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TechRepublic Tutorial: How to diagnose a faulty power supply

Determine the causes of failures of server and workstation power supplies

Most computer failures come from one of two places—hard drive failures or power supply problems. Most power supply problems usually don’t result from internal component failure, but rather from external events that place unreasonable demands on components within the power supply. For example, if lightning or a large power surge were to hit the electric lines, you would expect this to damage the high voltage section. Here’s what makes up the typical power supply and how you can figure out what’s wrong with it.

Just the basics
In this article, I’ll show you how to locate power supply problems using a minimal understanding of electronics, and a basic understanding of the multimeter. You can get a multimeter at most any electronics shops. The scope of this article is limited to the least complex portions of the power supply. Fortunately, these account for almost all the problems I have ever seen, so odds are very good that your problems will be similar.

Electricity 101
Electricity is the flow of electrons from a lower potential (more negative) to a higher potential (less negative). When Benjamin Franklin discovered electricity, he mistakenly assumed that electric current flowed from positive to negative, so you honor this mistake to this day by describing current flow in this manner. This is called conventional current. It’s important since all the electronic components are drawn according to this convention.

Electrons flow through components, and you call this flow current. Current is measured in Amperes. Current only flows when a potential difference exists. The measurement of potential is based in Volts.

Lastly, you must understand the differences between alternating current (AC) and direct current (DC). Alternating current changes polarity following a sinusoidal pattern. Electrons initially flow in one direction, and then they reverse to flow in the opposite direction. Power companies distribute AC power because it’s easier to generate and transmit. Unfortunately, in the United States, our power standard calls for 60-Hz oscillation between current polarities. This is the absolute worst frequency for the human heart, since it is perfect for causing fibrillation. For this reason, extreme safety precautions should be taken when performing any of the debug procedures described in this article!

As I said before, house current is very dangerous, and it claims a number of lives every year. I will include safety instructions along with each debug step I offer. If you stick with the method discussed, you will not get hurt. If you plan to deviate from my suggestions, I hope you will at least have a loved one stand nearby with a broom. In the event that you become part of a 110V circuit, the broom will be useful for knocking you free from the electricity!

Basic electronic components that make up your power supply
The components that make up your power supply include:
  • Capacitors – These have many shapes and sizes, and they can be made from any number of materials. They always have two wires (leads). They are typically round or cylindrical, but sometimes rectangular. They are usually made with a ceramic, plastic, or even metal exterior. Markings on the device will usually declare the capacitance in mF, uF, or pF. Typically, you will find markings on the printed circuit board such as C1, C2, etc. where these components are mounted. For some photos of capacitors, please check out:, and click on Capacitors. Capacitors pass higher frequency alternating currents while blocking lower frequency currents (DC being the lowest frequency of all).
  • Inductors – These are really just coils of wire, sometimes wound around a metallic or ferrite core. These can take on a number of appearances such as a donut shape, a small cylinder with axial leads, or numerous other packages. Typically, you will find markings on the printed circuit board such as L1, L2, etc. where these components are mounted. For some photos of inductors, please check out:, and click on Inductors. Inductors block higher frequency alternating currents while allowing DC to pass through unobstructed.
  • Diodes – These devices are usually long cylinders with leads sticking out of both ends axially. Usually, they are painted black and have a silver or white stripe on one end. Normally, there will be additional markings on the printed circuit board that identify these parts using symbols such as D1, D2, etc. Diodes allow current to pass in only one direction. You are most concerned about the diodes that will be used to rectify AC current into DC current.
  • Transformer – you will only be concerned with the power transformer that steps down the AC house current to a lower voltage. This component should look very much like the drawing on this page: It should have an iron core and coils of wire wrapped around the core. The coils will have some kind of insulator coating around them.
  • Voltage regulators – These devices will look very much like transistors, in that they have three leads that connect to the printed circuit board. These devices are typically mounted to a heat sink, which is a large metal plate that allows heat to dissipate away from the package. The voltage regulators insure that the power supply outputs never exceed the desired voltage levels. The power supply might generate a slightly higher voltage, which enters one terminal of the voltage regulator. On the output terminal, the voltage regulator would then supply a regulated +5V or +12V to the rest of the computer.
  • Metal oxide varistor – Really well designed power supplies will have these somewhere very close to the socket where AC power enters the power supply. They look very much like capacitors in that they usually have a round body with two leads. Almost always, they will have a plastic enclosure, and they will usually be denoted by symbols such as MOV or V on the printed circuit board. These devices are normally used as surge protectors. You can mentally imagine them as being two diodes that face opposite directions. Normally, no matter which way the current is going, one diode or the other will block the current from flowing. When a voltage spike hits the MOV, it causes one or the other diode to break down and conduct the wrong way. This would be analogous to flood waters overtaking a dam. When this happens, the MOV conducts current and hopefully shorts out the voltage spike before it harms other system components. If the surge lasts too long, this will usually damage the MOV, which makes them a very likely cause of power supply failure!

