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How to measure 12v on motherboard with multimeter?

M

mange_

Gawd
Joined
Jan 1, 2001
Messages
534
I'm trying to measure what 12v value I have on the motherboard when the computer is on but I don't know on what spots to measure to get a reading on the 12v.
I can't get mm to the pins on the atx-connector so it has to be another spot to measure on..

Help greatly apreciated,
Mange
 
Does this PSU have dual 12 V rails? If not, you can just use any of the molex connectors for measuring purposes :)
 
The fan connectors on the mb all shows 12v, as does the molex.
It is a dual rail psu, hiper type-arrrr 480w.
The weird thing is that mbm, speedfan and epox own usdm all report 5,72v on the 12v rail.. Could this be because of the dual rail some how?
 
mange_ said:
The fan connectors on the mb all shows 12v, as does the molex.
It is a dual rail psu, hiper type-arrrr 480w.
The weird thing is that mbm, speedfan and epox own usdm all report 5,72v on the 12v rail.. Could this be because of the dual rail some how?
Click to expand...


Are you sure you are looking at the +12V rail and not the -12V rail in MBM, speedfan, and epox? My -12V rail also reads like that, because it's not used for anything except oolder prots like a serial port. If you don't have anything plugged into the serial port, the -12V rail will read completely wrong.

If you are indeed looking at the +12V rail, then I don't know why it's reading flike that. Maybe your +12V sensor is reading your -12V rail and vice versa for some reason?
 
Speedfan reports my +12V wrong as 12.71V. Everest and the BIOS report 12.1V, as does my multimeter. Generally, you can measure your +12V rail by placing your multimeter in the yellow/black wires in a 4 pin molex.
 
mange_ said:
The fan connectors on the mb all shows 12v, as does the molex.
It is a dual rail psu, hiper type-arrrr 480w.
The weird thing is that mbm, speedfan and epox own usdm all report 5,72v on the 12v rail.. Could this be because of the dual rail some how?
Click to expand...
The possibility of a voltage sensor being faulty is an order of magnitude higher than that of the PSU being faulty when talking about mainboards.

Unless you're using some cheap generic PSU, that is :p
 
So what do I do now.. the pc works like a charm and when measured with a mm I see really good +12v values. Should I just ignore the strange readings and game on or is it important that I find the problem and start rma'in stuff?
 
mange_ said:
So what do I do now.. the pc works like a charm and when measured with a mm I see really good +12v values. Should I just ignore the strange readings and game on or is it important that I find the problem and start rma'in stuff?
Click to expand...
A DMM is usually considered more reliable than onboard voltage sensors, so those strange readings are therefore the result of a faulty sensor.

In other words, there's no problem :)
 
One item of note is some motherboards , like my Abit AN8, will give slightly low 12v readings off the fan header purely due to their whole speed control deal, in this case 11.7v at header, 12v dead (by DMM) on the 12v rail.

Secondly what is the easiest method to read the 3.3v rail?
 
StoneNewt said:
One item of note is some motherboards , like my Abit AN8, will give slightly low 12v readings off the fan header purely due to their whole speed control deal, in this case 11.7v at header, 12v dead (by DMM) on the 12v rail.

Secondly what is the easiest method to read the 3.3v rail?
Click to expand...
Easiest way I can think of is simply opening the PSU and measuring directly on the PCB :)

Second easiest way would be to measure the bottom of the ATX-connector on the mainboard's PCB, on the soldering points.

Though perhaps easier, especially if you want to measure multiple systems, would be to construct some kind of ATX-connector adapter which would plug inbetween the PSU and the mainboard and provide easy to use contact points.
 
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