As I delved more into making a power supply for the ITS1A thyratron, the design became more complex. For example, to produce 100V from the inductor I would need an external FET. To switch the FET properly, I would need another transistor. If I was going to do that, I would use a completely different chip in the first place. So I re-considered what I was trying to achieve, which was simply to light up one of my tubes, just to prove that I could. So I used an existing 50V power supply I had built using the MC34063, and just built two Cockcroft -Walton ladders – a regular voltage doubler for the +100V, and a ridiculous ladder with 12 diodes for the -300V. Actually the data sheet (which I translated with the help of an online OCR and google translate) says that should be -250V. So that is what I used. Here is a picture:
I verified all the voltages, then the next step was to figure out what pins did what. Careful examination of the tube showed that two pins were cut short – this correlated with two pins described as ‘free’ on the data sheet and that allowed me to figure out what went where:
So with this I was able to wire the tube up and get it to glow:
You can clearly see the detail of how the phosphor is activated.
What I haven’t been able to do is to control which segments are on an which are off! It is clearly something to do with grid two, but I haven’t been able to figure it out yet.
I have been meaning to get some ITS1A thyratron display tubes for some time, and finally bought some a few weeks ago. These are a seven segment display tube that looks a little like a VFD tube when on – they use the same phosphor – but they are driven entirely differently.
Although they can be controlled with logic-level signals (roughly 1V to 5V), they require a bizarre set of voltages to actually activate them. The data sheet specifies around 40V, 100V and -240V. Others have apparently driven them with 50V, 100V and -300V. Yes, that’s right, that is minus 300V.
Now I don’t happen to have a power supply lying around that can produce that range of voltages, but it is surprisingly easy to build one. Or at least design one. I haven’t built it yet. The principle is to first build a simple boost converter, then use a Cockcroft-Walton voltage multiplier driven from the un-rectified output of the inductor, to get the negative voltage. I simulated one in LTSpice. I set the output voltage to 100V. Built a diode/capacitor ladder for the -300V and used a 50V zener diode voltage clamp to create the 50V. This is what it looks like:
This is what the simulation looks like:
The part numbers for the diodes are just examples. I haven’t actually chosen them yet. Both the diodes and the capacitors in the ladder need to be able to handle over 100V. The capacitors should be low ESR types. The inductor needs to be able to handle the expected current, though I haven;t figured out what that is yet. However my aim with this is to just be able to test that the tubes work, and maybe have a little fun with them. An actual clock will come later.