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Test your power supply with a multi meter

148737 Views 4 Replies 4 Participants Last post by  nork
NOTE: This testing applies only to the standard ATX PSU, it does not apply to proprietary PSU's (ie: older Dell's ) which use a different pinout arrangement for their ATX connectors (non-standard).

20 and 24-Pin Connectors: The 24-Pin connector has an additional 4 pins attached to one end of the connector. The top 20-Pin section is electrically identical to the original 20-Pin ATX connector. Due to the additional connections the numbering sequence changes, so if you have a 24-Pin connection then make that adjustment when comparing any numbers shown below. (Refer to the pictures below.)


If you suspect a part has failed but not sure if it's the Power Supply (PSU), Processor (CPU), or motherboard at fault, you can use a multimeter to aid in locating the probable cause.

Testing the DC voltages
With the 20-pin ATX connector in the motherboard socket and AC power on, the DC voltages have to be probed from the backside of the connector by inserting the probe tip alongside the wires to contact the connector pins.

Set meter to read 20Vdc. Refer to ATX 20-pin connector pinout chart (below) and put the black probe tip to a GND pin (ie: black wires on pins 15, 16, 17).

With the Red probe tip...
1. Check Pin 9 (Purple, VSB) is about 5V. This is the standby voltage and is always on when the power supply is live to an AC source. If not 5V then problem with PSU, or a possible short circuit in motherboard or a peripheral device has caused the PSU to auto-shutdown.
2. Check Pin 14 (Green, PS_On) should be about 3~5V. If PS_On is zero Volts and VSB okay then disconnect the pc case Power On switch to see if voltage comes up, if so then bad switch.
3. Press the pc case Power On switch and PS_On should drop to ~0V.
If no change, the suspects are faulty switch or CPU. If the motherboards PS_On pins are accessible by removing the pc case's power on leads then short the 2 pins with a light tap from a screwdriver tip to trigger the power on. Another way is to use a jumper wire to short PS_On to GND. If no change in PS_On then probable fault is CPU.

With PS_On at ~0V...
4. Check Pin 8 (Gray, Power_OK) should be 3~5V to signal the CPU that power is okay to start. If not above 2.5V then signal not high enough to trigger CPU for start.
5. Pressing the Reset button (or shorting the reset pins) will make Power_OK go low (0V), and when released should go back up to 3~5V.
(Note, this may not happen if the manufacturer has used a 'soft' method to trigger the Reset.)
NOTE: If CPU doesn't POST when Power is first turned on but does POST when Reset triggered, then the initial power demand (by all devices) is to high for the PSU to handle, need to replace PSU with a better one.

With Power_OK at ~5V...
6. Refer to chart and check voltages are at proper levels on the 20-pin connector and all the peripheral power connectors inside the case.
+3.3v pins 1, 2, 11 (Orange wires)
+5V pins 4, 6, 19, 20, (Red wires)
+12V pin 10 (Yellow wires)
-5V pin 18
-12V pin 12

Note: Depending on the quality of the PSU, there may be nominal voltages detected on the +5V and +12V lines with AC live and pc power off. Always remove AC power, and wait at least 30 seconds to allow residual power to drain before working in pc case.

Testing for Continuity
UNPLUG THE COMPUTER FROM THE AC POWER, and wait 1 minute to allow residual power to drain off.

Set the multimeter to the lowest Ohms ( Ω symbol) value, a typical value is 200.
Touch the probe leads together to zero the meter, note the value if it's not quite zero (live zero), this value will be used to indicate a short circuit during testing. Touch the two probe tips to bare metal inside the pc chasis, should get same zero reading.

Testing the PSU for short and open circuits...
Remove ATX connector from motherboard.
1. Keeping one probe on chasis, use other probe to check PSU's AC ground pin and the DC connectors black wire pins are reading zero. Any value other than zero indicates faulty grounding-replace PSU.
2. Keeping one probe on chasis, use other probe to check the connectors non-black wires are non-zero (if a colored wire reading less than 50 suspect a problem).

Testing the Motherboard for open circuit...
Remove the CPU from motherboard socket before doing this test.
Refer to ATX 20-pin connector pinout chart for GND pin locations. Use PSU's 20-pin connector to familiarize yourself with the board's connector. Only test the GND pins, the multimeter's internal voltage may damage a component connected to the other pins.
3. Keeping one probe on chasis, check GND pins 3,5,7,13,15,16,17 should have the zero reading, any other value suspect a fault-check board is properly installed and repeat test.
If still non-zero remove board and test GND pins again, with one probe touching the metal ring at a mounting hole, if still non-zero then motherboard faulty-replace it with another board.