In Figure A, I have drawn a possible schematic for the high voltage section of a typical power supply. I placed a number of capacitors and inductors near the power input to show typical options that power supply designers might have used. C1, C2, and C3 are meant to suppress noise that the power supply might radiate back into the wall outlet.

Figure A
Here’s the basic layout of a power supply.

The FCC sets standards for how much noise is allowable, so designers use capacitors like these to achieve compliance. Likewise, inductors such as L1 and L2 also serve to block high frequency noise from sneaking out of the power supply. Notice that the two MOVs connect both of the high voltage lines to the outlet ground. Normally, the MOVs will act like they aren’t even there. However, if a surge comes in on either AC line, the MOV will short the surge to the outlet ground.

Determining whether the power supply is failing
First, you have to be sure that the computer problem is even a power supply issue. The easiest way to determine this is to make a few observations. After having made sure the computer is plugged in and the wall outlet works, switch the computer on. If the fans and lights don’t come on, this definitely points a finger at the power supply. Before condemning the power supply, make sure the power cord is connected and intact. You can swap power cords between the monitor and the computer to insure that the cord properly delivers house current to the power supply.

If none of the lights come on and the fan doesn’t spin up, this is almost certainly a power supply issue. However, you cannot be certain that the power supply is at fault yet, since a number of problems could cause this exact same symptom. For example, if a floppy drive shorted out, it would essentially create a problem for the power supply, which would ideally blow a fuse!

Shorting out means that a device fails in a manner that causes it to conduct electricity so well that no potential difference will occur across the device. You can mentally represent a shorted out device as being a wire or a metal paper clip. Now imagine what happens when you shove a metal paper clip into both slots of an electric outlet? If you have ever dropped a socket wrench on top of your car battery, you have experienced such a short circuit. Please don’t go out and try this, since it could kill you in any of several ways!

In order to determine whether the power supply has failed, you must disconnect the power supply from all other components. Before doing this, I highly recommend that you take a roll of masking tape and label each wire before removing it. The specific connections that must be removed are: 
  • The four-wire connectors to each of the peripherals like floppies, hard drives, etc.
  • The big cluster of wires that connects to the motherboard (might be two bundles, or might be just one).
  • Most older power supplies have two connectors that attach to the motherboard. These connectors could easily be reattached incorrectly, if you don’t pay attention to where each one of them goes. So make sure you know how to reconnect the motherboard properly, before you remove the connectors!

Once you remove all of the connections, repeat the same test. If the fan doesn’t spin up, this probably means the power supply is failing. To be certain, you can verify the failure with the multimeter. If you set the meter to measure DC voltage, using a scale that handles at least 15V, you can measure the voltage going to any of the peripheral connectors. By inserting the negative probe into one of the black wire contacts in the connector and the positive probe into the red wire contact of the connector, you should see the meter read 5V. If you move the positive probe from the red contact to the yellow contact, you should see 12V on that contact. If neither of these occurs, you know the power supply is not functioning.

In the event that the fan does spin up upon having removed all the connections, then you can easily figure out which component caused the failure. Just remove the power cord, and then reconnect the motherboard and test again with power. If the fan doesn’t spin up, you know the motherboard is hosed. If everything is ok, again remove power, and then reattach power to each of the other peripherals one at a time, and test. Whichever component prevents the fan from spinning up is the guilty party.

Remove and inspect
Assuming you determined that the power supply was defective, you must first remove the power cord and then remove the power supply from the chassis and remove its metal cover. Upon opening the cover, inspect the power supply circuit board for charred components. This is often the easiest way to identify a problem. If you find burn marks anywhere on the printed circuit board, this usually identifies components that got very hot just before something failed. This doesn’t mean that these are the components that caused the failure, but they are most likely defective now.


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