TheMatt

I know I have the Laptop article going on, but that has currently stalled while I wait for a book to come in from the public library. In the mean time, I will be using my brand new multimeter to help write an article to test various aspects of the PSU (and its envoirnment) including:

Testing the AC outlet for proper electrical potential (Voltage)
Testing the power cord for Continuity/Resistance (Ohms)
Testing the +3.3v, +5v, and +12v lines for proper electrical potential (Voltage)
Testing the Power_Good signal for proper range

Before I begin, does anyone else have any suggestions for other topics to include?

JohnthePilot

Hi Matt,
Have you asked this in Tweakers?
Regards,
John

TheMatt

Yep, I have posted there. I think I will get more suggestions when the initial draft is posted.

JohnthePilot

Send a copy to Glas.

Deleted090308

Testing the case switches with the Ohm meter.

TheMatt

I thought of that one Nick, but when I went to do it, my probes were too big to fit in the header connector. Have you done this before?

Deleted090308

Yes, I've done it before. If the probes are too big you could stick pieces of a paper clip into the switch plug to get contact.

TheMatt

Thats a good idea. I will add that.

Testing the case power switch for Continuity/Resistance (Ohms)

TheMatt

Testing your Computer Including the Power Supply with a Multimeter

I. Background Info

The power supply in your computer is perhaps the most important single component. Quality power supplies will increase system lifetime as well as run systems in environments where cheaper power supplies would not. Additionally, quality power supplies do thorough self checks to make sure they are working properly before even powering on the PC.

A. Terms to know - Whilst not completely necessary, it will be helpful to know these terms that are mentioned throughout the article.

• Voltage (Unit is volts or V. Formula variable is E or U.) - Voltage is the measure of electrical potential between two different points. A component must always be powered with the voltage it requires to function properly. Voltage tolerances are expressed in percentages (%) and in computers, power supply voltages for the most part must be within a ±5% tolerance.
  
  
• Amperage (Unit is amperes, amps, or A. Formula variable is I.) - Amperage is the measure of electrical current, the flow of electrons, between two different points. Computer components will draw only as much current as necessary to achieve getting the proper power level to the device. In electronic circuits, however, components can be overloaded by increasing the voltage over the maximum rated voltage, thus causing a component to draw more current than it can handle.
  
  
• Ohmage (Unit is ohms or Ω. Formula Variable is R.) - Ohmage is the measure of electrical current resistance. Anything that conducts electricity has resistance. However, some materials such as copper have less resistance than other such as aluminum. The element silver has the least resistance of all naturally occurring elements. Most quality wires used inside and outside computers will have a resistance of less than 5 Ohms. Resistances such as those on a resistor are usually expressed in Ohms with a percent tolerance similar to voltage.
  
  
• Continuity - Continuity is simply measuring if current is flowing between two points. In context, testing for continuity refers to testing if there is electrical flow between two points in a circuit, trace, or wire.

B. Extra terms - These terms are extra reading and are good to know if you are interested in electronics.

• Wattage (Unit is Watts or W. Formula variable is P.) - Wattage in electronics is the measure of electrical power in a resistive load. In resistive loads such as simple electrical circuits, wattage is the product of voltage and amperage. In power supplies such as the switching type computers use, it is the product of voltage, amperage, and power factor (PF). Wattage is usually varied by varying the amperage while the voltage is held constant. Wattage should not be confused with Volt-Amperage (VA) which is the measure of power in an inductive load as opposed to a resistive load and does not account for power factor. It is important to note that in physics wattage is a very general measure and can be derived from many different formulas depending on how it is being used. For example, dynamic power in an IC, also measured in Watts, is calculated by the frequency multiplied by the capacitance multiplied by the voltage squared.
  
  
• Volt-Amperage (Unit is Volt-Amperes, Volt Amps, or VA. Formula Variable is P.) - Volt-Amperage is the measure of apparent power in an alternating current (AC) circuit or the electrical power in an inductive load. It is similar to Wattage but does not take into consideration the power factor. Volt-Amperage is often used in combination with Wattage to determine the efficiency and power factor of a power supply. UPSs are also measured in both Wattage and Volt-Amperage to show that the true power produced is lower than the apparent power drawn and thus that some is lost due to capacitance and inductance. The lost power is called the Reactive Load.

C. Safety precautions - There is always some risk to the user and the equipment involved when working with electrical equipment. Before beginning, it is important to evaluate these risks and determine if any extra precautionary steps should be taken or if any extra safety equipment should be used.

