way, and monitor the current to see what, if any, transition occurs at the moment of lamp starting.

Steve Conner

Thu Apr 19 2012, 10:26AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Switched mode power supplies can do just about anything when hit with RF.

I agree that stoppers on the tube electrodes would be a good thing to try, but I don't see the need for chokes. The currents in the circuit are pretty small compared to a RF amplifier, so those cement block wirewound resistors should do fine. Put a 100 ohm one in series with the grid of the 3-500Z if you haven't already: right at the tube socket. If that doesn't help, you can also try one on the plate terminal, or a higher value like 1k.

Of course, the oscillations could also be coming from the op-amp or MOSFET stages, or they could be "Nyquist" oscillations encompassing the whole feedback loop. But the symptoms suggest a pretty high frequency, so the tube is the most likely culprit.

I also suggest not wasting your money on a low-end USB oscilloscope. Get a cheap second-hand analog one instead.

sngecko

Thu Apr 19 2012, 01:25PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

It sounds like I should buy/borrow/steal an oscilloscope and try to determine the identity of these illicit electron trafficking oscillations.

I'm pretty sure that the dropping out of the +24V SMPS is tied directly to the oscillations, as everything seemed to work fine when the small neon discharge lamp was working. But as soon as the oscillations started acting up, it dropped out.

Also, during the last HeNe laser test, the frequency of the audible oscillation changed as I adjusted the set current. This seems to suggest, as is almost obvious, that the oscillation is in the op-amp driver or the current regulation chain.

Would it make any sense to try and shield the SMPS? Or its input and output lines with ferrite beads?

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