n-FET Drain to Ground Resistance
sngecko, Fri Jan 27 2012, 08:21PMI'm being a bit lazy asking for your input here, but I work weird hours and I can't get into my lab.
Given the circuit below, if this circuit were un-powered (no grid or anode voltage or op-amp supply), would anyone expect resistance at the drain to ground of only ~500 Ohms?
This seems really low to me, but I had to head out of the house before I could tear out this circuit and test it. I think there might be a mistake in how I mounted the FET, given that the IRF830 that I'm using has a flange that is tied to the drain. I have shown, though, that the drain is isolated from the chassis, so I can't figure out why resistance is so low.
Scott.
Re: n-FET Drain to Ground Resistance
m4ge123, Fri Jan 27 2012, 09:11PM
Did you get 500 ohms by measuring the MOSFET with a multimeter? If so, you may have connected the probes backwards so that you're measuring the body diode.
m4ge123, Fri Jan 27 2012, 09:11PM
Did you get 500 ohms by measuring the MOSFET with a multimeter? If so, you may have connected the probes backwards so that you're measuring the body diode.
Re: n-FET Drain to Ground Resistance
sngecko, Fri Jan 27 2012, 09:23PM
By the Power of Gray Skull! I may have done just that. I will have to recheck it later...
On the other hand, the forward voltage drop is supposed to be only 1.6V for the IRF 830's diode.
Note: It turns out that I measured it correctly. What this thread may not be aware of is that I recently accidentally discharged a >10kV, 2uF capacitor through this circuit... So, it's no surprise that these FETs are toast.
sngecko, Fri Jan 27 2012, 09:23PM
By the Power of Gray Skull! I may have done just that. I will have to recheck it later...
On the other hand, the forward voltage drop is supposed to be only 1.6V for the IRF 830's diode.
Note: It turns out that I measured it correctly. What this thread may not be aware of is that I recently accidentally discharged a >10kV, 2uF capacitor through this circuit... So, it's no surprise that these FETs are toast.
Re: n-FET Drain to Ground Resistance
Proud Mary, Fri Jan 27 2012, 09:32PM
Have you considered what might happen in the case of gate drive failure, Scott? Your MOSFET would almost - but not quite - stop conducting, and you would have a potential divider consisting of the laser tube, the triode valve, the MOSFET, and R across your high voltage line. Could the voltage drop across drain and source exceed the maximum permitted for the device - 500V, I see from the data sheet?
Faults proliferate, and it can sometimes be difficult to identify the first cause from which all the others flow.
Proud Mary, Fri Jan 27 2012, 09:32PM
Have you considered what might happen in the case of gate drive failure, Scott? Your MOSFET would almost - but not quite - stop conducting, and you would have a potential divider consisting of the laser tube, the triode valve, the MOSFET, and R across your high voltage line. Could the voltage drop across drain and source exceed the maximum permitted for the device - 500V, I see from the data sheet?
Faults proliferate, and it can sometimes be difficult to identify the first cause from which all the others flow.
Re: n-FET Drain to Ground Resistance
sngecko, Fri Jan 27 2012, 09:56PM
I do typically start at near 0V on the gate. You're exactly right, Proud Mary.
As Steve Connor pointed out a while back, I did consider the clamped situation. I calculated a maximum Vds of ~150V given the 3-500Z's charts. I explain my reasoning at Note 3 on my Current Regulator page.
sngecko, Fri Jan 27 2012, 09:56PM
I do typically start at near 0V on the gate. You're exactly right, Proud Mary.
As Steve Connor pointed out a while back, I did consider the clamped situation. I calculated a maximum Vds of ~150V given the 3-500Z's charts. I explain my reasoning at Note 3 on my Current Regulator page.
Re: n-FET Drain to Ground Resistance
Proud Mary, Fri Jan 27 2012, 11:00PM
This is what you say in Note 3:
2) Now, we’re going to try to make the plate as tantalizing as possible to the electrons, but deny them to flow freely by clamping the FET. If Vp-g equals 20000V and 15V is on the grid, this puts us way off the chart. We can approximate the near-linear 1ma line with Vf-g = .006452 x Vp-g + 2.5V. In this case, if we were to put 20000V on the plate and allow 1ma through the FET, there would be ~130V on the FET drain. Of course we have to add the 15V on the grid, for a total of ~145V on the FET drain. The IRF830 laughs at this, with its 500V max voltage rating. This is assuming a 1mA current, which is by no means negligible. But I think we can assume that if it were to drop to near-zero, we should still be within voltage tolerances. When I receive the tubes, I’ll test the actual no-load cathode voltage.