• Testing AC Voltage from the Wall - This is likely to be the most dangerous test in this article. A potentially fatal 115v or 230v is coming from the wall. Because the voltage remains constant and the amperage determines the amount of total power, more voltage will equal more power because that always stays the same and your body determines the amount of current flow. When performing this measurement, always be sure to firmly hold the probes by the grips and never touch any metal part of the probes. Be sure the probe is all the way in the socket so no metal is exposed.
  
  
• Testing DC Voltage from the Power Supply - This has less risk involved because of the lower voltage. The major precaution here is being careful not to damage any components inside the computer such as the motherboard. Most DC voltage tests are performed with the computer on, so keep in mind that it is possible to short circuit something and possibly damage it if electricity is applied to the wrong place. Always ground yourself to the chassis when working inside the computer to avoid electrostatic discharge (ESD).
  
  
• Testing Continuity/Resistance on Wires - This is likely to be the least risky test because it involves injecting either 1.5V or 9V from the multimeter's own battery into the wire or circuit for the test. Again, when working with electrical equipment, be sure to test the component outside the PC and to keep the object being tested isolated from the attached component if possible. If it is a wire connecting a switch and header for example, try and remove the switch from the case as well as the header from the motherboard.

II. Testing Procedures

Testing the power supply and other aspects of a computer using a multimeter is one of the most valuable skills a computer technician can have when troubleshooting a computer that will not start or that is experiencing other problems. A multimeter is a very versatile tool that can be used for many tests and purchased for relatively little money. Having one around is a must for any computer technician.

When testing the power supply itself, we use a technique called back probing. This involves inserting one of the probes into the back of a connector to make contact with a terminal. Because we cannot test a power supply when it is not connected to a computer and because we want to test voltages when the power supply is under some load, it is important to be able to measure voltages while the power supply is running in its actual environment.

A. Testing Equipment - For this article the following equipment will be used. It is not necessary to have all or even most of this equipment or the exact type listed. The only non-computer device needed for each test is a single multimeter, however other devices were used to ensure reliability of the tests when they were performed.

• RadioShack 42-Range 10-Function Digital Multimeter (±0.8% DC, ±1.0% AC, ±0.8% Resistance)
  
  
• CRAFTSMAN 10-Range 5-Function Analog Multimeter (±4.0% DC, ±5.0% AC, ±4.0% Resistance)
  
  
• Kill-O-Watt P3 AC Electrical Monitor (±0.2% Power [W/VA], ±0.2% Power Factor)
  
  
• PC Power & Cooling Silencer Quad 750w EPS12v Power Supply (+3.3 +5v +12v ±5%, Power_Good 3V - 6V)
  
  
• Standard US AC Outlet (115v ±4.5%)

B. Testing your AC Outlet for Proper Voltage - This involves using the multimeter to test whether your outlet is putting out the proper voltage, if any. Your outlet should be either 115V or 230V (depending on where you live) with a ±5% tolerance being acceptable. Most computers however can run on 100V/200V, and high quality power supplies can even run on 90V/180V.

1. Turn on your multimeter and set it to AC voltage with the proper range if necessary. Your meter should now read less than 1V.
   
   
2. Insert both the black and red probes into the two non-ground power ports on the outlet at the same time. The two power ports will always be separate from the ground port.
   
   
   DANGER: A potentially fatal amount of power is present in AC lines. Use caution when testing any active AC line. Always push the probes all the way in and do not touch any metal part of the probe if exposed.
   
   
   Fig. 1B: How to insert the probe. Do not only have one probe in the outlet.
   
   
   Fig. 2B: Both probes correctly inserted into the outlet. No metal on the probes should be exposed.
   
   
3. Look at the multimeter and compare the reading to the acceptable tolerances. If no voltage is present, remove the probes and repeat steps 1 and 2.
   
   
   Fig. 3B: The multimeter reading the AC voltage from the outlet. Note the small wave symbol on the left of the display indicating the multimeter is set to measure AC voltage.
   
   
4. Remove both probes from the outlet at the same time. When finished, turn off the meter (digital meters only).

C. Testing the Power Cord for Resistance/Continuity - This procedure is relatively simple and involves almost no risk to people or equipment. On analog multimeters resistance will be measured, and on digital multimeters resistance or continuity can be measured. A resistance of less than 1 Ω is generally considered acceptable. When measuring continuity, the meter will beep and/or display "SHRT" to indicate that there is a complete circuit and the cord is good.

1. Turn your meter on and set it to Resistance or Continuity (digital meters only). Your meter should read 0 Ω, OF, or OPEN.
   
   
2. If you are using an analog meter, touch the two probes together and use the variable resistor knob to adjust the meter until it reads 0 Ω.
   