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Power Supply Information and Selection

If you're trying to decide which power supply to get or want more information about power supplies then read This Thread.
If your PSU lacks an 8 Pin CPU power

then this is what you buy. just dont use both 4 pin molex plugs off the same rail.

if you need a new PSU and you need the 8pin motherboard AUX connector please ask for assistance in the Power Supply Section; we can point you in the right direction.
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Tutorial: Repair your noisy cpu and system fans

This is not a short-term fix but a fix that will last for a long, long time.

Buy some graphite powder and some oil at an auto or hardware store. They come in different packages. For oil, the best is the pen-type as they are easy to handle and only give a small amount at a time. Just about any oil in a pen format will work fine. The graphite powder comes in small plastic tubes. But make sure you get graphite powder, not graphite oil, its too thin and runny for this job! So you need graphite powder in a small tube and oil in a round "pen" that can be squeezed out and placed just where you need it. Also, when the oil is used up in the "pen" you can open it and add your own oil.

First, clean the fan blades off real well. If you have a heatsink with the fan then clean the heatsink off as well. You can even use a bit of your oil and some q-tips for cleaning if needed. There is usually a small label in the middle of the fan. It will have a name and other info on it. Peel one edge up carefully and put a very small amount of oil in where you lifted the label up. Now add some graphite powder in the same place where you put the oil, add a fair amount more of the graphite powder than the oil. Clean off excess oil and graphite with a q-tip and push the label back where it was. You may need a very small amount of duct tape to hold the label in position and to prevent leakage. This is important if the fan is going to "sit up" as opposed to lying flat.

Now put the fan back where you want it and when it starts spinning it may be noisy for a few seconds until the graphite\oil works its way in. Then it will stop and it will stay stopped for many years and you may very well not need to buy a new fan after all! Just adding oil will only work for a while.

I experimented a lot with this and this is the very best way to quiet all those fans and have them work well. I have always been a big fan of those removable ide hard drive trays that you can buy. But the fans are the pits, they get noisy real fast. So i had to find a way to fix them as they are very expensive and hard to replace. Now i dont have to. Combo of oil and graphite is the very best solution by far. One caution, graphite stains worse than oil, so be careful! Im talking about staining your clothes and hands.

You can use this oil and graphite powder system with all cpu fans, video card fans, system fans, and even fans inside power supplies. But one word of caution, its easy to open a power supply and take out the fan, but do be careful as there are voltages there can can hurt you badly. If you dont know what you are doing then wear rubber gloves to get the fan out and to put the fan back in after the oil and graphite repair!

It doesn't matter whether the fan uses a ball bearing or a sleeve bearing, long as you can find a way to get some oil and graphite powder inside it.

Trust me, this will work. If the fan spins my method will work for years and I have even fixed fans that were stuck and wouldnt spin! This is better done with the fan on the table but in those cases where you cant or dont want to take the fan out of the system, thats where the "pen" with the needle comes in again, but i still find a way to get some graphite in as well, graphite is what makes the oil last a long time! And graphite is also an oil, but you need both together for maximum effectiveness. Oil by itself will only last a few weeks or so and then evaporate, and the fan will get noisy again. Its the graphite powder that does the long term job. The oil really only helps to spread the graphite powder around really well so the graphite can do the job!

Graphite, i imagine is conductive, but i have never run into a problem. I imagine its because its not conductive enough to cause any problems in that area.

Another thing it does. If you cant get a part to fit in, instead of using vaseline or some other lubricant, use graphite powder. Graphite powder can withstand heat better than vaseline. The only thing, as i said above, it does stain, so you have to be careful with it, wipe up any spills, etc. And, in some cases, you might want to use it with a touch of oil, just enuf to help spread the graphite powder around a bit better and faster.

You can pick up a tube of graphite powder for about $3 or so and theres enough to fix maybe 100 fans or more. Course it works on noisy and sticky doors in the house, anything that needs to slide like windows in wood and aluminum frames. I got an email from a person who used it to fix a sticky key on an old favorite keyboard. It works on just about anything, but, again, be careful as it does stain so its hard to get it out of clothes.

Hope you do as well as i have with this info!

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