To avoid receiving an electric shock from the power supply unit, make sure you are wearing shoes with rubber soles, and avoid touching anything metal while handling the electronic components.
To avoid receiving an electric shock from the power supply unit, make sure you are wearing shoes with rubber soles, and avoid touching anything metal while handling the electronic components.
Warning: Some people suggest attaching a 10-ohm resistor between a black and red wire (from the power cables on the output side), however, this is only guaranteed to drain the low voltage capacitors on the output - which really aren’t dangerous. It could also leave the high-voltage capacitors charged, resulting in a potentially dangerous—or even lethal—situation. Warning: If you suspect the power supply is damaged, do not use it! If it is damaged, the protection circuitry may not work. Normally, a protection circuit will slowly discharge the high voltage capacitors - but if the supply was connected to 240V while set at 120V (for example), the protection circuits have probably been destroyed. If so, the power supply might not shut down when it is overloaded or when it begins to fail.
6 Binding posts (terminals). A power resistor (10-ohms). 2 LED (recommended one green and one red). 2 dropping resistors (330-ohms). An SPST switch. A drill A Soldering iron Wire cutters Heat shrink tubing
Warning: Be very careful around the black capacitor caps and all the wires leading to them. They may discharge a strong electric current. Warning: This will certainly void any warranty you may have on the power supply unit. Warning: Do not remove the circuit board unless you must. The traces and solder on the underside could still have high voltage on them if you didn’t let the PSU sit long enough. If you must remove it, use a meter to check for voltage on the pins of the largest capacitors. When you replace the board, make sure that the plastic sheet goes back under the board. Only power supply technicians should attempt this. Warning: Anything above 30 milliamps/volts can kill you, or at the very least, give you a painful shock. Make sure that you have removed the power cord before doing the conversion and have discharged the capacitors by letting it rest for a few days. If in doubt, use a multimeter.
Red = +5V. Yellow = +12V. Blue = -12V. Orange = +3. 3V. White = -5V (older power supply units only). Purple = +5V Standby. Black = Ground (0V), Gray = power is on (output). Green = PS_ON# (turn DC on by shorting to ground).
To get more room you can mount the fan on the outside of the PSU case or remove it. You can also attach more fans if you are using a high-wattage power supply. If you don’t have enough room inside the power supply, you can mount all the components to a separate board outside the power supply unit.
Make sure you don’t put any holes where the binding posts will be touching anything on the inside of the power supply. Be careful not to leave any metal fillings or debris inside the circuitry of the power supply unit.
You can also consider using a lighted 12v switch, which will act as the load necessary to turn on the power supply. If you aren’t afraid of some soldering, you can replace the 10w power resistor with the cooling fan that was originally inside the PSU, be careful with the polarity though - match the red and black wires to each other.
Some power supplies need the grey and green to be connected together in order to run. If you don’t want to use an additional switch, just connect the green and black wires together. The PSU will be controlled by the rear switch, if there is one. You also don’t need an LED, just ignore the gray wire. Cut it short and insulate it from the rest.
The +5VSB line is +5V standby (so the motherboard’s power buttons, Wake on LAN, etc. work). This typically provides 500-1000 mA of current, even when the main DC outputs are “off”. It might be useful to drive an LED from this as an indication that the mains are on.
Look for ATX power supplies with a 20-pin connector, a 20+4-pin connector, or an AT power supply if you need -5V.
If you only have three red wires, another wire (sometimes pink) must be connected to them.
Note that some power supplies may have either a gray or brown wire to represent “power good”/“power ok”. (Most PSU’s have a smaller orange wire that is used for sensing— 3. 3V—and this wire is usually paired at the connector to another orange wire. Make sure this wire is connected to the other orange wires, otherwise your lab power supply won’t stay on. ) This wire should be connected to either an orange wire (+3. 3V) or a red wire (+5V) for the power supply to function. When in doubt, try the lower voltage first (+3. 3V). Some newer power supplies will have “voltage sense” wires that need to be connected to the actual voltage wires for proper operation. If you only have two or less orange wires, you should also have a brown wire which must be connected with the orange.
Check for loose connections by gently tugging on them. Inspect for bare wires, and cover them to prevent a short circuit. If you don’t feel like soldering nine wires together to a binding post (as is the case with the ground wires) you can snip them at the PCB. 1-3 wires should be fine. This includes cutting any wires that you don’t ever plan on using.
The voltages that can be output by this unit are 24v (+12, -12), 17v (+5, -12), 12v (+12, GND), 10v (+5, -5), 7v (+12, +5), 5v (+5, GND) which should be sufficient for most electrical testing.
