How to use a computer ATX power supply as a lab power supply

From ArticleWorld


Computer ATX power supplies are currently being phased out in favor of newer models. This makes them very cheap and quite easy to find, thus being a good alternative for the expensive lab power supplies used in schools. ATX power supplies have the advantage of providing a very tight voltage regulation and good short circuit protection, making them ideal for experiments.

You will need an ATX power supply that can go to more than 150W of power. You will also need wire cutters, a drill, soldering wire and iron, electrical tape, some needle nose pliers and heat shrink tubing. You may also want to use LEDs, a case in which you will also need resistors. A low wattage switch and a power resistor for loading the power supply are also required, as well as binding posts for output terminals. The power resistor should be a 10-ohm resistor able to carry at least 10W.

[edit]

Steps

1. Get the power supply from the computer's case, if you have not already done so, disconnecting anything that may be connected to it. Cut off all the connectors, leaving a few inches for later use.

2. On the power supply, you will se a black and a red wire. Strip the insulation of the ends and connect them for a few seconds, in order to discharge the power supply. Now gather all the parts you need.

3. Remove the screws that connect the top and the bottom of the power supply unit's case. Now take all the wires that have the same colors and bundle them together. The standard colors are:

  • Red: +5V
  • Yellow: +12V
  • Orange: +3.3V
  • Purple: +5V/Standby
  • White: -5V
  • Blue: -12V
  • Black: Ground
  • Brown: Sense
  • Green: DC on/Input
  • Gray: Power on/Output

Newer power supplies have some extra wires. There should be four red wires and three orange wires in the main power bundle, out of a total of 20 wires. If you can only find two orange wires, you also have a brown wire, which you should connect to the orange one. If you only find three red wires, then another wire (usually pink) has to be connected to them.

4. Now drill some holes in the free area of the case. Make sure that you do not leave any metallic parts inside the power supply (and that you only drill the case, of course). It is a good idea to mark the holes before you drill them. Drill 5 holes for the connectors (one for -12V, +12V, -5V, GND and +5V respectively), and two other holes for a LED and a power switch if you need them. You may not care about the LED, but it is a good idea to use a power switch, so that you can prevent accidents.

5. When you are sure that you have drilled the holes correctly, screw the binding posts in their holes and attach the nut to the back.

6. Now connect the components as following:

  • One of the red wires goes to one end the power resistors. The other are tied to the red (+5V) binding posts.
  • One of the black wires goes to the other end of the power resistor; another black wire goes to a 330 ohm resistor attached to the anode of the LED (if it is installed). A third black wire should go to one end the DC-On switch, and all other black wires go to the black binding post (Ground).
  • The green wire goes to the other end of the switch.
  • The white wire goes to the -5V binding post.
  • The yellow wire goes to the +12V binding post.
  • The blue wire goes to the -12V binding post.
  • The gray wire goes to the LED's cathode (if it is installed).

7. Insulate the soldered ends in heat-shrink tubes and organize the wires with electrical tape to allow a good airflow. This will be useful because the power supply may heat seriously during experiments.

8. Once you are sure you have done everything correctly, plug the power cord in the back to an AC socket. Flip the supply's main switch and, if needed, the switch placed to the front. Test everything with a 12V light bulb.

[edit]

Notes

A good way to check your wiring is to see what happens to the supply's fan. If the fan is spinning, then you probably did something wrong. You should do the light bulb test even if the LED does not light up: there is a chance that only the LED does not work, either due to an incorrect wiring or simply because it is broken.

It is very important that you do not touch any of the lines that go to the large blue tubes. These are the capacitors, and their discharge can be lethal. If you do not have much experience in working with power supplies, you should not try to build this power supply. Ask someone who is more experienced to do it, or at least to assist you while building it.

Retrieved from "http://articleworld.org/index.php/How_to_use_a_computer_ATX_power_supply_as_a_lab_power_supply"