Hello all. New here. I ran across this thread and felt I needed to post some helpful information. Sorry for the long post. I am long winded. Computer power supplies are 'switch mode' type power supplies. Wall-warts, like the one pictured in the previous post are 'linear' power supplies. Switch mode power supplies are typically more expensive, but are far more efficient. They don't waste energy like linear type supplies. Both linear supplies and switch mode supplies come in a variety of voltages and currents. Both are readily available from literally thousands of suppliers online or in local stores.
A linear power supply generally consists of a transformer, a half wave (sometimes full wave) bridge rectifier and a filter capacitor. The transformer drops the line voltage (110-120 volts from the outlet) down to the voltage required, such as 12 volts in the case of computer type fans. But, this voltage is still AC... not the DC voltage required by such fans. To convert the voltage to DC, the bridge rectifier is used. It is a clever use of 'diodes', to steer the positive half of the AC wave to to one terminal and the negative half of the wave to the other terminal. Now you have DC. The filter capacitor acts as a 'filler', to fill in the 'bumps' in the voltage wave, smoothing out the DC output. (I am seriously generalizing) Otherwise, the output would look like a very bumpy road if viewed on an oscilloscope. Since a transformer is used, it is 'on' the entire time the unit is plugged into the wall outlet. Also, a transformer used to get 3 Amps of current is quite large and heavy. Basically, linear type supplies are hideously wasteful and overly bulky for what they put out.
More modern power supplies are 'switch mode'. This is a relatively complex circuit, whose explanation is well beyond anything practical for this post... or for most readers to enjoy. Switch mode supplies that can supply 3 Amps or more current are small comparatively and are extremely efficient. One does not really have to know how they work to use them. Most all switch mode supplies come ready to use. Plug in line voltage to the input and plug the device to be powered to the output. I use 3 Amp switch mode regulators in my RC planes. If such a regulator was of the linear type, it would be so wasteful it would not be worth it. If it were a transformer based linear supply... the plane would be to large and heavy to fly.
If a person plans to use a switch mode computer power supply to power computer fans or other hardware, it can be done. It is not extremely dangerous to modify them. There are no high value or capacity capacitors in them. As long as the unit is unplugged when working on it... there is no real shock hazard. To be sure the capacitors in the circuit are discharged enough to eliminate 'all' shock hazards... simply let the unit sit unplugged for an hour and you are good to go. The most that would happen (if any charge remains), is that you may short a capacitor to ground with a wire, tool or the soldering iron. The most you will get as a result... is a bit of a spark. Nothing serious like what was suggested earlier in this thread. But... what must be known is that some modifications 'would' need to be done to make it work. A computer power supply is designed to detect if it is connected to a load on the 5 volt line. A load is any electrical device that draws current. In the case of the computer... it is the computer's motherboard. If the circuit detects a load on the 5 volt line, it will allow the entire power supply to power up. (Some power supplies may differ) To remedy this, a simple fix it to solder a 6 volt automotive light bulb to the ground and 5 volt output wire. This will load the power supply's 5 volt line and will allow the power supply to power up every time the switch is on. Typically, a computer power supply is good for as much as 20 Amps of current. Different voltage outputs will have different current capabilities. These are listed on the computer power supply's case. So yes... a computer power supply can be invaluable to hobbyist use and can be done quite safely, even for individuals with minimal electronics knowledge or experience. I use my old computer power supplies as a bench top supply for many projects, including a regulated supply for charging Lithium Polymer batteries as well as other types of batteries. You would be hard pressed to find a small, light weight, high current power supply that is cheaper. Such power supplies are only considered scary, if a person knows little to nothing about them. In such a situation, it is always best to follow the general rule. If you don't know something... ask. Then you will know.
When it comes to going the simple route and using wall-wart type linear power supplies, expect higher energy usage for what you get out of it. If you are concerned with power consumption... they are not the best route. To get 3 Amps from one... it will be quite large and produce a good amount of heat comparatively. Also, they take up a good deal of space on the wall outlet. Not all power supplies such as this, have the same pin-out on the output cable. The center may be positive, or it may be negative. Depends on who made it. No matter what power supply is used, it is always a good idea for a person working with electrical circuits, to purchase at least a cheap DMM, (Digital Multi-Meter). Such a meter will have the basics one needs to read voltage, current, resistance and other features... depending on the complexity of the meter. Typically such meters can be purchased for less than $20 at Radio Shack or Sears. Typically, if the test leads of the DMM are placed across the output wires of the power supply, you will get a voltage reading that is either a positive reading, or a negative reading. If a '-' sign precedes the voltage reading... then you have the test leads hooked up backwards. This is a good thing, because it tells you that your polarity is wrong. For example, the positive test lead (red) is hooked up to the negative wire of the power supply. Now you know the polarity of the power supply wires. Switch the test leads around and read again. Make sure there is no negative sign preceding the reading. Then, simply mark the positive wire with tape and draw a plus sign on the tape. Now there is no mistaking which wire is positive and which is negative. Not all devices will have a standard color code, so don't 'ever' depend on color code to determine what a wire is carrying. Always test it with your meter. Don't ever assume that a wire has a specific voltage on it. Read it with the meter and be sure. Once you have a meter... you will find uses for it everywhere. Checking outlets to see if they are 'on'. Checking your car battery voltage. Checking all kinds of things that use power. After all, our whole world runs on it. It doesn't hurt to learn a little something about it and purchase at least a cheap meter to help you along the way. Especially if you plan to wire up high current devices for use in a grow cabinet/room, etc.
Mr. Ganja has it right. Don't shy away from working with something because you don't know anything about it. Everyone starts doing something for the first time. Nobody is born with the knowledge. So a person has to do some research. How much research is needed to properly construct a grow cabinet, then grow something interesting in it?
Just make sure they say 12 volts...I have quite a few of these laying around the house from answerting machines, portable game systems, ect.... there are two wires on the plug, the black wire on the plug that has a stripe is power..wire the striped black power wire of the adapter to the red wire of the fan. then wire the black wire from the adapter to the black wire of the fan.. good luck with your grow...