Understanding and Troubleshooting the PC Power Supply

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Troubleshooting

Now that you’ve chosen the right power supply, lets put some of your knowledge to practical use. At the beginning of this article we listed several functions of the power supply and corresponding trouble. Let’s review this list of symptoms again:

  • System is completely dead
  • Intermittent Power-on or system startup failures or difficulties
  • Smoke from the power supply
  • Blown circuit breakers
  • Lockups
  • Reboots
  • Random memory errors (not at the same address)
  • Hard drive and fan failure
  • Overheating
  • Electric shocks when touching the computer case or connector
  • Static discharge disrupts normal operation

The most common symptom a technician will run into is a PC that had been working for days, months, or years no longer powers up. The assembler will not hear any noise, there will be no LED lights, and there will be no video.

First, the obvious should be checked. Change where the power supply is plugged. A switch will control some wall outlets, therefore make sure someone didn’t turn the power off at the switch. Of course, at the same time make sure that your cat or dog didn’t play a trick on you and unplug the power supply.

Second, check the overflow switch on the back of the power supply. The rocker should have the bar flat against the power supply – using an analogy of a bridge – the power is able to flow when the bar is against the power supply. The circle is ‘open’ or if it helps to remember – ‘off.’

Next, determine if the motherboard has an LED built onto it. If the LED is present but not showing light, then you can safely assume the power supply isn’t providing voltage to the board. If the LED does show power, hit the reset button. Does the unit now power up? A reset signal is very different from a power on signal. Therefore, the power supply could be the culprit.

Finally, simply replace the power supply with another one. If the system works after replacement, then you can safely conclude that the original power supply was bad.

But what if you do not have access to another power supply? As a Do-It-Yourself assembler, you should have access to a multi-meter. No self-respecting technician would be without it. Measure the voltages on the 20-wire motherboard connector. Gently insert the black lead into any of the black wires. Then touch the red lead into the wires, taking the measurements. Compare your readings to the following table:

Pin

Color

Signal

Pin

Color

Signal

1

Orange

+3.3

11

Orange

+3.3

2

Orange

+3.3

12

Blue

-12

3

Black

GND

13

Black

GND

4

Red

+5

14

Green

PS_On

5

Black

GND

15

Black

GND

6

Red

+5

16

Black

GND

7

Black

GND

17

Black

GND

8

Grey

Power_Good

18

White

-5

9

Purple

+5Vsb

19

Red

+5

10

Yellow

+12

20

Red

+5


Table 1: ATX Power Supply Connections

Don’t let this table overwhelm you. The color designations are quite simple; black is ground, red is +5, orange is +3.3, and the purple is the standby voltage signal.

A common error in power supply troubleshooting is plugging the power supply into the wall outlet and not into a motherboard. The fan on the power supply will not spin and therefore the person assumes the power supply is bad. Never do this. Always plug the ATX power supply into a motherboard before plugging it into the wall.

The trickiest troubleshooting is done when someone complains about reboots and lockups. The obvious components are the RAM modules, however a poor power supply can cause this symptom as well. Early AMD Slot A systems were notorious for reboots. According to Gateway, they determined the problem was a poor voltage regulator on their choice of motherboards.

We found with our clients, that changing the power supply to a unit with greater power would solve the reboots. Actually, many of the reboots were due to a poor combination of components; such as using the wrong memory modules (mixed timings, multiple PC133 modules, etc) as well as noise-levels that increased as the processor speed increased (poor motherboard design).

In fact, we ran into systems that wouldn’t install Windows 98 properly unless a power supply with enough wattage was used in the chassis. We only discovered this by accident after replacing everything else, and since we replaced the power supply as a last resort, we realized that the problem lay in not having enough power on the +5Vsb signal.

The last symptom to consider is overheating. Make sure that nothing is blocking the airflow – keep the system away from a wall – and always feel free to add fans inside the chassis.

A final word regarding power supplies. The fans inside these units can be loud and annoying to anyone sitting in a room filled with computers. Therefore, if you are replacing a system, consider purchasing a quiet model that exceeds your present requirements. Your ears will thank you.


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