Lab 8-7 Testing Power Supplies

 

Objectives

7  Identify some dangers involved in Power Supplies.

7  Identify and diagnose common power supply problems.

7  Test a power supply using a power supply tester.

7  Test connectors on the power supply.

 

Materials Required

7  Power supply

7  Power supply tester

7  Different connectors on power supply

7  Multimeter

 

Directions

  1. Get a power supply tester from me (check it out).
  2. Read the accompanying instructions on how to test your power supply.
  3. Get three power supplies from the cupboard. Do not plug it in!
  4. Test each of the power supplies in the following manner
    1. Test the P1 connection
    2. Test each of the molex connectors
    3. Test the ATX connector, if present
    4. Test each of the floppy connectors
  5. Fill out the table below

 

Power Supply Brand

Wattage rating

P1 test

# Molex connectors

Molex tests (pass/fail, note any that fail)

# ATX connectors

ATX test

# floppy connectors

Floppy connectors Test

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7  If any of the power supplies completely fails, throw it away (ensuring that you’ve tested others first to make sure you’re using the tester correctly).

 

7  If any of the power supplies has bad connectors but the P1 works, get a piece of tape and mark the bad connectors.

 

7  If any power supplies has all of a specific kind of connector fail, throw it away

 

  1. Now open up your lab computer, return your power supply tester, check out a multimeter.
  2. The first step in using the multimeter to perform tests is to select the proper function. For the most part, you never need to use the current function of the multimeter when working with computer systems; however, the voltage and resistance functions can be very valuable tools.
  3. In computer troubleshooting, most of the tests are DC voltage readings. These measurements usually involve checking the DC side of the power-supply unit. You can make these readings between ground and one of the expansion-slot pins, or at the system board power-supply connector.
  4. It is also common to check the voltage level across a system board capacitor to verify that the system is receiving power. The voltage across most of the capacitors on the system board is 5V (DC). The DC voltages that can normally be expected in a PC-compatible system are +12V, +5V, –5V, and –12V. The actual values for these readings may vary by 5% in either direction.

WARNING

Normal practice is to first set the meter to its highest voltage range to make certain that the voltage level being measured does not damage the meter.

 

10. The DC voltage function is used to take measurements in live DC circuits. It should be connected in parallel with the device being checked. In a PC system, this typically means connecting the reference lead (black lead) of the meter to a ground point and the measuring lead (red lead) to a test point to take a measurement, as illustrated in the figure below:

11. Note that on the side there is a label of what each of the cables in the P1 does.

12. Set your multimeter to DC volts 3.3. You’re telling it what you’re testing when you do this.

13. With the power off, connect the meter lead probe to the 3.3v connector on the power supply connector as shown in the second picture.

14. Connect the meter ground probe to the ground on P1.

15. Ensure that the multimeter is turned on and set to the correct settings. Set it down on the table with the probes in the power supply.

16. Plug in the power supply and turn it on. Without touching anything, write down the reading you get below:

 

17. Turn off the machine, unplug the power, remove the probes.

 

18. Do the same for the following positions and record the readings

 

Voltage

Reading

Voltage

Reading

Voltage

Reading

3.3

 

-5

 

5

 

12

 

-12

 

5

 

Remember to reset the multimeter each time to the proper reading.

Testing Resistance

1.      The second most popular test is the resistance test, or continuity test.

2.      Failure to turn off the power when making resistance checks can cause serious damage to the meter and can pose a potential risk to the technician. Resistance checks require that you electrically isolate the component being tested from the system. For most circuit components, this means desoldering at least one end from the board.

3.      The resistance check is very useful in isolating some types of problems in the system. One of the main uses of the resistance function is to test fuses. You must disconnect at least one end of the fuse from the system. You should set the meter on the 1 k ohm resistance setting. If the fuse is good, the meter should read near 0 ohms. If it is bad, the meter reads infinite. The resistance function also is useful in checking for cables and connectors. By removing the cable from the system and connecting a meter lead to each end, you can check the cable's continuity conductor by conductor to verify its integrity.

NOTE

4.      An electrical short is a condition where electrical current is given a path of flow around a designated component. In some cases, the short may be a complete bypass to a ground point so that no components receive current to operate with. On the other hand, an open is a condition that is created when no path for electrical current is present, such as when a connector comes loose or a component burns out.

5.      Get one of our older motherboards.

6.      Carefully locate a fuse and remove it.

7.      Set the meter to 1k ohm and test it.

8.      What is the reading?

 

Testing Speakers

9.      You also use the resistance function to test the system's speaker.

10. To check the speaker, simply disconnect it from the system and connect a meter lead to each end. If the speaker is good, the meter should read near 8 ohms (although a smaller speaker may be 4 ohms). If the speaker is defective, the resistance reading should be 0 for shorts or infinite for opens.

11. What reading did you get?

 

Only a couple of situations involve using the AC voltage function for checking microcomputer systems. The primary use of this function is to check the commercial power being applied to the power-supply unit. As with any measurement, it is important to select the correct measurement range; however, the lethal voltage levels associated with the power supply call for additional caution when making such measurements. The second application for the AC voltage function is to measure ripple voltage from the DC output side of the power-supply unit. This particular operation is very rarely performed in field-service situations. We’re not going to do it, lest we die.