AC Filament Supply for an XRay Tube

[sixtimesseven]

Hi

I'm looking for a adjustable AC filament supply for an Xray tube.
It should be pretty accurate since my normal operating point at 1.15A is pretty close to instant failure of the filament at 1.35A. Voltage is about 2.6V.
Are there any good schematics for "normal tube filaments" which could be adapted?

DC works fine so far. But apparently DC will lead to brittle filaments over time and I do not want to damage my micro tube

[jonpaul]

Is the cathod near ground or at HV +/-?

If at HV what is the isolation voltage filament to ground?

Jon

[sixtimesseven]

If at HV what is the isolation voltage filament to ground?

I don't think you can get indirectly heated X-ray tubes (too much cathode bombardment).

The filament and the cathode are at the same potential and are ground referenced. Anode is at 60kV.

[sixtimesseven]

I mean a variac / transformer with tabs seems to be the classic approach.

Inverters seem to be the modern solution but are much more complicated to get right.

For my needs (2.5V 1.2A, 3W) an class b amplifier (and a audio transformer) would probably do it. With a rms current measurment chip for feedback and supervision, just to be on the safe side. I was just wondering if there is a better solution...

[Terry Bites]

Usually there is large choke in series with the filament transfomer primary to limit the maximum current.

[NiHaoMike]

Just add a H bridge to the DC supply to turn it to AC.

[Zero999]

A simple self-oscillating H-bridge can be made with a few transistors. The downside is it will have some shoot-through, which can be mitigated by adding an inductor, or current limiting.
https://www.eevblog.com/forum/beginners/mini-h-bridge-for-driving-vfd-filament-very-warm-to-the-touch/msg567741/#msg567741
https://www.eevblog.com/forum/projects/power-supply-low-v-dcac/msg1105483/#msg1105483

[james_s]

If at HV what is the isolation voltage filament to ground?

I don't think you can get indirectly heated X-ray tubes (too much cathode bombardment).

The filament and the cathode are at the same potential and are ground referenced. Anode is at 60kV.

Not only are they the same potential, they are one and the same thing. The cathode in an xray tube is a directly heated plain tungsten filament. Typically in older machines they are NOT ground referenced though, the HV transformer secondary will have a grounded center tap which is also where the tube current is monitored, the cathode will be at the same potential below ground as the anode is above. Modern inverter machines often have a grounded cathode and use a HV multiplier to power the anode with closed loop feedback to maintain the voltage.

On the old iron transformer machines the filament transformer is a conventional iron transformer with the windings on separate bobbins for HV isolation since the cathode is floating at a high negative voltage. On the modern (dental) machines I've dealt with it is a simple open loop buck regulator that provides a regulated DC voltage.

Normally the filament is run at about incandescent lightbulb filament temperature, it should not be THAT close to instant failure.

[sixtimesseven]

Actual datasheet is here:
https://www.eevblog.com/forum/projects/x-ray-control-grid-voltage/msg3086597/#msg3086597

I seem to have lost it locally, good thing I posted it 

Normally the filament is run at about incandescent lightbulb filament temperature, it should not be THAT close to instant failure.

I usually run it at 1.15-1.2A, then it glows reasonably bright, pretty close to the 1.5A or so for instant failure.

A simple self-oscillating H-bridge can be made with a few transistors. The downside is it will have some shoot-through, which can be mitigated by adding an inductor, or current limiting.

Looks interesting as well.

[sixtimesseven]

So, I thought about how to get a variable AC supply running for a simple filament. Decided to do a quick test with a Arduino, two PWM outputs, 180 degree to each other and a drv8871 H-Bridge Motor driver.

The breadboard still lacks the LC filter as well as monitoring and maybe a isolation transformer.

Pretty happy for a 5min test.

EDIT:
On the scope: CH1 and CH2 are PWM outputs and IN1 / IN2 inputs on the drv8871 and CH3 is the H-Bridge output.

[james_s]

Am I missing where it says it needs an AC supply on the filament? Old iron transformer xray systems typically used AC because it was easy, they would power the filament with a simple transformer and use a large adjustable power resistor to set the filament voltage to calibrate the output of the machine. Some time back I reverse engineered a relatively modern Kodak inverter dental head and that system used a DC filament supply, it was nothing more than a constant voltage buck regulator with the output voltage set during calibration.

[sixtimesseven]

Am I missing where it says it needs an AC supply on the filament? Old iron transformer xray systems typically used AC because it was easy, they would power the filament with a simple transformer and use a large adjustable power resistor to set the filament voltage to calibrate the output of the machine. Some time back I reverse engineered a relatively modern Kodak inverter dental head and that system used a DC filament supply, it was nothing more than a constant voltage buck regulator with the output voltage set during calibration.

I used DC so far which worked. Apparently, DC does reduce lifetime of the filament and can distort the focal spot.
Apparently, the aging problem is easily fixed by adding some ripple on top of the dc current. But I would like to try if ac will improve resolution with the smaller focal spot.

https://www.spellmanhv.com/en/Technical-Resources/Application-Notes-X-Ray-Generators/AN-02#DC/AC-Filament