WARNING: The computer switching power supplies contain lethal voltages. Do not open them. Do not use the parts inside. Use an isolation transformer. Do not connect them to an old radio.
There are many parts in old computer power supplies that are useful. The Power Supply Units (PSU)s from old computers can be used for building projects. One of the projects that they have usable parts for are inverters. The computer supply uses a switching control Integrated Circuit (IC) some switching transistors, a transformer and some output filtering.
HOW A COMPUTER SWITCHING POWER SUPPLY WORKS
The mains is rectified (be it 115 VAC or 240 VAC) and the DC applied to the switching transistors. When the power supply is switched to 240 volts, the mains is rectified and applied to 2 capacitors in series, giving about 300 volts DC. When it is switched to 115 volts, the mains is voltage doubled using the 2 capacitors and this generates about the same voltage. This section uses some filtering, a bridge rectifier, and 2 large capacitors. The switching transistors switch the high DC voltage into a ferrite transformer. The output of the transformer is rectified, filtered and connected to the output sockets. The switching transistors are controlled by an IC which regulates the output by turning ON the transistors for a short time. If the output voltage is too high, the switching transistors are ON for a shorter time, and if the output volts are too low, they are switched ON for a longer time. The switching is done at a constant frequency at about 100 kHz and the width of the transistor ON time is varied. This is called Pulse Width Modulation (PWM). Operating at this high frequency makes the transformer small and the output filter choke and capacitors small. The drawback is that all components are operating very fast and at high currents, so they use schottky diodes (low forward drop and fast switching = low heat) and special output capacitors (high current).
IS THE SUPPLY ANY GOOD?
Visually inspect the insides for burning. If all seems clean and undamaged, test the PSU. Some will not run without a load, so connect a 1 ohm 10 watt resistor between the red wire and the adjacent black wire on one of the 4 pin output connectors (5V output). Connect a meter across the resistor to measure 5 volts DC. Plug a mains cord in, and turn the PSU on. If the meter reads 5 volts, the PSU is good, and a check of the yellow wire should read 12 VDC. All the parts should be usable. The fan may run depending on the state of its bearings and if the 12 VDC is good.
If the PSU goes (has gone) bang, only some of the parts will be usable. Open it and inspect the power transistors for holes in them, and any burnt parts. If nothing is obvious, check the fuse.
If the PSU does nothing or buzzes a little, then it has a fault. This fault may be simple, like a blown fuse, a high resistor in the switching transistor base circuit, or a short on the output. The short is usually due to a shorted filter capacitor (obvious by one bulging) or a shorted rectifier diode. The diodes are in a 3 pin package on a heatsink and have 2 diodes in the package. There are other individual diodes on the board.
USEFUL PARTS
The useful parts for inverter projects are the controller IC (quite often a TL494 sometimes labelled IR3M02), the driver transistors for the main switching transistors, the transformer (11:1 ratio), the output capacitors, the big output diodes, the little output diodes, the filter choke (a toroid), and generally the resistors and capacitors. There is sometimes an Op-Amp as well.
IN A FAULTY PSU, WHICH PARTS ARE GOOD?
It is easy to check the output diodes, small diodes, capacitors and resistors with an ohm meter. The small black diodes located around the output are usually the schottky type and the small glass types used elsewhere are usually normal types. You can check them with an ohm meter. You can determine what type they are by connecting them in series with a 10k resistor across a 12vdc power supply. If their forward bias is about 0.7 volts, they are a normal diode. If it is about 0.4 volts, then they are a schottky type. The toroid, input filter, and transformer will be OK. A catastrophic failure will usually have blown the switching transistors, but the IC and driver transistors often survive. Test the transistors with an ohm meter, they should have a high resistance both ways (measured by reversing the meter leads) between collector and emitter. They should have a high and low resistance between base/emitter and base/collector (measured by reversing the meter leads). Test the IC by building a circuit and see if it works, or test it in a known good inverter. The main functions are the oscillator, the 5 volt reference, the error amplifiers, and the output transistors.
I have built an inverter by only using parts from a computer PSU and a piece of perf board. It uses any DC voltage in the range 7 to 40 volts DC as input and gives 95 VDC out.