How does my Desktop Power Supply Unit ,work? (the PSU )
How can I test it?.
Who makes a PSU better, besides silly weight statements answer, FULL PROTECTIONS WITH OCP, SCP, OVP, UVP, OPP
Jump to my, Laptop power supply section here.
The PSU is sized by Watts and Ampere's ratings on each RAIL.
Power supplies , like TIME, keeps everything from happening all at once. (like the above uncontrolled lightning shows) The power supply is actually a regulator.(besides converting AC to DC)
Safety first, do not play inside the unit (dismantled) with the power cord still connected to wall A.C. or the 170volts inside will shock you very badly, the PSU is never actully off, unless unplugged from the wall.
The PSU not opened inside is safe, mostly, at 12vdc will not shock  you ever but if you short the 12vdc to ground it will blow the wires to kingdom come and throw burning copper chunks in to your eyes or catch fire.
If you must open a PSU up, disconnect the line cord first, then wait 15minutes, and if smart discharge caps with a chicken stick like seen here. (but use a 5K 5watt resistor instead (best for 170 to 300v)

A Better DESKTOP (DT) PSU?  
The #1 symptom of a  dead, or near dead PSU internal FAN,  same goes for up to 7 fans in servers or workstations (see Z600 spec. for huge number of fans)
If the PSU fan and all other fans are dead that means the PSU is dead or the rails of the PSU are shorted (overloaded.)
The PSU power supply
you choose is for the needs of the whole PC now and say 10 years from now, even with the hope of running 150 +watt GPU cards now or later. (even gaming)
Newer more modern PSU
have added features not even shown in the ATX spec. (like FULL  over voltage and over current protection, note some only do under voltage on 5/12vdc, rail 2 pins ! as seen on most old cheap grade PSU's.)
A new PSU
costs $15 to $50. (avoid all cheapest cost PSU's as most of them are pure junk)
 The PSU is easy to test in situ 
or can be bench tested easy. (one good buy for  PSU is the ATX12 v2.3 Thermaltake as seen here for only $35)

Question #1 how does it turn on? (if the fan is dead, the PSU is dead)
clean it? (PC and PSU?)
Is the RTC 3vdc battery coin cell dead? (max life is 7 years says the pink bunny) I mention this as it fools folks, with dead PC and is not PSU caused at all.

Level one testing , calls for the hard power reset proceedure.(any desktop) We call this the long Power RESET.
  1. Make sure the optional rocker power switch is in the "1" On position.
  2. Remove the Thick A.C. Line cord at either end.
  3. Push the power button for 10 seconds. (I do it  3 times, to see the fans stop twitching)
  4. Release the release the power button,
  5. Wait 5 minutes, touch nothing. All the above is to get the large capacitors fully discharged.
  6. These steps can clear up 2 issues, a PC chip latches up or PC is stuck in sleep or hibernate modes(the latter 2 are bad chip software drivers)
  7. Plug the line cord back in to the PSU rear
  8. Plug the line card to the AC wall jack one that is not dead, and do not use any  power strip.(for now)
  9. The PC will turn on  now, if the PSU is good  (baring any internal shorts inside the PC) the fan in the PSU and CPU must spin and actually blow air. (if filthy inside clean it)
Level 2 testing is below:

Factoids on Desktop PC's

  1. The PSU +5v-standby_Power feed, most times shows up on the motherboard with a green LED (mine is yellow),  If this LED is dead, the PSU is dead.(in most cases)
  2. A bad PSU if it fails in the worst way can blow up $300- $1000 worth of expensive electronics inside that CASE, so buy a good PSU, and prevent it, get one with larger fan ,quieter.
  3. The PSU is safe to work on and the PC ,if the PC is not plugged in to the wall  AC power jack. (push power button on PC to discharge capacitors inside now, then wait 5 minutes time), 100% safe.
  4. A PSU can fail to turn on even if good or brand new, if  ANY module inside the PC is shorted. (shorts happen) we do isolation tests below to find  those problems. (shorted HDD, RAM, CD drive, or PCI-express cards shorted.)
  5. The PSU can be dead even for a simple error like the front power switch wire fell off of the Motherboard power jack (mine not yours). (just a look there first is wise)
  6. A PSU that fails the bench test is a bad PSU. (easy no?)
  7. The PSU or PC can shut off  fast for overheating,  overheating can take hours, or even seconds, for sure if the CPU fan is dead or its heatsink packed solid in lint.

