12 Volt Power Supply Made From Random Junk – Part 01 [Design]


[Note, this is Part One. The project is completed in Part Two.] I need a power supply that can supply about 14V at loads of at least 10 amps. This need came about because of my 45 ft sailboat, which last year lost all its batteries. It had been moored on an inaccessible piling without power for its trickle charger. So, we bought a new deep cycle battery, but it needs more than a trickle charger to keep charged. Actually, the battery may already have been damaged. Ugh. I can’t afford to keep buying these big batteries. This power supply not only needs to be able to trickle, but to pump out a lot of amps when it is loaded by, say, the 12V pumps.

And, my challenge to myself is to make this power supply out of junk I have lying around my workshop. To be honest, I do have a LOT of such junk, so making a decent power supply should not be too hard. I shouldn’t have to buy anything to make this.

In designing it, the first question is – switcher power supply or linear? I have lots of linear parts, so that’s easily answered. For the transformer, I’ll use a Variac core, with a secondary winding done by hand. I’ll sketch out a design that is simple and easy to throw together. And uses no integrated circuits. Just for fun. Preliminary sketch — something to start with: (See my final design, which is very different.)

A quick sketch, just to get started.

I relented on my “junk only” resolve and bought the diode bridge – I ordered a couple of these from Mouser: KBPC5004W-G. Having a rating that is 100% higher than the design goal is just a few cents more than one that is just good enough. The power FETS came out of an old junked Pioneer Stereo Receiver. The current sense resistor is a hand made and calibrated one. (More on that later.) The Variac is one that had been on the boat for 25 years, that I’d used to correct for bad harbor AC volts (often as low as 105VAC).

The unit needs a heatsink and a fan. As I thought of simple ways to accomplish this, I remembered an old Packard Bell PC in the closet. I found it had a nice clean boxy power supply complete with fan and HP-type 120V power receptacle. I told myself, well I’ll just take it apart and use its fan and especially use its air flow venting as part of the design. Below, is a shot of the old supply with its cover off. I tore out all its electronics (saving the heatsinks and the two capacitors) and cut up the metal.

Old computer supply from a 1998-era PC. On the left is the Variac core showing the hand-wound secondary.

For the main chassis, I have this 12x10x6 18G steel box. (Ugh. I hate having to cut steel.) So, I joined the halves of the old 1998 supply together to make part of the chassis front, and installed it into the steel box, like this. Imagine that the fan is pushing air INTO the box. Air will emerge from the grillwork. So, under the grillwork is the preferred location for the heatsink.

Steel box, plus the PC supply panels

Then finally, I laboriously cut the original steel box front panel with a Dremel-type tool, and voila, we have a usable chassis. Again, imagine that the heatsink will go behind the fine grill. Then by running the fan backwards it will “suck”. (Heehee) That will cause the hot air to leave the top of the unit. One should never use a fan to force hot air to descend — hot air always needs to be allowed to rise. The blank side of the panel will still need an ammeter, a voltmeter, banana jacks, a couple of potentiometers, and maybe an indicator light. I don’t have a plan for those parts yet. This ends this update — more to come.

So, here is a usable chassis to start mounting parts onto and into.

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One thought on “12 Volt Power Supply Made From Random Junk – Part 01 [Design]

  1. Pingback: A Bench Power Supply That You Can Build Out of Junk – Part 2 | barbara4tech

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