Now, if we remove the grid drive, then the drain current will fall to ~25 μA, which then becomes the maximum current that can flow through the whole potential divider series string of laser tube, triode valve, MOSFET and R.
The laser tube starts to fire, so a big portion of the 20kV supply will appear as a pulse on the triode anode. The triode conducts, so its equivalent resistance becomes quite low, but still it can only pass the 25 μA limited by the switched-off MOSFET. What do you think the drain-source voltage would be then?
What would happen in your laser tube if ionisation were to start (and its resistance start to fall) but then stop again after some nanoseconds when the voltage across it collapsed? It would stop conducting and the voltage build up on it again at the rate allowed by the 25 μA. Could this account for the oscillatory behaviour? Which components would determine the time constant?
Added afterthought: If you were to replace your MOSFET and R with a single resistance, across which, for example only, 100V was dropped during conduction of the triode, and you were to tie the grid to Earth by a high resistance 'grid leak' then you would have a control grid voltage of -100V, that is, within the normal range of grid bias for this valve. But you have tied the grid to +15V, removing the fail-safe design feature for this particular valve of having a low quiescent current at zero bias. The electrons will naturally think they are looking at an anode, and some of them will jump ship rather than go all the way to the intended anode, as we saw before. In normal use of 3-500Z as an RF power amplifier, grid current will only flow on positive half cycles, but in your case it will flow all the time. The impedance of the valve will be low, not much of your 20kV will be dropped across it, and your 500V drain-source limit will be exceeded.
My instinct is that you should have fixed negative bias on the grid.
Proud Mary, Fri Jan 27 2012, 11:00PM
This is what you say in Note 3:
2) Now, we’re going to try to make the plate as tantalizing as possible to the electrons, but deny them to flow freely by clamping the FET. If Vp-g equals 20000V and 15V is on the grid, this puts us way off the chart. We can approximate the near-linear 1ma line with Vf-g = .006452 x Vp-g + 2.5V. In this case, if we were to put 20000V on the plate and allow 1ma through the FET, there would be ~130V on the FET drain. Of course we have to add the 15V on the grid, for a total of ~145V on the FET drain. The IRF830 laughs at this, with its 500V max voltage rating. This is assuming a 1mA current, which is by no means negligible. But I think we can assume that if it were to drop to near-zero, we should still be within voltage tolerances. When I receive the tubes, I’ll test the actual no-load cathode voltage.
Now, if we remove the grid drive, then the drain current will fall to ~25 μA, which then becomes the maximum current that can flow through the whole potential divider series string of laser tube, triode valve, MOSFET and R.
The laser tube starts to fire, so a big portion of the 20kV supply will appear as a pulse on the triode anode. The triode conducts, so its equivalent resistance becomes quite low, but still it can only pass the 25 μA limited by the switched-off MOSFET. What do you think the drain-source voltage would be then?
What would happen in your laser tube if ionisation were to start (and its resistance start to fall) but then stop again after some nanoseconds when the voltage across it collapsed? It would stop conducting and the voltage build up on it again at the rate allowed by the 25 μA. Could this account for the oscillatory behaviour? Which components would determine the time constant?
Added afterthought: If you were to replace your MOSFET and R with a single resistance, across which, for example only, 100V was dropped during conduction of the triode, and you were to tie the grid to Earth by a high resistance 'grid leak' then you would have a control grid voltage of -100V, that is, within the normal range of grid bias for this valve. But you have tied the grid to +15V, removing the fail-safe design feature for this particular valve of having a low quiescent current at zero bias. The electrons will naturally think they are looking at an anode, and some of them will jump ship rather than go all the way to the intended anode, as we saw before. In normal use of 3-500Z as an RF power amplifier, grid current will only flow on positive half cycles, but in your case it will flow all the time. The impedance of the valve will be low, not much of your 20kV will be dropped across it, and your 500V drain-source limit will be exceeded.
My instinct is that you should have fixed negative bias on the grid.
Re: n-FET Drain to Ground Resistance
sngecko, Sat Jan 28 2012, 01:11AM
As I read your post for the first time, I was reminded of something from The Hitchhiker's Guide to the Galaxy. As a bowl of petunias is falling through the atmosphere, it thought, "Oh no. Not again." It was speculated, as I try to follow your excellent explanation, that if I understood exactly what you were saying, I would know a lot more about the universe than I do now.
