So, after a long hiatus I am back in the lab and back to try and finish at least one damn project before I die. Started innocently enough, was bored on an evening and not knowing what to do with myself I thought I’d catch up on some of Dave Jone’s excellent video blog entries. Naturally, his energy and charisma stoked the flames of interest and had me missing the heady smell of flux and the hum of an energized transformer. I looked over what I had on the go and found with delight that I actually remembered a lot and was able to get back to where I was intuitively in no time at all (read: a couple of weeks of review).
Here I’ll summarize the projects I have on the go before I get to the good stuff – the experiments.
Almost finished. Kinda. Sorta. The schematic I reviewed for any errors and made sure I got everything right – no problem. I gave the half-finished board a good look-over and found it just fine, needing only to be populated. I know for certain I will be needing a second board on top of it, which is fine, to house the ridiculous number of high voltage transistors as well as the remainder of the 4017 counter ICs. I mapped out a plan of how i’m going to do my interconnects as well and puzzled over the problem of having something like 30 connections going from one board to the other. This is what happens when you use data that isn’t multiplexed and no microcontroller, you basically end up with a lot of wiring. For the power connectors and a few other things I don’t mind using molex style PCB connectors but I was faced with a challenge of how to route the 16 connector outputs from the bottom board 4017s to the top. I briefly flirted with the idea of keeping them all on the same board but then I would end up with 28 odd connections I would have to take to the second instead of 16. So with that in mind it dawned on me that the old IDC connector and the ribbon cable are ideal for this application! I’ll pick some up at the shops when I finally make it down there. As always, I’m s a few parts short on every project so it’s a worthwhile journey. All this one needs apart from this is a few switches to set the time some way to mount the nixie tubes securely and safely and of course a box to put it all in.
The power supply is, and always was, a beast of a project. Some do their first power supply simply but I wanted something flexible, cool, powerful, and more or less something I will want to use years down the road. The inevitable revisions take forever and the whole project is quite complex by this point. As I’ve mentioned before I’ve broken it into modules to make it easier on myself: AC Power, Pre-regulator, Current Limiter, Voltage Regulator, Control and display.
The AC Power board, as shown from previous posts, is complete and overall I’m quite pleased with it. It does have a rather high DC voltage output of ±42.6V which necessitates the inclusion of the pre-regulator block (mentioned previously) but otherwise performs just fine and will take more than I could possibly throw at it before it dies. Subsequent video watching and research has made me want to add the rather important addition of MOVs (Metal-Oxide Varistors) to add some over-voltage protection to the AC input of the supply though this is a rather trivial addition and simply have to add one each between the hot-ground, neutral-ground, and hot-neutral. I will probably add them to the rectification board or may have them on a separate board or hanging off the terminal block, I have not yet decided. Over-current protection is already present in the 5A hot fuse in the terminal block, as well as redundant case fuses I will employ in the final build. There is also the question of the ground lift. Research has shown me the wisdom of not tying the centre tap of the transformer (the 0V rail) to mains earth so with that in mind I will keep it floating by default with an aircraft switch on the front to enable mains earth referencing should I need it. Easy.
The pre-regulator has seen much progress since the last time I took a poke at it. I had previously included it, discarded it, then included it again in a much more workable form. After much fiddling in circuit lab, I settled on using a zener diode/darlington transistor regulator and crunched all the numbers into a workable solution. I did build it up and test it but will expound on my results in a subsequent blog post. The upshot is I can knock that crazy 42.6V down to much more usable 28-29V and have it work over a variety of loads which is nice. There still is the question of stability and whether or not it will play nice with the rest of the circuit.
The other modules are untouched from last check. It will be a long time before a completed product. I’m still fuzzy on a bunch of things and I’m expecting pitfalls along the way which could be both frustrating and highly amusing. Of course, that is why i’m doing this in the first place – I’m learning, and that is its own reward.
Of course this ties in closely with the power supply project as I need some practical way to test the thing under working conditions not to mention calibrate it. It has taken a back seat to other projects yet I will have to build it to build my power supply. Projects always lead to more projects. It all started, as mentioned previously, with Dave Jone’s excellent example, but I’ve been further spurred on by the discovery of Martin Lorton’s excellent version which will probably be much more suited to my needs. Naturally, I will add my own modifications to both make it my own and to suit my needs. Martin’s has a 2A cap (I believe he reduced it to 1500mA by the end though) and I need mine to sink 3A to properly test my power supply. It should be a simple matter of selecting the right mosfet and/or using mosfets in parallel.
Given that this project is likely to take me as long as the others and I still have not been able to leave my house to grab the appropriate parts, I went back to why I need this damn thing in the first place. The easiest solution for a dummy load is to of course whack the right value resistor that can handle the power you want to dump through it. This has proven most frustrating since not only do I not have a collection of power resistors on hand, but finding ones with the appropriate tolerance and power dissipation capability has proven to be difficult.
I decided for a more low-tech approach and see if I could make my own power resistor to act as a static dummy load just for now. That is a subject of the next blog post which I will marry with the power supply pre-regulator test. Quick answer: I did, and it not only worked but I’m still alive and my house is still standing.
Always something new an shinier on the horizon, isn’t there? This is why I never get anything done.
Keeping with the theme that projects beget other projects, the power supply needs a dummy load, the dummy load needs a precise high power low-ohm resistor, I need a way to measure low resistances. It’s commonly known that most DMMs do a woefully awful job of measuring low resistances. One has to dump enough power into it to see a measurable result, and things like the leads now have a nontrivial resistance. So I started building an adaptor for my multimeter that fixes these problems. A post will be written on this also. No, I didn’t finish this either.
This is merely an idea and a helpful schematic posted by someone on a forum somewhere. One of my eGo chargers for my ecig is malfunctioning and not doing it’s job and I sit here rather nervously waiting for my one functioning one to die and deny me my fix. I’m merely thinking about this one for now, if I get it wrong i’ll have explody batteries on my hands so you can bet I’m going about this one carefully.