How to Repair a Computer Power Supply

How to Repair Computer Power Supply

We live in a society where most items are disposable. Just like we have disposal lighters, pens and several other types of disposable stuff, operators often believe power supplies are also disposable. What happens when a power supply fails?

We immediately look for a replacement without thinking about repairing it. Many of us are apprehensive about repair charges which may be huge. As a result we save ourselves the trouble and get a new one.

While buying a replacement may seem like a good solution for many, you can actually save a lot of money by repairing your computer supply. If you have some technical knowledge, put it to use and read on to find out how to repair a computer power supply. Sometimes a repaired PSU can have a longer life than a new one.

The PSUs used in computers are known as switching power supplies. In this case, the 110 volt AC input is rectified by two diodes and they are filtered by using a pair of capacitors. This results in two different high-voltage sources – positive and negative.

A pair of transistors is used to switch the high voltage supply into the required output. This switching takes place at a much faster pace, at the rate of 40,000 cycles per second. It typically uses an integrated circuit to control the transistors.

What causes power supply failures? How to Repair Computer Power Supply

There are only a few components that cause power supply failures, and the most common type of failure is the switching transistors. The transistors often short-circuit resulting in a lot of power flowing through the transformer, hence this blows the fuse.

A transistor failure is usually caused by bad quality capacitors. If you check a non-working power supply, you may expect to find swollen or leaking filter capacitors. Any capacitor that has gone bad should be immediately replaced with low ESRcapacitors. The ESRs are capable of filtering power in a switching supply.

Most of the manufacturers do not use low ESR capacitors while making power supply units because they are slightly more expensive. Hence, they use conventional capacitors in their PSUs. We recommend that you replace the leaked or swollen capacitor with a low ESRcapacitor. While it might cost you a few extra dollars, it’s worth the money in the long run as a good quality capacitor can extend the life of a power supply.

Another common cause of power supply failure is a diode not functioning properly. There are quite a number of diodes in a switching power supply and if any one of them stops functioning, this will result in the supply shutting down. Some of the common types of diode failures are -5 volt output rectifiers or a shorted +12 volt. While the failure of these diodes will not result in a blow out of the fuse, the PSU will identify the short and shut down automatically.

Testing and repairing the computer power supply

Before commencing the tests, make sure you switch off the system completely. Now, start with testing the switching transistors that you will find mounted on a heat sink. You may use a digital multimeter set or ohmmeter to test the transistors. Look for signs of a short between a collector and emitter and replace the transistor if it is damaged. Some technicians suggest that you replace both transistors even if you find only one of them is damaged.

Next you need to test the three pairs of diodes which are fairly small in size. Like the transistors, the diodes are also mounted on heatsinks. If you identify a diode failure, make sure you replace all the diodes with high quality replacements to reduce the noise levels.

Replace all the output capacitors with low ESR caps and then switch on the power supply. If the PSU still does not work then there might be an issue with the integrated circuit. Test the IC by installing it in a power supply that you know is good.


Some people choose to toss out the non-functional power supply because they believe that the price of replacement parts is almost the same as getting a new PSU. However this is not true. Repairing a computer supply and installing high quality replacement parts will not only save you money but also increase the life of your PSU.

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