Anyway, it is certain that the grid drive failed at some point. I have not yet deduced whether it failed before or after the big shock or before or after the initial (doomed) tube lighting. At any rate, I had started the process by clamping even that 6.12mA grid current, so the rest of your analysis should hold at least until the current is allowed to increase past a point where the discharge can be sustained.
I wish I was better prepared to answer your questions... Here goes:
(1) What might the drive voltage be as soon as the discharge tube begins conducting with insufficient current to sustain itself? I can only answer by trying to measure it in some way. So much relies on characteristics of the 3-500ZG that are hard to find in the literature.
(2) I think your explanation of oscillation makes sense insofar as I understand it. I noticed that the pitch of the oscillation increased as I increased the current. This makes pseudo-sense to me, as the new current level was allowing the tube to start more frequently. I don't think I understand why there is a "minimum sustainable current," though. Is it a result of the ionized gas having a lower resistance?; thus, if the current is sufficiently restricted, there is insufficient power to maintain the ionized gas channel?
(3) As for the time constant, I imagine that there are any number of structural capacitances in series with resistive elements or coupled to the chassis in some way.
So, in retrospect, I should have attempted to start the tube with sufficient gate drive on the FET to permit 6.12mA (current from grid) + 6.5mA (optimum current for the laser) to flow (12.62mA) in order to prevent the oscillation from beginning in the first place?
Afterthought Response: The +15V arrangement derives directly from a paper: Michael J. Posakony, “A High Voltage Current Regulator for Laser Gas Discharge Tubes,†The Review of Scientific Instruments Vol. 43, No. 2, pp. 270-273 (February 1972).
sngecko, Sat Jan 28 2012, 01:11AM
As I read your post for the first time, I was reminded of something from The Hitchhiker's Guide to the Galaxy. As a bowl of petunias is falling through the atmosphere, it thought, "Oh no. Not again." It was speculated, as I try to follow your excellent explanation, that if I understood exactly what you were saying, I would know a lot more about the universe than I do now.
Anyway, it is certain that the grid drive failed at some point. I have not yet deduced whether it failed before or after the big shock or before or after the initial (doomed) tube lighting. At any rate, I had started the process by clamping even that 6.12mA grid current, so the rest of your analysis should hold at least until the current is allowed to increase past a point where the discharge can be sustained.
I wish I was better prepared to answer your questions... Here goes:
(1) What might the drive voltage be as soon as the discharge tube begins conducting with insufficient current to sustain itself? I can only answer by trying to measure it in some way. So much relies on characteristics of the 3-500ZG that are hard to find in the literature.
(2) I think your explanation of oscillation makes sense insofar as I understand it. I noticed that the pitch of the oscillation increased as I increased the current. This makes pseudo-sense to me, as the new current level was allowing the tube to start more frequently. I don't think I understand why there is a "minimum sustainable current," though. Is it a result of the ionized gas having a lower resistance?; thus, if the current is sufficiently restricted, there is insufficient power to maintain the ionized gas channel?
(3) As for the time constant, I imagine that there are any number of structural capacitances in series with resistive elements or coupled to the chassis in some way.
So, in retrospect, I should have attempted to start the tube with sufficient gate drive on the FET to permit 6.12mA (current from grid) + 6.5mA (optimum current for the laser) to flow (12.62mA) in order to prevent the oscillation from beginning in the first place?
Afterthought Response: The +15V arrangement derives directly from a paper: Michael J. Posakony, “A High Voltage Current Regulator for Laser Gas Discharge Tubes,†The Review of Scientific Instruments Vol. 43, No. 2, pp. 270-273 (February 1972).
Re: n-FET Drain to Ground Resistance
Proud Mary, Sat Jan 28 2012, 02:03AM
As a practical proposal, if you were to substitute the laser tube for a resistance equivalent to the tube's at its optimum working current, then you can adjust the triode grid bias to whatever value and sign is needed for it to draw that amount of anode current. Given the variable impedance in the cathode line, this procedure can only point you in the right direction, rather than provide a final answer.
I have noticed one small error in the circuit diagram, where we see that a tantalum capacitor shown decoupling the grid of one triode is not shown on the other, so there may be other errors too. Who can say?
