HasPower

Behold, shiney giver of electrons! This is one of those projects that had been kicking around in the back of the cranium for a while and came together all at once, mostly by accident. First, in the summer (of ’05) my cousin’s laptop power brick was deemed useless for it’s intended purpose when it caught fire because the ground wire frayed totally and made sparks. I was commissioned to get rid of said power brick. And the following Spring (’06) I took a metalworking class and was required to make a box. Perfecto chronologium. (No, that’s not actual latin for “good timing”.)

Unfortunately, I fried it accidentally (my sloppy wiring on my Wah pedal shorted out) and so it has sat and rusted, soon to be cannibalized for (working) parts when I build a new one. So much for good ideas.

Specs:

  • Input: 120-240VAC
  • Output: 2×9.7VDC @ 200+ma each
  • Very clean power (no ripple, <5mv of noise on the output – less than my Boss PSA)
  • Compact size (approximately 2.25″x2.5″x5.375″)
  • Cheap (I spent ~$10 for stuff I didn’t have in my collection)

The box was fabricated at the school shop using the bar-folder and finger brake and the corners spot welded. Most holes and other fitting stuff were done at home.

The real power supply is the guts of a Dell PA-9 laptop power brick. The only modifications were removal from it’s plastic case, conversion to standard IEC power connector and change of wire on the output terminals.

CAUTION: the copper foil sheild around this supply actually had about 60VAC potential on it when the unit was plugged in. Don’t let it contact other circuitry, metal casings, or your fingers, it gives one a bit of a surprise.

Because the Dell supply is rated for an output of 20V @ 4.5A, it obviously won’t work very well for running 9V stuff, so I stuck a couple linear regulators on it. It’s a standard 7809 regulator with a 22uf filter cap across the output and a diode on the ground leg to lift the output to 9.7V. I stole the values from my Boss PSA-120 this rig is replacing – the only difference is that the PSA uses a 1N4148 diode and I only had 1N914s on hand. That may be why the PSA is rated for 9.6V and I’ve got 9.7, not sure. Either way, it’s close enough to simulate a nice fresh 9V battery. (Incidentally, DigChip is a great resource for data sheets on almost any semiconductor you can think of, including regulators.)

In retrospect, this supply not very good for this type of application in general – you end up sinking 11V as heat, which is 2-3W extra your regulator has to get rid of.

As you may have noted above, the Dell supply has more than enough current to spare and I’m not using anywhere near capacity. I currently have two regulator circuits wired with (I’m guessing here) a capability of around 200-300ma each. One runs the ME-33, the other just the Wah at the moment. The regulators are bolted to the case via a piece of 1x1x0.125″ aluminum angle stock, the rational being 1) the PSA has no airflow and uses a little chunk of aluminum for a 200ma load, 2) I also have no airflow but can use a bigger hunk of aluminum and a metal case for a similar load on multiple regulators, and 3) the aluminum is a lot flatter than the bottom of my box – better heat transfer from the regulators – and I needed an 1/8″ shim anyway. The bottom did get a little warm under normal use and I didn’t really gunk up the junctions with heat-paste.

The regulator board was built to be as compact and modular as possible. I was planning on having space for four regulator circuits but I’m not sure I left enough room. The board can actually be disconnected from power input/output and unplugged from the regulators for ease of service. I used RCA jacks for output for two reasons: they were available for free and they don’t come unplugged as easily. The 2.3mm barrel connectors have a tendancy to yank out easily and are harder to get. Things to fix: The wires to the output jacks need to be both longer and more flexible, the 3/4 pin connectors need to be cemented to the board so they don’t wiggle so much, and I need to trim the case a little so it goes together better without so much care and feeding.

My total cost for this project was about $10US and a weekend – I bought two regulators for $1.25, 9 rubber feet for $2.99, and 2 RCA plugs for $3.50. The box I made in a couple hours, cutting holes in it took half of a Saturday, the rest of the parts I scrounged up. If you do this from scratch and don’t have the junk I do you’ll probably end up paying more like $15 in parts and hardware plus whatever the box costs, assuming you get a power brick for free. Even so, it’s pretty good compared to the $100+ for some commmercial effects pedal supplies.

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