# The PSU "rail" checking guide.



## de.das.dude (Jun 17, 2011)

This is a thread dedicated to check how the 12V and 5V cables are connected on your PSU and if it is actually a dual rail like the manufacturer claims it to be.


First a list of the tools you will need.
*1. Tools:-*

A multimeter(specification afterwards)
tweezers
some masking tape
some spare wires
anti static gloves

*1.1 Multimeter*
This doesnt not need to be those expensive ones. Heck, you can even do this with some wires and an LED light and a 3V cell.







Just in case you do get a multimeter, make sure it can measure at least 20V and has the connectivity testing feature which makes a *beep* if theres is a connecting path between the multimeter terminals
OR just use the resistance measuring mode. 
"0" means no resistance = connected.
"1 with a left indent" means infinite resistance = not connected.


*1.2 Tweezers*
You will need this to short the 14th(green) and 16th(black) wire of the ATX motherboard connector to switch the PSU on manually.
Alternatively you can bend a suitable length of 1mm thick copper wire/ or a paper clip to do this.

*1.3 Masking tape*
not really used, but always good to keep this handy.

*1.4 Wires*
not used but good to keep it handy.

*1.5 Anti static Gloves*
in case youre on a carpet. or youre not grounded.


*2. Now What to Do?*
First of all. *YOU DO NOT NEED TO OPEN YOU PSU FOR THIS!!! DONT TOUCH THAT SCREW DRIVER!*

*2.1 Disconnecting your PSU*
1. Turn of your PC.
2. Turn off the power to the PSU (not the PSU switch)
3. There probably is some LED on you motherboard which goes off after you turn the power off. This means its safe to disconnect the PSU jacks from the mother board now.
4. Remove the power cable and screws connecting the PSU to the chassis and motherboard.

*Note: do not switch on your PSU. You dont need the PSU powered on for this test.*

*2.2 Selecting correct multimeter mode*

1. If there is a connectivity testing mode, select it. it looks like the symbol of a buzzer or has the symbol of a diode.





2. Alternatively select the resistance measuring mode. I use 2K ohm, which means it measures till 2K ohm.

*2.3 The Concept behind this Test*(please read)

*The Meaning of "RAIL"*
This is the long line of 12V and 5V cable that powers your peripherals like HDD, fans ODDs SSD, lights, and even powers GPUs etc. Each rail has a particular amount of max current that can flow through it. If more than that is required it will fry the transistors connected to that rail or simply wont supply enough power at all.


*I has many rails  / I has one rail  , what does that mean?*
Multiple rails mean that the 12V and 5V are connected to different groups of transistors.
This means that all the 12V and 5V cables are not connected to the same "source" of power.
Which means that they may or may not allow the same amount current (and hence power) to each rail.

Having a single rain means that all the 12v and 5v lines originate from the same source.

Even if you have two physically visible rails, they maybe connected to the same source meaning there's actually one rail. (manufacturer just lied to you)
Thus even if the manufacturer specifies max 18A and 17A for the 12V two rails, since they are connected to the same source, either of the rails can carry the total of 18+17 = 35A irrespective of what the manufacturer claims.


However if there are two distinct rails whose max currents are 12A and 40A respectively,
they will allow only what is specified. connect your GPU to the 12A rail and get ready to tear your hair off  (actually there wont be any GPU connector on the 12V rail  )


*Is there any way i can know which one i have?*
Fortunately, im here to the rescue. This tutorial will show you how to know if you have one rail or multiple rails or if the manufacturer just lied to you.


*THE BASIC CONCEPT.*
If two things are connected to the same thing (i.e single rail) then they are connected to each other.

In other words, for a single rail PSU all the wires of a same voltage are connected by conducting paths. Hence all the wires are "equal".




*3. Just When You Thought I wont Write about the Real Test...*

okay time to get your hands dirty ! (not really  )

*3.1 The Easy But not Very Scientific Way*
1. Simply look at the ampere rating for the two "rails".
2. If they are almost the same value with a max tolerance of 2A, like 18A and 17 A, 25A and 23A, it probably is a single rail, just two different set of wires.
3. If theres is a huge difference between the ampere ratings like 18A and 25A, they sure belong to different rails.

*3.2 The Scientific way*

Its time to finally get your multimeter.

First lets test the peripheral rails, the ones with one yellow, one orange/red and two black wires.
We will test only the 12V rail as 5V really doesnt matter, 12V takes the real punch.

1. Insert the black multimeter terminal into one yellow wire's molex pin of one rail.
2. Insert the red terminal into the yellow wire's molex pin of another terminal.