Proud Mary, Sat Jan 28 2012, 02:03AM
As a practical proposal, if you were to substitute the laser tube for a resistance equivalent to the tube's at its optimum working current, then you can adjust the triode grid bias to whatever value and sign is needed for it to draw that amount of anode current. Given the variable impedance in the cathode line, this procedure can only point you in the right direction, rather than provide a final answer.
I have noticed one small error in the circuit diagram, where we see that a tantalum capacitor shown decoupling the grid of one triode is not shown on the other, so there may be other errors too. Who can say?
Re: n-FET Drain to Ground Resistance
sngecko, Sat Jan 28 2012, 03:21AM
You'll notice that the tab above the tantalum decoupling cap connects directly to the other grid bias, indicating that both grid bias voltages are driven from the same +15V supply. Thus, the single decoupling cap.
sngecko, Sat Jan 28 2012, 03:21AM
You'll notice that the tab above the tantalum decoupling cap connects directly to the other grid bias, indicating that both grid bias voltages are driven from the same +15V supply. Thus, the single decoupling cap.
Re: n-FET Drain to Ground Resistance
Proud Mary, Sat Jan 28 2012, 07:08AM
gotcha!
Proud Mary, Sat Jan 28 2012, 07:08AM
gotcha!
Re: n-FET Drain to Ground Resistance
sngecko, Sun Jan 29 2012, 04:01PM
m4ge123: It turns out that both channel's FETs have around 500 Ohms of resistance to ground (even when tested properly). I'm going to rebuild the FET mounts, since they are likely blown...
sngecko, Sun Jan 29 2012, 04:01PM
m4ge123: It turns out that both channel's FETs have around 500 Ohms of resistance to ground (even when tested properly). I'm going to rebuild the FET mounts, since they are likely blown...
Re: n-FET Drain to Ground Resistance
Proud Mary, Sun Jan 29 2012, 04:32PM
Scott, as per my email, where I wrote that avalanche failure looked likely, your R readings seem to confirm this. This isn't one of my strong areas, so
I consulted 'MOSFET Failure Modes' to harden up my opinion.
I suggest you include in your re-build over-voltage protection at drain and gate. On the drain, I'd go for two or three levels of protection - perhaps an avalanche diode in parallel with a MOV, and a gas arrestor of 300V as final fall back. MOSFETs can also suffer dV/dT failure as you know, so suppression of fast transients propagated by the laser tube would seem prudent.
Proud Mary, Sun Jan 29 2012, 04:32PM
Scott, as per my email, where I wrote that avalanche failure looked likely, your R readings seem to confirm this. This isn't one of my strong areas, so
I consulted 'MOSFET Failure Modes' to harden up my opinion.
I suggest you include in your re-build over-voltage protection at drain and gate. On the drain, I'd go for two or three levels of protection - perhaps an avalanche diode in parallel with a MOV, and a gas arrestor of 300V as final fall back. MOSFETs can also suffer dV/dT failure as you know, so suppression of fast transients propagated by the laser tube would seem prudent.
Re: n-FET Drain to Ground Resistance
sngecko, Sun Jan 29 2012, 06:57PM
In addition to your excellent advice above, I'll probably put a similar level of protection on the triode grid, as back-feeding the power supply is also bad juju. The only arc protection I'd originally included was the neon lamp, which actually was doing its job before I pulled it out like a knucklehead.
sngecko, Sun Jan 29 2012, 06:57PM
In addition to your excellent advice above, I'll probably put a similar level of protection on the triode grid, as back-feeding the power supply is also bad juju. The only arc protection I'd originally included was the neon lamp, which actually was doing its job before I pulled it out like a knucklehead.
Re: n-FET Drain to Ground Resistance
Proud Mary, Sun Jan 29 2012, 09:57PM
I was slightly surprised to see the arc-over protection in the grid line in the original paper, without mention of how often such a bad thing might be expected to happen with the circuit given. In transmitter practice, I'd think of PA arc-over as an outcome of a major mis-match, for example, but certainly a fault.
Of course, it's good practice to build in precautions and circuit strategies for coping with even an uncommon fault, if such a fault could cause costly damage, or hazard to the operator.
Proud Mary, Sun Jan 29 2012, 09:57PM
I was slightly surprised to see the arc-over protection in the grid line in the original paper, without mention of how often such a bad thing might be expected to happen with the circuit given. In transmitter practice, I'd think of PA arc-over as an outcome of a major mis-match, for example, but certainly a fault.
