Too Busy For Words - the PaulWay Blog

Mon 16th Apr, 2007

More projects, more sticks

Two projects occupy my idle time at the moment. The first is ringing around to find a place that will cut the metal end pieces for my laptop case cover design. I've got a very promising lead out of Serafin P/L in Queanbeyan, who do laser and water cutting, and have also given me the names of two companies who can do the folding and pressing of the cut pieces. So that's good.

The other project is to make a power converter box, a rehash of an old project that I've decided to bring off the back burner. I have a perfectly good 50W 12VDC power supply from my old Via EPIA firewall that died (capacitor bloat - watch out for it) that runs off my APC power line filter to stop it going down from transient power spikes or drops. It's running the Yawarra firewall, and I also want it to power the ADSL modem (15VDC at 1A - WTF?) and the network switch (6V at 0.75A). I've been scouring the net for compact DC-DC converters and have found several companies - V-Infinity, Recom and Analog Devices, as well as another supplier who I don't have to hand (I took the spec sheets home to study).

My further aim is to have a solar panel charging a 12V battery array which can keep these, and other essential items like MythTV, going in the event of a power failure. Although regular solar panel systems involve an inverter and a grid interface system, I was thinking of also having a direct 12V feed cabled to where it was needed. The above components all take direct DC, and I'm sure a bit of searching will turn up some suitable power supplies that run off 12VDC input. I understand that this kind of direct power is popular for computers run off big UPS systems, as you don't take a double loss of converting from DC to AC and back to DC. They all seem to take screw-in terminals, which is a bit ... shock-prone for my liking - I'm surprised there isn't some 'IEC'-style connector designed for this purpose. But then I suppose they usually live in locked cabinets in secure facilities.

The original project was a 'wall wart eliminator' that could be run off a computer's 12V supply to run the various peripherals connected to the computer that didn't need to be on when the computer was off. Apart from being more efficient - a lot of older wall warts are simple linear power supplies and use power even when the device they're attached to is off - it also eliminates a lot of the unsightly clutter on your desk or wall. I personally hate the things - they're never a convenient width so that you can put two side by side, and on vertical walls I'm always afraid that they're going to fall out and my network switch is going to go down unexpectedly.

The problem is, of course, what voltages does one need to supply? For a generic device that suited everyone, the answer would seem to be "a huge range" - I've got devices that I use that take 3V, 4.5V, 6V, 7.5V, 9V, 12V and 15V. This had me stymied for a long while - being able to choose your preferred voltage for each output would imply a very complex, custom switched-mode converter. But gradually an idea congealed in my subconscious, and when I read the key words "industry standard pin placement" on one spec sheet I realised: you could simply have a number of blank spaces or sockets on the board, and the user simply orders and obtains the necessary converters for their desired outputs and everyone's happy. About the only thing that wouldn't require this is the 12V output, which would just be a direct feed from the 12V input. While it might sound like this would need some kind of regulation, we can be reasonably confident that the input is also fairly well regulated, so this would just make that circuit less efficient. The 12V-12V converter just becomes a couple of wires.

So: more simple hardware hackery coming soon.

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