*If multimeter in connectivity testing mode:-*
If you get beep, it means that both "rails" are connected to the same rail. Which means the PSU is single rail.
If no beep, they are individual rails.

*If multimeter in resistance measuring mode*
If your multimeter shows anything less than 100 Ohms resistance, then both "rails" are connected to the same rail. Which means the PSU is single rail.
If your multimeter shows infinity or "1" with a left indent, it means that the rails are different.

*3.3 The Ghetto Scientific way* (for those without a multimeter)

1. Take two wires of 6" length.
2. Strip both ends for both the wires. Strip off around 1/2".

Lets name the wires A and B.

3. Take one end of wire A and jam it into the yellow wire's molex pin. Take the other end and connect it to the positive or longer leg of the LED. Just wrap around the naked wire around the LED leg and use masking tape to hold in place.

4. Take one end of wire B and jam it into the yellow wire's molex pin of another rail. take the other and connect it to the positive of the 3V battery/cell. Use plenty of masking tape as required.

5. Now take the free end of the LED and touch it to the negative terminal of the battery.

IF the LED glows, its single rail. If not its two distinct rails.

*Note:* Best 3V battery?? Your old BIOS battery!

*3.4 Moving Forward*
Now that you have tested the peripheral rails, next you need to check the CPU ATX 4 or 8 pin power rail.

*You may want to check:-*
1. The power rail with the peripheral rail.
2. Individual 12V wires of the power rail i.e. if all the 12V power jack wires come from same source or not.


*4. How to Strip your PSU and find out if its Single or Dual Rail*
*4.1 Safely Switching off your PSU. damnit i dont have a camera and cant finish *


_*finishing soon....*_
how to open up the PSU and find out if its a single or dual rail!



so youre gonna thank me or not ?
i didt copy paste, i had to write this whole thing you know..... ^_^
[taking pics and generating drawings, will put them up soon!]


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## Jetster (Jun 17, 2011)

Ok, sure, leave me here with these tools and nothing to do 

Edit: Nice Job


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## de.das.dude (Jun 17, 2011)

Jetster said:


> Ok, sure, leave me here with these tools and nothing to do



back


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## slyfox2151 (Jun 17, 2011)

wait, so i went searching for my screw driver for nothing ? 





KEEP POSTING


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## _JP_ (Jun 20, 2011)

Nice thread. I haven't read the whole thing yet, but it sound good so far. 
Keep going!


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## slyfox2151 (Jun 20, 2011)

TBH i have never seen a true multi rail PSU... care to link one?






Yellow +12 from molex + yellow +12 from CPU 8 Pin connector showed they were connected. 
Using a Multimeter.


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## xBruce88x (Jun 20, 2011)

Looking forward to the steps for how to tell by taking the psu cover off, I mean what to look for. I have an idea (all yellow wires soldered to the same point) but just want to be sure. My psu has a lot of connectors and I think it'd be quicker to just look inside. And I have no multimeter. (Yet)

-posted using BB 8310


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## de.das.dude (Jun 20, 2011)

thanks for all the replies.  i dont know why this thread didnt show up in my subscriptions ?



slyfox2151 said:


> TBH i have never seen a true multi rail PSU... care to link one?
> Yellow +12 from molex + yellow +12 from CPU 8 Pin connector showed they were connected.
> Using a Multimeter.


true dual rail PSU, coming right up..
okay you are right its hard to find.
but mod Tatty One used one tagan 550w dual rail. Apparently he seems to have had problems with dual rail.

and yes, they are both connected to the same thing, which means its a single rail.

One reason why dual rails are extinct is because of load balancing. You had to be careful of the load you put on each rail, you must make sure the lower amped rail does not receive most of the load. 



xBruce88x said:


> Looking forward to the steps for how to tell by taking the psu cover off, I mean what to look for. I have an idea (all yellow wires soldered to the same point) but just want to be sure. My psu has a lot of connectors and I think it'd be quicker to just look inside. And I have no multimeter. (Yet)



yes they all originate from the same place.
i will post that portion as soon as i can get my hands on a camera as explaining that requires some pictures.


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## streetfighter 2 (Jun 20, 2011)

de.das.dude said:


> 1.2 Tweezers


You probably shouldn't be testing continuity with the PSU switched on 

IIRC it's not a good idea to switch on the PSU without providing it a small load; a couple fans should do.


de.das.dude said:


> 1. If there is a connectivity testing mode, select it. it looks like the symbol of a buzzer or has the symbol of a diode.