Of course, it's good practice to build in precautions and circuit strategies for coping with even an uncommon fault, if such a fault could cause costly damage, or hazard to the operator.
Re: n-FET Drain to Ground Resistance
sngecko, Mon Jan 30 2012, 02:34PM
Does Posakony's paper make sense regarding the requirement that "no current" be allowed to flow in the grid? I just can't imagine how, at least before the tube lights, there would not be current flowing if the cathode/filament is powered and the grid biased. Once the tube lights, I presume that the plate voltage would completely take over.
Also, that arc-protection is for plate to grid strikes, right?
sngecko, Mon Jan 30 2012, 02:34PM
Does Posakony's paper make sense regarding the requirement that "no current" be allowed to flow in the grid? I just can't imagine how, at least before the tube lights, there would not be current flowing if the cathode/filament is powered and the grid biased. Once the tube lights, I presume that the plate voltage would completely take over.
Also, that arc-protection is for plate to grid strikes, right?
Re: n-FET Drain to Ground Resistance
Proud Mary, Mon Jan 30 2012, 03:42PM
I'll look again at the Posakony circuit with especial reference to the grid and the 3-500Z datasheet. We know already that this valve was designed for zero bias operation, so that grid drive failure wouldn't burn it out. Maybe his stipulation is addressed to this, but we shall see.
Yes, the arc-protection resistor is there to protect the semiconductors from malicious anode transients doing violence to the grid circuit, but it doesn't look very reassuring to me given that there is 20kV just upstream.
Proud Mary, Mon Jan 30 2012, 03:42PM
sngecko wrote ...
Does Posakony's paper make sense regarding the requirement that "no current" be allowed to flow in the grid? I just can't imagine how, at least before the tube lights, there would not be current flowing if the cathode/filament is powered and the grid biased. Once the tube lights, I presume that the plate voltage would completely take over.
Also, that arc-protection is for plate to grid strikes, right?
Does Posakony's paper make sense regarding the requirement that "no current" be allowed to flow in the grid? I just can't imagine how, at least before the tube lights, there would not be current flowing if the cathode/filament is powered and the grid biased. Once the tube lights, I presume that the plate voltage would completely take over.
Also, that arc-protection is for plate to grid strikes, right?
I'll look again at the Posakony circuit with especial reference to the grid and the 3-500Z datasheet. We know already that this valve was designed for zero bias operation, so that grid drive failure wouldn't burn it out. Maybe his stipulation is addressed to this, but we shall see.
Yes, the arc-protection resistor is there to protect the semiconductors from malicious anode transients doing violence to the grid circuit, but it doesn't look very reassuring to me given that there is 20kV just upstream.
Re: n-FET Drain to Ground Resistance
Proud Mary, Tue Jan 31 2012, 05:14AM
I believe I have found the solution:
All this guff about 15V+ bias on the triode grid should only be understood in the context of Posakony's use of BJTs, and their need to have their base currents controlled by the potentiometer also fed from the 15V+ line. This 15V+ is also communicated to the cathode, so there is no
PD between grid and cathode in the quiescent state when the zero set pot has been suitably adjusted.
To make your circuit viable, you should not apply any external bias voltage to the grid, but connect it directly to Earth by a grid leak resistor, as I suggested elsewhere, so that the relative grid bias is completely controlled by your MOSFET in the cathode line. I would suggest a grid leak of 47kΩ be tried at first, and then optimised up or down as necessary. A bypass capacitor of (provisionally) 500nF should be connected in parallel with it.
Once you have made good the damaged parts, added the protection circuits, and modified the grid circuit as I suggest, I expect to hear that it is all up and running!
Proud Mary, Tue Jan 31 2012, 05:14AM
I believe I have found the solution:
All this guff about 15V+ bias on the triode grid should only be understood in the context of Posakony's use of BJTs, and their need to have their base currents controlled by the potentiometer also fed from the 15V+ line. This 15V+ is also communicated to the cathode, so there is no
PD between grid and cathode in the quiescent state when the zero set pot has been suitably adjusted.
To make your circuit viable, you should not apply any external bias voltage to the grid, but connect it directly to Earth by a grid leak resistor, as I suggested elsewhere, so that the relative grid bias is completely controlled by your MOSFET in the cathode line. I would suggest a grid leak of 47kΩ be tried at first, and then optimised up or down as necessary. A bypass capacitor of (provisionally) 500nF should be connected in parallel with it.
