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n-FET Drain to Ground Resistance

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sngecko

Sun Jan 29 2012, 04:01PM

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

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...

Proud Mary

Sun Jan 29 2012, 04:32PM

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

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.

sngecko

Sun Jan 29 2012, 06:57PM

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

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.

Proud Mary

Sun Jan 29 2012, 09:57PM

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

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.

sngecko

Mon Jan 30 2012, 02:34PM

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

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?

Proud Mary

Mon Jan 30 2012, 03:42PM

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

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?

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

Tue Jan 31 2012, 05:14AM

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

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 47kÎ© 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!

sngecko

Tue Jan 31 2012, 01:09PM

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

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!

Proud Mary

Tue Jan 31 2012, 03:00PM

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

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!

sngecko

Tue Jan 31 2012, 04:43PM

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

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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