Saturday, September 24, 2011

Garage Sale Handplanes #2: PSU Mod for Electrolysis

This is Part 2 of my previous post Garage Sale Handplanes

I have tried using Naval Jelly for rust removal and never liked the staining that occurs. If you miss a spot during application, you'll clearly see the outline of the stain. Successive coats won't hide it. The only way to remove them is to sand away the stain. A friend of mine had great success using electrolysis to remove rust from an old gun so I thought I would give that a try.

Electrolysis rust removal is essentially the process of using electricity and an electrolyte to move the rust from the part being cleaned to a sacrificial electrode. There's a bit more going on there, but you get the picture.

The first step then, is to provide the electricity. Pretty much any DC current will do so long as you have some sort of protection for short circuits. Most people use a 12v car battery charger/starter since they are cheap and most people have one. It should be able to charge at rate of a couple amps. The trickle chargers won't work. I unfortunately don't have one, but I do have an old computer power supply. A PC power supply (PSU) is basically a regulated AC to DC converter that provides multiple voltages ("rails"). The only difficulty is that turning on a PSU requires you to jump a wire (short 2 wires together) and ensure there is a load on the 5volt rail.

On any PSU, there should be a nameplate which gives you the number of amps each rail can produce. Here you can see the +12V rail can produce a good 18 amps! That will be more than enough for our needs


My goals:

  • Add a switch so I don't have to jump wires to start this thing
  • Add binding posts to make connecting wires easier
  • Add a fuse to protect the PSU in case I cause a short circuit
  • Add resistors to stabilize the voltage

Here is a view w/ the cover removed. As you can see there's not much room for me to add anything.

One thing to note, is that based on what I saw, the PSU is laid out so that the high voltage AC work is done on one side (top in the pic above) and the lower voltage DC work is on the other. Since I'm adding wires and components for DC, I'll want to stay towards the DC side as much as possible.

I don't need no stinking wires! Well, actually I do. These wires are color coded. The green wire will power on the PSU if I connect it to one of the black wires. The yellow, red, and orange wires represent the +12V, +5V, and +3.3V rails respectively. The black wires are all ground. I removed all the wires except the yellow, black, green, one red, and a pair of wires for an extra fan. Of the colors I did keep, I still had too many wires so I thinned them down so I would have more room in the case.

To stabilize the voltages, I needed to put a load on the +5V rail (red wires). I used 2 20Ohm 5 watt resistors wired in parallel. This gives me effectively a Since they would get hot dissipating that power, I zip-tied them to the case grill. 

Here you can see the binding posts for banana plugs and the switch installed on the case. The biggest pain was making a 1/2" hole for the switch. A stepped drill bit (the kind that looks like a cone) is what you want. I tried using a dewalt twist drill bit with a smaller "starter" bit at the tip. That failed miserably because the transition between the larger part of the bit would catch on the metal, bend it up and jam the whole kaboodle. The next major pain was my fault as you have to be aware of which nuts/sleeves need to be on which part of the wire before you solder. I ended up soldering the binding posts 3 times because of this.

Here's a look from the inside. Notice I used shrink wrap tubing to minimized the exposed metal. Hopefully nothing will short out.
Here you can see that I added battery clamps.Again remember to thread the clamp handles onto the wire before attaching them otherwise you wont be able to get them back on.
 Here is the 15 amp inline fuse. The PSU probably has some sort of protection also, but these are cheap to install and fuses are cheap to replace.

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