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Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I've built over the past year a triode/FET-based current regulator (and 20kV, ~3kW power supply) for low-pressure gas plasmas. Please see here.
I ordered a HeNe tube from eBay of questionable heritage to test the regulator. Each time I lit the tube (up until the time an exploding resistor blew a power supply in the regulator), the cooling fans would slow, the laser tube would emit a high-pitched whine, and a NE-2 neon lamp at the triode grid would almost continuously light. An ammeter in series shows >100mA is flowing from the tube filament to the FET drain; however, the current regulator shows only ~6mA during this test.
I'm not sure what is causing the problem... I've listed a couple of project questions on my site, including:
(1) Does turning up the voltage to the starting voltage until the tube strikes, then turning it down to the operating voltage (relatively slowly compared to a HV starter), cause the tube abnormal stress/operation? I wouldn't think so if the tube would be dropping the same voltage anyway.
(2) Does the 6.12mA that flows from, apparently, the triode grid alter the overall circuit current, or does the plate current dominate the circuit current as soon as the laser tube is lit?
(3) Does having the power supply referenced to earth ground, or having this ground act as a return, cause black magic in the regulator's chassis ground to which it's connected that might lead to the behavior shown above?
Is the HV on the plate going into the grid? killing the power supply? I just don't know.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Both circuit diagrams shown on your link indicate an indirectly heated cathode, while 3-500Z has a directly heated filament, requiring a little more circuit complexity.
How have you configured the 5V filament supply not shown in the circuit diagrams?
Why have you stuck 15V+ fixed bias on the grid of a zero bias thermionic valve? What is this 15V+ relative to - the filamentary cathode, the circuit Earth etc etc ?
It's not easy to give an opinion without seeing a more complete circuit diagram.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The whining noise means your circuit is oscillating. Could be that the feedback loop can't cope with the laser tube's negative resistance, or capacitive pickup from the HV output is getting back into the op-amp board, or the tube itself is oscillating at RF.
Do you have stopper resistors right on your tube terminals? (Control grids at a minimum) Are there high impedance nodes in your op-amp circuit that ought to have electrostatic shielding? (Maybe the board needs a metal shield and some of the wires to the front panel pots need to be screened?)
And like Proud Mary points out, are the filament supplies properly floating?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve Conner wrote ...
The whining noise means your circuit is oscillating. Could be that the feedback loop can't cope with the laser tube's negative resistance, or capacitive pickup from the HV output is getting back into the op-amp board, or the tube itself is oscillating at RF.
Do you have stopper resistors right on your tube terminals? (Control grids at a minimum) Are there high impedance nodes in your op-amp circuit that ought to have electrostatic shielding? (Maybe the board needs a metal shield and some of the wires to the front panel pots need to be screened?)
And like Proud Mary points out, are the filament supplies properly floating?
I too thought it might be oscillating, but didn't want to cause offence. 3-500Z is designed for grounded grid configurations, where Miller effect is not a concern.
But the way it is set up in this regulator circuit, there could well be enough Miller capacitance to cause instability and oscillation. Other oscillatory modes also look possible, but its best to leave further analysis until sngecko has posted a complete circuit diagram of the filament supply.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Sorry, I forgot to attach a direct link to the full diagram.
My overall circuit topology is basically the same as that of John Doran. His full schematic can be found here.
I understand that the positive grid bias is supposed to assure that full current is passed from the triode anode to cathode. Yes, those diagrams of an indirectly heated cathode are from LTSpiceIV. I used that diagram in lieu of a more detailed one with the integrated heating element.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Back to your original questions:
(2) Does the 6.12mA that flows from, apparently, the triode grid alter the overall circuit current, or does the plate current dominate the circuit current as soon as the laser tube is lit?
A positive grid will always draw current. The electrons departing the cathode have a broad energy distribution. Some are highly energetic and will zip past the grid no matter what voltage of either sign we put upon it, The laziest electrons can't be bothered to go all the way to the anode and are glad to drop out at the first opportunity - your positively charged grid - which thus begins to act as an anode, draws current, and dissipates heat.
Maximum allowable grid current is a major parameter in power valve circuit design, but I don't think this is at the bottom of your troubles. 3-500Z was designed for zero-bias grounded grid applications, so the grid is tough as old boots, with substantial heat capacity.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Thanks Proud Mary. I hope you and 4hv don't mind, but I quoted your response on my project website, with citations/links of course.
That's what I thought was going on. The neon lamps were lit during the unpleasant operation, indicating that >120VDC was at the grid, but I have no idea why this should be.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
sngecko wrote ...
Thanks Proud Mary. I hope you and 4hv don't mind, but I quoted your response on my project website, with citations/links of course.
That's what I thought was going on. The neon lamps were lit during the unpleasant operation, indicating that >120VDC was at the grid, but I have no idea why this should be.
I assumed the neons were there to protect the circuit in the event of a failure leading to a large positive voltage on the grid. Does this seeming over-voltage travel back into your bias supply?
There is one other reason why the neons may be glowing. If the circuit is oscillating vigorously, then the neons - if sufficiently close - could ionise just as they would when bringing them into the field of a Tesla coil, given the voltages you are using.
What we really need to get to the bottom of this are some measurements - anode voltage and anode current, grid voltage and grid current, and the voltage on the cathode.
Do you have any means of detecting unwanted oscillations in the circuit? Oscillation in an unstable circuit need not be confined to a single frequency, but at any and all frequencies where there is gain and the output is 180° out of phase with the input and can feed back.
Anyway, best not to speculate ahead of the evidence, and let's see what can be learned by taking some basic measurements.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I am presuming at the outset that a relatively large positive voltage on the grid is indeed hitting the bias supply. This might explain the slowing of the fans and destruction of the +/- 15V power supply.
Only one side of the neon was glowing, indicating a DC signal, right? Does a strong nearby oscillation cause this behavior in neons?
I don't know how to detect oscillations (I don't have an O-scope yet), but I can take any kind of voltage/current measurements. It's a little tough without a dummy load that's light-able, but I can (and have) put the power supply directly on the plate.
In this case, the presumed oscillatory behavior does not occur.
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High Voltage Current Regulated Discharges