Once you have made good the damaged parts, added the protection circuits, and modified the grid circuit as I suggest, I expect to hear that it is all up and running!
Re: n-FET Drain to Ground Resistance
sngecko, Tue Jan 31 2012, 01:09PM
That has got to be the missing piece of Fig. 2 that isn't clearly explained in the paper. I did not notice the +15 being directed to the cathode in the cutoff state!
Also, I certainly understand better requirement (2) in Posakony's paper now. In his circuit, that 15V PD always had to be added back into the collector voltage in cutoff. Excellent.
Wish I could send a pint over the internet for you!
sngecko, Tue Jan 31 2012, 01:09PM
That has got to be the missing piece of Fig. 2 that isn't clearly explained in the paper. I did not notice the +15 being directed to the cathode in the cutoff state!
Also, I certainly understand better requirement (2) in Posakony's paper now. In his circuit, that 15V PD always had to be added back into the collector voltage in cutoff. Excellent.
Wish I could send a pint over the internet for you!
Re: n-FET Drain to Ground Resistance
Proud Mary, Tue Jan 31 2012, 03:00PM
Arcing: With the triode grid at Earth potential, anode-grid arc-over becomes more likely as the anode voltage rises i.e. when the valve is drawing the least current. The Eimac datasheet gives Va max as 4,000V, so you may be pushing it with 20kV. Make sure the filament is good and hot before switching on the HV supply or the full supply voltage will appear on the anode at the instant of switching on. You may want to add an HV time delay circuit if you haven't already done so. 60 secs should be ample to ensure that the filament has time to stabilise fully at its working temperature.
X-rays energetic enough to transit the glass envelope will start to be produced at ~15kV and these escapees will be able to get out of the aluminium case in quantity at ~20kV. If this is just a brief pulse during laser ignition it will have little significance, but a fault leading to a sustained high PD (> 15kV) between triode anode and cathode must be considered a health risk. 80% of 20 keV X-ray photons will still be alive and well after a journey of 100 cm through air at STP.
Good luck, Scott!
Proud Mary, Tue Jan 31 2012, 03:00PM
Arcing: With the triode grid at Earth potential, anode-grid arc-over becomes more likely as the anode voltage rises i.e. when the valve is drawing the least current. The Eimac datasheet gives Va max as 4,000V, so you may be pushing it with 20kV. Make sure the filament is good and hot before switching on the HV supply or the full supply voltage will appear on the anode at the instant of switching on. You may want to add an HV time delay circuit if you haven't already done so. 60 secs should be ample to ensure that the filament has time to stabilise fully at its working temperature.
X-rays energetic enough to transit the glass envelope will start to be produced at ~15kV and these escapees will be able to get out of the aluminium case in quantity at ~20kV. If this is just a brief pulse during laser ignition it will have little significance, but a fault leading to a sustained high PD (> 15kV) between triode anode and cathode must be considered a health risk. 80% of 20 keV X-ray photons will still be alive and well after a journey of 100 cm through air at STP.
Good luck, Scott!
Re: n-FET Drain to Ground Resistance
sngecko, Tue Jan 31 2012, 04:43PM
Thanks for the reminders. I should be okay, as the big laser will drop around 10kV in operation and I've rigged a momentary HV starter (~20kV) for a few cycles, instead of the manual Variac adjustment. I will definitely never try to start the thing at very low currents, though. Sheesh.
As for arcing, the 3-500Z warms up very quickly (one of its virtues), but I will certainly be careful. There is no failsafe on my current regulator's instrument panel to prevent switching on the HV without the cathode turned on at all, let alone a delay to prevent it for a safe amount of time. Maybe later.
As for now, I will make the changes and repost when there is something to report.
sngecko, Tue Jan 31 2012, 04:43PM
Thanks for the reminders. I should be okay, as the big laser will drop around 10kV in operation and I've rigged a momentary HV starter (~20kV) for a few cycles, instead of the manual Variac adjustment. I will definitely never try to start the thing at very low currents, though. Sheesh.
As for arcing, the 3-500Z warms up very quickly (one of its virtues), but I will certainly be careful. There is no failsafe on my current regulator's instrument panel to prevent switching on the HV without the cathode turned on at all, let alone a delay to prevent it for a safe amount of time. Maybe later.
As for now, I will make the changes and repost when there is something to report.
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