HOW TO: Measuring PSU Rails

[CrazeD]

I, at one time, didn't know how to accurately measure the power my PSU was giving out. I learned how to and this is a guide for anyone who doesn't know how to measure your PSU rails.

Ok, first you need a multimeter that can handle 15 DC volts. You can get a cheap one (like mine) at Radio Shack for about 15$.

Next, we need to understand which wires do what on the power supply. You will be dealing with only four different wires, here they are:

YELLOW - This is your +12V rail
RED - This is your +5V rail
ORANGE - This is your +3.3V rail
BLACK - This is a ground

** Before we continue, please make sure that you NEVER touch the two test probes on your multimeter together while the unit is switched on. This could damage the multimeter. Also, never allow the test probes to touch the incorrect wires (IE: negative probe onto a positive lead, or the positive probe onto a negative lead) as that could damage the unit and/or whatever you are testing. **

There is two ways to do this. Method one is easier than method two, but it requires a power supply with a 20+4 pin connector. Method two works just as well, but takes a tad longer.

METHOD ONE:

With your PC off, switch the power supply off and unplug all devices connected to it (except the motherboard). Take your multimeter and set it to 15DCV. Now, grab a molex plug and stick the black (negative) test probe into either of the two black pins on the molex. Make sure it stays in place, you don't want it to fall out. Also make sure that it doesn't contact any metal or anything conductive.

Disconnect the 4pin connector from the 20pin ATX connector block. You will have one orange, one yellow, one black, and one red wire coming out of this plug.

Switch on your power supply. and turn on your PC. Make sure you don't allow the red (positive) test probe to touch any metal or wires.

Take your red (positive) test probe and decide which rail you'd like to test. As mentioned above, you can test the +12V, the +5V, and the +3.3V rails with this plug.

To test, simply touch the end of the red (positive) test probe to the corresponding wire. IE: To test the +12V rail, touch the test probe to the tip of the YELLOW pin.

METHOD TWO:

If you don't have a 20+4 pin ATX connector, or don't feel like taking the 4pin connector off, use this method.

For method two, we have to use the molex and 20pin ATX connector.

Insert the black (negative) test probe into either black pin. Insert the red (positive) test probe into the yellow pin. This will give you your +12V rail measurement.

Insert the black (negative) test probe into either black pin, and insert the red (positive) test probe into the red pin. This gives you the +5V reading.

For the +3.3V reading, it gets a little tricky. Stick the black (negative) test probe into either black pin on the molex connector. Then, take your red (positive) test probe and cram it into the fitting on the back of the orange pin on the 20pin ATX connector. You should have enough room to get the test probe in there. Make sure you use the ORANGE wire (there are several orange ones, any will do).

Remember that having unplugged all devices, this is your voltages are idle. Jot this down, and then plug all your devices back in and test it again. Write this down, and then run prime95 (In-place large FFT's) and then test voltages again. You could even run 3DmarkXX or play an intense video game and again check the voltages.

The point I'm trying to get across is that as the system strains, it draws more power. IE: A video card consumes a lot more power more in 3D mode than in 2D mode, etc.

Nitestick mentioned there being different +12V rails, so I'm going to
add to the guide how to check those as well.

In your typical dual +12V rail power supply, the first rail will be the 20(+4)
pin ATX connector, and the 4/8pin CPU power connector. Rail 2 will be everything
else.

To measure the ATX connector you'd have to stuff your test probes into the back
of the connector. With my powersupply and multimeter, that wasn't quite possible
(it didn't fit). So, what I did was took a pin and wedged it into the back of
the connector. If you do it right, you should get it wedged in pretty far. Now,
this is dangerous because if it happens to fall out and lands on your PCB,
you're going to have problems, so make sure you stuff it in there real far. Put
them in with your system OFF and power supply OFF or unplugged, and when they're
stiff and ready you can turn it back on.

How I did mine was I stuck one pin in the 4 pin CPU power connector, and one in
the actual ATX connector. I did this so I didn't accidently connect the two and
short it out.

You can see the pins marked by white arrows.

Now, some of the newer power supplies can have three or more +12V rails. I'm not
entirely sure what will be hooked up to what rail, but I'm guessing it would go:
+12V1 = ATX and CPU power, +12V2 = molex and floppy, +12V3 = PCI-e and
SATA....etc. If you look in the user manual that came with your power supply, it
should tell you which rails do what...just remember that yellow = +12V, and
black = ground.

Here is how you measure the PCI-e connector. Now, just for the picture I have my
test probes through the back of the connector. This is because you can't insert
it into the front like you can with a molex, so just so they wouldn't move for
the picture, I did it this way (I don't have three hands, lol). Though if you
can't get it through the back, you can prop the connector up and touch the test
probes to the end of the pins on the front.

I was bored, and needed to check my old system due to some problems....so I figured I'd make a guide while I was at it.

Hope someone learns something.

Updated 10/01/07

 

[Lord AnthraX]

Sticky!!!

 

[Snake-Eyes]

Sticky!!!

Ditto. Useful, and well done!

 

[CrazeD]

lol, thanks.

 

[Nitestick]

i was going to make a criticism only to find you had included what i was thinking of further down the page.

a couple of things of note:
it's good to take a reading during POST and powerup because HDDs draw a lot more current when spinning up. so you want to know how far, if at all your rails drop during spin up as an upgrade may mean it drops too far to power up

i like taking readings while running F@H (or prime95) on both cores and 3Dmark at the same time. that way you can be certain your cpu and graphics systems are at full load.

good guide, i want to sticky it but if you've noticed the hardware pages are all getting a bit cramped with stickys (especially overclocking). i'll see if i can do some shuffling to make it fit.

remember pretty much all PSUs these days have multiple rails that run virtually independant of each other, you may want to include something about how to measure each 12v rail. the first 12v rail to my understanding is always through the ATX connector. the second rail will be through the molexes (i think) and definitely the PCI-E connectors, at least in a two rail system. i'm not sure how it goes from there but you should be able to look it up.

edit: going to give it a try in the hardware troubleshooting forum.

 

[CrazeD]

That's a good point about the different rails. I also believe the first rail is always the ATX. I think the second rail is all the molex connectors, and then any more would most likely be PCI-e connectors.

Also about the HDD's spinning up... you could also (if you have a fan controller) flip off your fans and then turn them back on, as they take about double the power to turn them on initially as it does to run them after they've started. You want to always make sure you have consistent power, because that is what separates a *decent* powersupply from a *great* powersupply.

 

[Nitestick]

well it depends on how many rails you have. 2 rails means presumably 12v1 is in the ATX connector and 12v2 is the molexes, SATA power and the pci-e connectors (if any). 3 rails would have 12v1 in the ATX connector, 12v2 on the molexes and SATA power and 12v3 on the pci-e connectors. you should probably be able to find out which each rail is from your psu manufacturer and hence determine how to test each rail.

in any case 12v1 remains a dedicated rail for the ATX connector.

 

[Burnselk]

PSU Voltages

Why was there no mention of the -12v output on a typical PSU?

Also, how about the -5v output on older PSU's?

What are they used for.....and how do you measure them?

 

[Ste]

Unless stated already, you don't have to disconnect the Wires to Take readings.

 

[Nitestick]

Why was there no mention of the -12v output on a typical PSU?

Also, how about the -5v output on older PSU's?

What are they used for.....and how do you measure them?

because those readings are fairly insignificant and can be taken in exactly the same way as the other readings if you identify the correct wires in the ATX connector.