The buzzer symbol is for the continuity test and the diode symbol is used for the diode test.  These may be separate modes on some multimeters.


de.das.dude said:


> Even if you have two physically visible rails, they maybe connected to the same source meaning there's actually one rail. (manufacturer just lied to you)


As I understand it relates to overcurrent protection and all the rails should allow for a short burst of current above the rail's maximum current rating.  In principle this is no worse (or better) than having multiple independently regulated 12V rails.  Furthermore the limitation on current in 12V rails is a relic of an older ATX spec (ATX 2.3+ does not have this limitation).  (source)


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## de.das.dude (Jun 20, 2011)

streetfighter 2 said:


> You probably shouldn't be testing continuity with the PSU switched on
> 
> IIRC it's not a good idea to switch on the PSU without providing it a small load; a couple fans should do.
> 
> ...



first thing was writing all the stuff needed. tweezers will be used in the last section, TIPS and Tricks.

and the buzzer and diode thing is same.
both check if current is flowing or not. and these are same on multimeters. (i have seen around 20 diff types till now )

for a diode to be okay, it needs to be conductive i.e. closed circuit.
and i dont see the difference between what connectivity test i gave and you linked. i eve gave the ghetto way as shown in WIKI later!


and last, i will write about PSUs a little at the end.

the power hazard thing you said, you know what these bastards do nowadays to make sure too much current dont flow, without the use of too many fuses?
they connect the fuse at the end to the ground! i have opened 3 PSUs and they were like this. this saves multiple fuses as no matter what the voltage, current has to be grounded in the end, in this case it happens through the fuse.

okay i might still be wrong, i am doing mech engg, not electronics, but i have pretty decent knowledge. enough for the ECE prof to flirt with me  but thats another story 


and the max 240VA thing means that at max a current with 12 volts may carry not more than 20A. This is why even for single rails they connect multiple wires to distribute the current (~ 1:1 ratio). To reduce heating. thats all


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## qubit (Jun 20, 2011)

That's interesting about the manufacturers lying about having dual rails. If anything, the better power supplies have beefy single rails that will take everything, without the user having to worry whether they are going to burn out one of the rails with excess load.

My Corsair HX850W is a top-dog PSU and that has a single rail. This is the only type I'll buy.


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## slyfox2151 (Jun 21, 2011)

streetfighter 2 said:


> You probably shouldn't be testing continuity with the PSU switched on
> 
> IIRC it's not a good idea to switch on the PSU without providing it a small load; a couple fans should do.



PFT you soft-cock... i tested mine while my whole system was running


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## jsfitz54 (Jun 21, 2011)

I liked the post. Thank you.

Waiting for second installment.


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## de.das.dude (Jun 21, 2011)

slyfox2151 said:


> PFT you soft-cock... i tested mine while my whole system was running



actually that is totally electrically okay to do, as long the voltage in the power cables is not somehow leaking to the ground.
even if it did, you would only blow the silly fuse in the PSU, which can be fixed by merely unplugging it from you AC power supply for around 20 seconds, and plugging it back in.


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## xBruce88x (Jun 21, 2011)

qubit said:


> That's interesting about the manufacturers lying about having dual rails. If anything, the better power supplies have beefy single rails that will take everything, without the user having to worry whether they are going to burn out one of the rails with excess load.
> 
> My Corsair HX850W is a top-dog PSU and that has a single rail. This is the only type I'll buy.



technically they're not... well not always

"A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU) relative to some understood ground."

so technically even if they draw power from the same source, if they have separate grounds then they separate rails. The main source is capable of more, but due to some ATX standards, they had to limit how much in amp each rail could hold. so they split up just before coming out of the PSU to the inside of your comp. I'm guessing they were more worried about the actual exposed wires in the case than inside the PSU. Perhaps a safety concern or a bleed over thing or something. who knows.

http://en.wikipedia.org/wiki/Power_supply_rail


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## qubit (Jun 21, 2011)

xBruce88x said:


> technically they're not... well not always
> 
> "A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU) relative to some understood ground."
> 
> ...



Ok, but it sounds like this split is within the PSU only, is that right? The PSU still has one physical 12v wire* coming out of it, which you can load up to the PSU's capacity?

*Of course there's more than physical wire, but they will all be soldered to the same point in the PSU to be electrically equivalent. An ohmeter should register a near zero resistance between any of the plugs on the end.


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## xBruce88x (Jun 21, 2011)

depending on the way they "split" the rails. I guess you could say some have a "hybrid" design.


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## de.das.dude (Sep 17, 2011)

GOT A CAM THE second update will be here soon!


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## HammerON (Sep 17, 2011)

Nice thread! Might come in handy one day


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