Anyone understand PSU readings ?

Can any one tell me if the following readings are correct. The + readings seem to be OK but I do not understand the - readings

+ 3.3V Voltage 3.28 V
+ 5V Voltage 4.89 V
+ 12V Voltage 11.80 V

- 12V Voltage 1.05 V
- 5V Voltage 1.99 V


The readings are from SiSoft Sandra. I also ran Aida32 and came up with similar readings. I have been trying to track down various little faults that keep cropping up, all started with the mouse that refused to move in the horizontal plane unless it was attached to a KVM switch and the other computer was turned on ( didn't work with a USB mouse either). Then the sound went, then the monitor wouldn't come on unless you did a full re-boot 3/4 times (funny enough that started working again this morning), then the odd setting would change, such as forgetting my automatic log on's when the computer starts up, open programmes getting stuck on the task bar when minimised, USB ports not recognising plug and play and other irritating little things that just don't make sense.
I have got so fed up with it that I have bought a new MB and will be installing it next week. I am running XP pro, Athlon 1.7, 256mb DDR and plenty of HDD room as I have recently reformatted and reinstalled XP in the hope that it would sort these problems out, it didn't.
It was only this morning that I seriously considered the PSU as a sort of after thought, everything else seems to be OK including memory. The system is also virus free.

Anyone got any idea's ?

 

best to confirm these readings with a meter but it looks as if the negative side of the PSU has packed up

software systems can only measure true if the power supply is OK! - even then I don't trust them

the two negative rails drifting a bit positive fits well with a ?open circuit rectifier within the PSU

this is one of the (few) PSU faults that is actually quite mendable by an electronics tech without special equipment

failing that a new PSU is indicated.

best wishes, HJ.

 

Hugh, it's always been my understanding that the negative rails are meaningless to a PC and should be ignored. Have I been misled all these years?

 

although used nowhere near as much as +ve rails, negative suplies do serve a few things, for example
- "proper" COM ports will drive outputs between ~+10 and ~-10... although there are also COM port outputs which toggle between +5V and 0V. (An input threshold of around +2V works for both.)
- analogue parts of sound card - often opamps powered between +12 and -12. (this is actually rather good news as it allows the sound card to be DC coupled)(you can use them as DACs and ADCs, and do other things with them:data logging, or turn your PC into a 'scope of sorts)

==

it's devices powered between +12 and -12 which make me unsurprised that Flybywyre's minus rails have drifted a bit postitive...

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times gone by (early '80's technology) it could be completely lethal if only the negative supply failed. Often even things like ROMs would have more than one supply (look up 2716T)...

-ve voltage used to "bias off" the active devices inside... so failure of the -ve bias would cause the current drawn from the +ve rails to rise, often catastrophically for the chip.

==

best wishes, HJ.

 

Thanks for the replies...........
I have now thoroughly cleaned the PSU by taking the cover off and blowing compressed air through it and installed it along with a new MB. Everything seems to be working OK at the moment and the current -readings are:

-12V = -9.15V
-5V = -4.70

The + rails are slightly lower now, within tolerance apart from the +12V rail which is reading 11.17

Leave alone or get a new PSU ?

 

whoa - too many changes at once! assuming that you've cleaned the PSU, swapped to a different motherboard, but kept the same plug-in cards it's looking like either PSU or one of your plug-in cards...

there's something distinctly dodgy going on. Please try to obtain meter readings and watch how they drift after turn on, as the various bits warm up.

watching for trends as the thing warms up is as useful as spot-check values. PSU capacitors on the way out (common failure mode) usually manifests as drooping voltage with increasing temperature.

the +12 is certainly a bit low, you don't want it drooping any lower...

==

from the way the -ve rails were slightly positive I suspect something between the +12 and -12 is taking too much current, this is probably going to be on a plugin card... so try pulling out all non-essential peripherals, see if it's a plug-in card which is loading the supplies.

good luck & best wishes, HJ.

 

WhhhHHHHooooOOOAAAaa Nellie! Are you sure your +12v rail is reading 11.17???

I've found that the absolute "bottom" at which a system will boot is about 11.5v - and most of the time that even fails.

You can use a multimeter to measure the 12v reading at one of your molex (HDD) connectors. Just do a google or altavista image search first on "molex connector" to show you which wires are which.