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RECYCLING OBSOLETE COMPUTER POWER SUPPLIES

Ray Robinson VK2ILV
7 Roland Ave, Wahroonga 2076
Sydney = N.S.W.=20 Australia

WARNING: The computer switching power supplies contain lethal = voltages. Do=20 not open them. Do not use the parts inside. Use an isolation = transformer. Do=20 not connect them to an old radio. =20

There are many parts in old computer power supplies that are useful. = The=20 Power Supply Units (PSU)s from old computers can be used for building = projects.=20 One of the projects that they have usable parts for are inverters. The = computer=20 supply uses a switching control Integrated Circuit (IC) some switching=20 transistors, a transformer and some output filtering. =20

HOW A COMPUTER SWITCHING POWER SUPPLY WORKS =20

The mains is rectified (be it 115 VAC or 240 VAC) and the DC applied = to the=20 switching transistors. When the power supply is switched to 240 volts, = the=20 mains is rectified and applied to 2 capacitors in series, giving about = 300=20 volts DC. When it is switched to 115 volts, the mains is voltage = doubled using=20 the 2 capacitors and this generates about the same voltage. This = section uses=20 some filtering, a bridge rectifier, and 2 large capacitors. The = switching=20 transistors switch the high DC voltage into a ferrite transformer. The = output=20 of the transformer is rectified, filtered and connected to the output = sockets.=20 The switching transistors are controlled by an IC which regulates the = output by=20 turning ON the transistors for a short time. If the output voltage is = too high,=20 the switching transistors are ON for a shorter time, and if the output = volts=20 are too low, they are switched ON for a longer time. The switching is = done at a=20 constant frequency at about 100 kHz and the width of the transistor ON = time is=20 varied. This is called Pulse Width Modulation (PWM). Operating at this = high=20 frequency makes the transformer small and the output filter choke and=20 capacitors small. The drawback is that all components are operating = very fast=20 and at high currents, so they use schottky diodes (low forward drop and = fast=20 switching =3D low heat) and special output capacitors (high current). =20

IS THE SUPPLY ANY GOOD? =20

Visually inspect the insides for burning. If all seems clean and = undamaged,=20 test the PSU. Some will not run without a load, so connect a 1 ohm 10 = watt=20 resistor between the red wire and the adjacent black wire on one of the = 4 pin=20 output connectors (5V output). Connect a meter across the resistor to = measure 5=20 volts DC. Plug a mains cord in, and turn the PSU on. If the meter reads = 5=20 volts, the PSU is good, and a check of the yellow wire should read 12 = VDC. All=20 the parts should be usable. The fan may run depending on the state of = its=20 bearings and if the 12 VDC is good. =20

If the PSU goes (has gone) bang, only some of the parts will be = usable. Open=20 it and inspect the power transistors for holes in them, and any burnt = parts. If=20 nothing is obvious, check the fuse. =20

If the PSU does nothing or buzzes a little, then it has a fault. This = fault=20 may be simple, like a blown fuse, a high resistor in the switching = transistor=20 base circuit, or a short on the output. The short is usually due to a = shorted=20 filter capacitor (obvious by one bulging) or a shorted rectifier diode. = The=20 diodes are in a 3 pin package on a heatsink and have 2 diodes in the = package.=20 There are other individual diodes on the board. =20

USEFUL PARTS =20

The useful parts for inverter projects are the controller IC (quite = often a=20 TL494 sometimes labelled IR3M02), the driver transistors for the main = switching=20 transistors, the transformer (11:1 ratio), the output capacitors, the = big=20 output diodes, the little output diodes, the filter choke (a toroid), = and=20 generally the resistors and capacitors. There is sometimes an Op-Amp as = well. =20

IN A FAULTY PSU, WHICH PARTS ARE GOOD? =20

It is easy to check the output diodes, small diodes, capacitors and = resistors=20 with an ohm meter. The small black diodes located around the output are = usually=20 the schottky type and the small glass types used elsewhere are usually = normal=20 types. You can check them with an ohm meter. You can determine what = type they=20 are by connecting them in series with a 10k resistor across a 12vdc = power=20 supply. If their forward bias is about 0.7 volts, they are a normal = diode. If=20 it is about 0.4 volts, then they are a schottky type. The toroid, input = filter,=20 and transformer will be OK. A catastrophic failure will usually have = blown the=20 switching transistors, but the IC and driver transistors often survive. = Test=20 the transistors with an ohm meter, they should have a high resistance = both ways=20 (measured by reversing the meter leads) between collector and emitter. = They=20 should have a high and low resistance between base/emitter and = base/collector=20 (measured by reversing the meter leads). Test the IC by building a = circuit and=20 see if it works, or test it in a known good inverter. The main = functions are=20 the oscillator, the 5 volt reference, the error amplifiers, and the = output=20 transistors. =20

I have built an inverter by only using parts from a computer PSU and = a piece=20 of perf board. It uses any DC voltage in the range 7 to 40 volts DC as = input=20 and gives 95 VDC out.