   
3. Unplug the cord from the wall and power supply, and old the cord so that at both ends the ground connector is facing towards the floor.
   
   
4. Insert one probe into the left voltage hole. Rest the cord and probe on a flat surface so the probe stays in the hole.
   
   
5. Touch the other probe to the corresponding left voltage prong at the other end of the cord. Note if the meter measures less than 1 Ω or reads "SHRT" on the display.
   
   
6. Repeat steps 2 through 4 on the right voltage and ground lines of the cord. If all three have good readings, the cord is good. When finished, turn off the meter (digital meters only).

D. Testing the +3.3V and +5V DC Outputs - This test will usually not produce results that show cause for concern, but it is good to check anyway, especially on older systems where the CPU voltage regulator ran off either the +3.3V or +5V lines. Older systems used the +5V for SIMMs, while modern and semi-modern systems use the +3.3V for DIMMs. Both the ATX and EPS standards define a required ±5% tolerance on the +3.3V and +5V, meaning the +3.3V must be between 3.14V and 3.47V while the +5V must be between 4.75V and 5.25V.

1. Turn on your multimeter and set it to DC voltage with the proper range if necessary. Your meter should now read less than 1V.
   
   
2. Lay your computer down so the motherboard is flat and parallel to the floor. Turn on the computer and boot into the operating system. Wait at least 5 minutes before beginning testing.
   
   
3. Locate an orange wire on the 20-pin or 24-pin main connector.
   
   
   Fig. 1D: Finding an orange wire on the main ATX power connector.
   
   
   Fig. 2D: Finding a red wire on the main ATX power connector.
   
   
4. Insert the red positive probe into the rear of the connector at the location where the orange wire enters the connector. The probe should sink into the connector by at least 2mm.
   
   
   Fig. 3D: The red positive probe correctly inserted into the back of the main ATX power connector. Note that the probe is in a terminal with an orange wire.
   
   
   Fig. 4D: The red positive probe correctly inserted into the back of the main ATX power connector. Note that the probe is in a terminal with a red wire.
   
   
5. Touch the black probe to any metal part of the chassis. Do not touch it to any metal components other than the chassis interior itself. Avoid using the painted metal exterior of the chassis.
   
   
   Fig. 5D: The black ground probe touching the metal part of the chassis and successfully creating a complete circuit.
   
   
6. Note the reading on the meter and compare it with the acceptable +3.3V tolerances listed above. If it is out of range, a new power supply will likely be necessary.
   
   
   Fig. 6D: The multimeter registering 3.3V from the power supply.
   
   
   Fig. 7D: The multimeter registering 5V from the power supply.
   
   
7. Remove the black probe from the case and rest it on a non-conductive surface, then remove the red probe. Repeat Steps 3 - 5 but with a red wire. Compare the reading to the acceptable +5V tolerances. When finished, turn off the meter (digital meters only).

E. Testing the various +12V DC Outputs - This test is critical on all modern computers. Fans, hard disk motors, optical drive motors, floppy drive motors, processors, video cards, and other high output voltage regulators all run off the various +12V rails in a modern system. The three main places to test are the ATX main connector, the CPU power connector, and the video card power connector. Both the ATX and EPS standards define a required ±5% tolerance on the +12V, meaning it must be between 11.4V and 12.6V.

1. Turn on your multimeter and set it to DC voltage with the proper range if necessary. Your meter should now read less than 1V.
   
   
2. Lay your computer down so the motherboard is flat and parallel to the floor. Turn on the computer and boot into the operating system. Wait at least 5 minutes before beginning testing.
   
   
3. Locate a yellow wire on the 20-pin or 24-pin main connector.
   
   
   Fig. 1E: Finding a yellow wire on the main ATX power connector.
   
   
   Fig. 2E: Finding a yellow wire on the CPU power connector.
   
   
   Fig. 3E: Finding a yellow wire on the PCIe power connector.
   
   
4. Insert the red positive probe into the rear of the connector at the location where the yellow wire enters the connector. The probe should sink into the connector by at least 2mm.
   
   
   Fig. 4E: The red positive probe correctly inserted into the back of the main ATX power connector. Note that the probe is in a terminal with a yellow wire.
   
   
   Fig. 5E: The red positive probe correctly inserted into the back of the CPU power connector. Note that the probe is in a terminal with a yellow wire.
   
   
   Fig. 6E: The red positive probe correctly inserted into the back of the PCIE power connector. Note that the probe is in a terminal with a yellow wire.
   
   
5. Touch the black probe to any metal part of the chassis. Do not touch it to any metal components other than the chassis interior itself. Avoid using the painted metal exterior of the chassis.
   
