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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Problem is, the heater supply doesn't float, it's grounded. The Hv supply sends +75kV to the anode at full blast, while the other end is attached to the same ground (via a 3mA meter).
The problem I have is, I can't float ground 200V below ground!
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Klugesmith wrote ...
2. Look at the connection schematic in the datasheet. If you want to use the third wire (grid/focus) as CEI intended, your battery-powered heater supply must float, with a resistor of about 20K between one side and the grid. Negative side of HV supply goes to the grid. When there is no anode current, grid and heater are at the same potential. At an instant when there is 10 mA of HV current, the 10 mA flows through the 20K resistor and makes the grid 200 V negative with respect to the heater. This tends to reduce (regulate) the HV current, as well as focus the spot.
You are absolutely correct. When a "grid type" of X-ray tube is used in an AC (60Hz) tube head, the grid (which is really the focusing cup) is tied to the "hot" lead of a 35KV winding, and the filament is heated with a low voltage winding on the hv transformer. The other 35KV winding is tied to the anode (in opposite phase), and so the tube effectively "sees" 70KV across it. The filament supply has to be very well insulated because that low filament voltage sits "floating" at 35KV above ground! When the tube draws current, the voltage drop across the grid resistor is what gives the tube the proper bias which not only controls the tube current, but it sets the proper voltage on the focusing cup that will create a very small focal spot on the anode target. The focusing cup acts like an electric "lens". In an ideal world, you want the focal spot to be very tiny so that it produces a "point source" of X-ray photons. But in reality, it's always a tradeoff between focal spot size (which determines image resolution) and the amount of power that the tube can handle (which determines image intensity and contrast).
In a DC (also called high frequency) tube head, the filament is at ground potential, and usually fed from a regulated DC power supply. The grid voltage is controlled with circuitry that not only sets the proper operating bias on the tube, but it is sometimes used to turn the tube on and off for precise exposure timing, and it provides much steeper rise and fall times than you can get by turning the hv power on and off, which is the usual method of timing the exposures.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Well I'm just trying to get a small focal spot, and the current will be set via the fixed heater voltage. This machine is designed for long exposures at low luminance, so precise control over the exposure duration is not needed.
If this tube requires the grid to be biased at a certain voltage then I'll have to pick a different tube since I have no way to bias the grid. All my tubes small enough to fit in here have some sort of grid, save for one which I have no data on. It has a focusing cup though, so I think I'll use the unmarked tube instead of the CEI one, even if it's anode spot is pitted.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Grenadier wrote ...
Well I'm just trying to get a small focal spot, and the current will be set via the fixed heater voltage. This machine is designed for long exposures at low luminance, so precise control over the exposure duration is not needed.
If this tube requires the grid to be biased at a certain voltage then I'll have to pick a different tube since I have no way to bias the grid. All my tubes small enough to fit in here have some sort of grid, save for one which I have no data on. It has a focusing cup though, so I think I'll use the unmarked tube instead of the CEI one, even if it's anode spot is pitted.
Pitted anode spot > large unfocused spot.
If you're planning to stick with the hobby for some time, it's worthwhile building an independent grid bias supply, which need not be complex or costly, because very little power is required.
As an alternative, you could wire 22 9V PP3s in series to get your 200V. So little current is drawn that the batteries will last for years. You also have the option of easy sign reversal if you get into grounded anode mode, where many of the Be window tubes are at.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
SOOO... I got it running agian. Worked fine at 100W, and for a while at 200W.
But then, it stopped radiating. Nothing exploded this time; just stopped making rays. Powered it up a few times after that and read 15kV on the kVmeter, 1.5mA on the other one. I decreased the heater voltage and mA decreased, so I assume that the tube is still working as it should. However, voltage didn't increase anything special so I thought the CW took a shit.
After tearing apart the head and getting oily again, I come to find out that both the CW ant tube were OK. Turns out the FBT died on me, internal arcing on the LV windings.
Upon testing the ZVS with another FBT to make sure that was not the issue, I managed to make it the issue by accidentally shorting a primary coil and burning out the volt regulator. Plan on fixing that soon.
Back to the failed FBT. I checked the secondary and it was intact, 470 ohms. That means it WAS the LV windings that failed, and lacking anything else to do I ripped 'em out.
I'm also replacing the core, since the other one was a bit too puny for 200W. This one might work...
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Way ta go, Grenadier! GOOD JOB on a rather complex project. You got my vote on BuildLounge. Now if every other 4hv member would vote for you, then you'd have a good chance of winning that laser cutter! Good Luck!
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Grenadier's big thread of Röntgen related shenanigans
