Avalanche

Video is up and public and the world is good. Of course, it'll be a while before there are any code repos up since that's on the other end of main development.

Next thing up is going to be the power supply and converting that to the flyback topology I'd like to use. Stay tuned!

The project details are much more filled in now (well, with what information exists to this point), including a very professional slide deck I paid a consultant BIG MONEY to make. Video soon.

A bit has developed about the project in my mind over the last few days thinking about it. I think it lends itself well to a highly modular structure with 4 comprising the full thing: 1) a power supply, 2) the pulse generator (both the namesake and most trivial component), 3) an embedded µC-based timing unit, and 4) software for that timing unit to pipe data to on a PC.

1) Is straightforward, if I think one of the most useful bits of the project. Generating HV is useful for lots of things, but doing it in a comfortable fashion is tricky since, you know, it can kill you if you mess it up. Having banana leads clipped to mains floating around on the table is the easiest option, but also the most insane. I've been shocked going this route... well, we'll leave it there.

2) Because a high-bandwidth pulse is sort of the point.

3) it turns out, there are a lot of useful side-effects of the pulse generator circuit. Most obviously, its timing is non-deterministic, and the timing probability distribution is quite related to the charge voltage it's fed. I'm really interested in working with that both experimentally and as a random entropy generator. I need something with pretty high timing accuracy to actually listen to and log pulses.

4) Gotta get that data into the computer. I imagine I'll also set up some web server software, probably with a RasPi, to make the random output available online as well. Nothing cooler than entropy sources accessible via API!

Partly fleshed-out details and a video coming soon.

Obviously this project is a bit more underway than "just starting," but for the sake of starting a proper project log, I'll update with where it's at now.

The pulser bit is done. I mean, it's like 4 components, it's trivial. Although really, it's not done. It needs a PCB, proper RF connector, and probably some kind of source impedance. Realistically that's actually a number of details to work out.

Even so, the much-harder part is the power supply, and that's where I really plan to put the value of this project. At present, the supply/pulser system is MOSTLY self-contained, meaning it only needs two power supplies. A 5V supply runs a 555 to generate a mostly-50% duty cycle mostly-square wave, and that is then used to drive a SN754410NE H bridge. This is where the second supply comes in - the H bridge is switched by the 5V from the 555, but it's PUSHING a much higher second supply of anywhere from 0 to 35-odd volts. In practice, I'm currently finding best functionality around 12 or so volts, but you'll see in the next part why it even matters. And actually, I lied, the H bridge isn't driven directly by the 555 but rather by a pair of signals coming from an inverter sitting in the middle - one of the signals is inverted from the other, basically driving two outputs of the quad half-H bridge in a push-pull configuration.

So this push-pull output of the H bridge, equal to the second supply voltage (so peak to peak, double that) is then fed into a Cockroft-Walton multiplier of (as it happens to be now) 7 stages. The exact output of this multiplier, set by the second supply voltage (since it's a multiple of that), is used to drive the pulser. By adjusting it up, one can tune the frequency that the pulser recovers and thus re-pulses, though the exact timing is random. In general, if the output is just past the avalanche breakdown threshold of the device used (here, a 3904), you'll get one, infrequent pulse. By increasing it, the output capacitor charges more quickly and thus the device avalanches more frequently, though again, the timing is quite non-deterministic. In the final design, I very much intend to maintain adjustability of the power supply so one can fine-tune the output pulse frequency, since I'd like to do some experiments with that as a generator of random timing.

Thing is, this power supply topology in general isn't particularly great. I don't really like using the multiplier, if only because it's terribly low-current. I've also had some issues with poor isolation breaking the H bridge device - for example, if you try to spark the multiplier output to its ground reference, you can do that, but the H bridge tends not to survive. I suspect it's probably because of some kind of high voltage ground bounce. I'd like the supply to be more resilient to this kind of abuse, so I'm working on a different topology. That'll be the next update, I think.