   
   Fig. 7E: The black ground probe touching the metal part of the chassis and successfully creating a complete circuit.
   
   
6. Note the reading on the meter and compare it with the acceptable +12V tolerances listed above. If it is out of range, a new power supply will likely be necessary.
   
   
   Fig. 8E: The multimeter registering 12V from the power supply.
   
   
7. Remove the black probe from the case and rest it on a non-conductive surface, then remove the red probe. Repeat Steps 3 - 5 but with a yellow wire on the CPU power connector and the video card power connector(s) in use. Compare those reading to the acceptable tolerances. When finished, turn off the meter (digital meters only).

F. Testing the Power_Good signal - The Power_Good signal (also sometimes called the PS_OK signal) is a 5V signal from the power supply to the motherboard indicating that the power supply has passed all internal tests and that all voltages are in range. If the voltages fall out of range, the Power_Good signal will be withdrawn and the computer will shut down until the signal returns. The normal acceptable range for the Power_Good signal is 3V to 6V, however some motherboards may run with the signal as low as 2.4V.

1. Turn on your multimeter and set it to DC voltage with the proper range if necessary. Your meter should now read less than 1V.
   
   
2. Lay your computer down so the motherboard is flat and parallel to the floor. If this test is necessary the computer at this point likely cannot be powered on, however as long as the switch on the power supply is on the Power_Good signal will be present assuming the power supply is working.
   
   
3. Locate the gray wire on the 20-pin or 24-pin main connector.
   
   
   Fig. 1F: Finding the gray wire on the main ATX power connector. Note that PC power supplies only have one of these wires, and they are always located on the main ATX power connector.
   
   
4. Insert the red positive probe into the rear of the connector at the location where the gray wire enters the connector. The probe should sink into the connector by at least 2mm.
   
   
   Fig. 2F: The red positive probe correctly inserted into the back of the main ATX power connector. Note that the probe is in a terminal with the gray wire.
   
   
5. Touch the black probe to any metal part of the chassis. Do not touch it to any metal components other than the chassis interior itself. Avoid using the painted metal exterior of the chassis.
   
   
   Fig. 3F: The black ground probe touching the metal part of the chassis and successfully creating a complete circuit.
   
   
6. Note the reading on the meter and compare it with the acceptable Power_Good tolerance listed above. If it is out of range, a new power supply will be necessary. If no signal is detected, repeat the testing procedure. If again no signal is detected, then the power supply is most likely bad and in need of replacement.
   
   
   Fig. 4F: The multimeter registering 5V from the power supply.
   
   
7. Remove the black probe from the case and rest it on a non-conductive surface, then remove the red probe. When finished, turn off the meter (digital meters only).

JohnthePilot

Do you mind if I make some grammatical and typographical changes to what you have written? Also, I'll make some suggested changes in wording which I'll write in red. Let me know if you agree, or not, with the suggestions.

TheMatt

Go ahead with the grammar errors and typos, but be careful not to change the wording. If you are unsure if something will change if it is correct or not highlight that in red.

Thanks John.

kbd_usr

DO NOT ground test (0 out ohms) on a negative terminal. Will lead to EM interference, which could cause shorts.

This is due to grounding out the trickle charge to prevent shorts.

JohnthePilot

I've marked all my suggestions in red. The items in brackets are what I think are better alternatives for the preceeding word or phrase. The items without brackets are what I think should be there. What do you think?

TheMatt

Do you mean use a GND pin on the 20/24-pin ATX main connector? If so then I agree. I always use the chassis as ground and not a GND pin.

TheMatt

I have fixed up some of the errors but there were two suggestions you pointed out that it would be incorrect for me to change.

1. Ohmage (yes, that is in fact a word ) is the measure of resistance. I am putting the unit as the term and then defining what it is a measure of.

2. I did not change wattage to power because technically wattage and volt-amperage are both measurements of power. They are just measurements of power on different types of loads (resistive is wattage, inductive is volt-amperage).

Deleted090308

Some users (i.e. me ) are used to U as the formula variable for potential difference.
I like your explanation of W vs. VA, but I have a feeling the "hardcore" physics will confuse most readers so much they'll go out and buy a Bestec PSU...

TheMatt

I will add it in. In Ohms law voltage is listed as E. Thats why I selected it.

Also, I will also add something referring to the PSU selection article for selecting a power supply. This article is for troubleshooting power supplies, not testing them.

Deleted090308

Isn't troubleshooting a kind of test? Only joking - I get your point.

JohnthePilot

That's fine. As I said, they were suggestions.

TheMatt

I was up too late. I meant troubleshooting them, not selecting them.