That's right, I built a 12V power supply for testing pretty much anything I need to that would need that type of voltage, specially electrical equipement for vehicles. It's pretty simple and can be built using pretty much things you already have. The actual cost will depend on how neat you want it to look and how you want it to work. I did mine for under $8, but it's also pretty basic and I had a lot of stuff laying around. Anyway, here a list of things you'll need:
- 1 Computer power supply (the lower the wattage the better)
- 1 switch
- 1 10 Ohm, 1W power resistor
- ? binding posts
That's pretty much it. You should already have the computer power supply from a computer that is just collecting dust. Don't forget the the lower the wattage the better. I say that because high wattage power supplies (such as the one I used, which was a 350W) puts out too much Amperage at 12V, but can be used if your careful and don't allow a circuit to be connected for too long and overheat. Everything you need can be found at Radio Shack, so shouldn't be a finding the material needed.
The Build (don't forget that some basic electrical circuitry knowledge is needed):
It's actually pretty simple since already have everything you need. I personally didn't want to spend the money on the switch so I just used the switch that switches between 110V and 220V at the back of the supply. It really didn't matter to me if it was there or not since I know I'm only going to be using 110V to power this thing. Because of this, the first thing I did was cut the wires going to this switch, and since the switch is on the ON position for 110V you will need to short the two wires. I did this as close to where the wires meet the circuit board as possible so it wouldn't be in the way.
The next thing you need to know is what each wire coming off the board does. Here's a table for ATX supplies (but it should be available on the side of your power supply):
Black ---- Ground
Yellow ---- 12V
Red ------- 5V
Orange --- 3.3V
Green ----- Power
Grey ------ "ON" (I'll explain this at the end, it's optional)
Some, really old, power supplies will not have the 3.3V wire available, and Dells from 96-02 (I think) doesn't follow the ATX standard.
Note: From a few years back on power supplies started being "switched". What that means is that they need to have a certain amount of load for then to stay on, and that's where the 10 Ohm, 1W resistor comes in. It'll need to be inline with a 5V wire and ground. Make sure it stays in a well ventilated part of the power supply because these tend to get extremely hot... I put mind right infront of the fan.
In my case, I only wanted to 12V, but 3.3 and 5V might be useful for you application. At this point, I just started cutting wires. They don't need to be really long, just make sure it's long enough for the wires to get were they need to go. Cut any wires you're not going to be using as close to the board as possible since you want as clean of a job as possible.
I then got the power (green) wire and a ground and connected it to the switch... This is your power button/switch. After this, it's just connecting the appropriate wires to the biniding post and making sure it doesn't touch the case since they'll be grounded. At radio shack, the binding posts are sold in a black and red combo, black for ground and red for hot (shouldn't be hard figuring this out). Use as many wires as you want.
Caution: At the beginning I mentioned how high wattage supplies have different amperage. As an example: my 350W supply puts out 12V @ 12A, a 145W supply will put something like 12V @ 8A. Be careful with this, at this much higher amperage it's very easy to fry a circuit if you're leave it connected for too long.
The "ON" wire (optional): The ON wire is there to tell the motherboard that the power supply passed it's POST (Power On Self Test) and is OK to run. The interesting thing is that it sends a constant 5V while on. This can be used (if you want) to power an LED and tell you that the supply is on and OK to be used. To make this work, you'll need to connect the grey wire to the anode (long) leg and a ground wire to the cathode (short) leg of the LED along with a 220 Ohm resistor.
WARNING:
Keep in mind that the power in a computer power supply is enough to kill, so BE EXTREMELY CAREFUL when working in it. Make sure it's properly ground before first use and you have someone near just in case a connection was not connected correctly.