Theory of operation of the PSU and the power managment chip. ( Do not read this , as this is for understanding more complex issues, not just a dead PSU) JUMP past this now.
The PSU follows ATX rules.(and color coded wires)

The PSU is a slave, to the motherboard PMC, power managment chip, like for example the Intel ICH9 or 10. chip.  (in the early days, this was ACPI, in 1996 and uses by BIOS and now UEFI new BIOS.)
This chip may be called many other things the South bridge and I/O controller Hub (or ICH) or Glue logic.(as seen on a real Intel motherboard (MOBO) here.
Later ICH became PCH, with Intels Series 5 processors. (2008 year) Nehalem CPUs up, but does all Power management now.
This chip does many things, but one small part (and complex) is the control of power. (and supports power managment logic for... see bullet list below...)
  • Hard power on (user pushes power button!)
  • Soft power on/off, including at least,  WAKE ON,  LAN, wake on mouse or USB device, or keyboard. (yes, you can turn the PC on using commands to the Ethernet port from 12,000 miles away, and advanced topic....skipped now.)
  • BIOS can even turn the PC on automaticaly using its onboard RTC time clock chip, it has  Calendar there and you can have it turn on only on some days or not. (all programmable by you, RTM read your manual on our PC)
  • Sleep mode.
  • Hibernate mode (a deeper sleep)
  • The Chip even allows you to define what the power switch does, (power on/off, sleep or hibernate)  (in Windows click power, click advanced power settings, and read what it can do)
For most folks not playing around (has less risk)with SLEEP modes or the horrors of Hibernate (many chip drivers are buggy here) then the power switch is programmed as simple as it gets  a power switch. (ICH rules always win here)
The first rule of Power managment is stand by  power.
If +5vsb Stand By, rail goes dead, the MOBO will be dead as will be ICH and all wake on features of this PC. We use a voltmeter on pin 9 and see 5vdc there. good. if not , the mobo can not turn on the PSU.

The PC Case has a power switch, this does not actually pass 120vac (or 240v) to the power supply, at all, ever.
It  is a 100% digital  power switch (momentary closure SPST switch seen on the front of most PC cases or front side wings)
You push the button and the mobo ( motherboard ) ICH (PCH) chip  saw you do that ( the pin to do that is just a port input  bit)  and the ICH, then sends POWER_ON (or PWR_ON) to the PSU.(pin 16)
Some PCs the BIOS can stop power on  due to , a dead fan at this instant, ( the fan reads  0  RPM fans, not allowed in this model PC)
It may also have  too hot sensor reading on the MOBO and fail to power on. (If say the PC just shut  off overheated and is still why to hot to run safely)
The ICH (PCH) chip then and now,  monitors the Power_OK pin on the PSU and the same signal from the MOBO onboard VRM (voltage regulator ) modules to see if these lines go FALSE, if they do the ICH then sets power_on line to false.
This can happen in 1 second flat , the PSU turns on then off in 1 second. (I most cases you have bad PSU or a shorted DEVICE connected to the MOBO, in rare cases the VRM is bad on the MOBO or shorted, parts there.
It can be shorted RAM. (CD/DVD/BR_D) HDD, SSD or a shorted Video card (PCIe ) or shorted NIC card, (PCI-e) or any card in ANY PCIe slot of any kind.
In  most cases it is not a shorted USB  device, as these have there  own current limiters to avoid this horror, from ever happening. (by Design)

There is more see that here.

If the PSU never turns on , see the next section below.
If it  PSU turns on and promptly turns off, then the PSU may have been told to do so by the ICH(PCH) chip, or the PSU saw overloads.
The PSU can not simply tell you way (some $300 PSU can) but it has no brain, no log files to read, sorry.
Only technician can find it or you playing as one. (go for it)
The BIOS can turn off the PCU, so if the RTC BIOS coin cell is below 2.9v, it is bad, get rid of it or BIOS can and will go nuts, the battery holds up the NVram , and if that scrambels it may set the power on schedules to off or set a false fan or overheat warning to the ICH and off she goes, turns off. or will not turn on.  This logic for power works like this.
It's not windows doing this, windows is not even running yet, so it can not be Windows or if Linux , then it. (can't)
What we do first is bench test the PSU  or try a spare,  (on cheap PC's non gaming PCs, a spare is like $15 cost, 300w)
As seen NEXT.

End theory of op.'s:

Checks Simple. ( this is how I do the tests, briefly.
  1. RTC coin cell not dead.
  2. Get rid of the dirt and lint as seen here first. (if fans stall its a very bad thing)
  3. If PSU internal fan is dead, as you push the  case POWER switch, (means no 12vdc exists.) make sure line cord is connected and wall power is good, (use a test lamp at wall jack?) and that the rear PSU rocker switch is set to "1" on.
  4. Make sure the power switch is wired to the mobo per the documents shown at HP.Com.  The MOBO wires to the PSU with the huge 24pin  connector seen below, and must be fully seated.  )
  5. Ok fan inside PSU case spins, nicely , and so do the PC chassis fans too, if fitted, (all Destopm   PCs have more fans) The CPU has a fan for sure and must spin, and on mine a GPU fan and  some have case fans.
  6. Ok, we can do more tests, we can in fact use a magical tool called  a VOLTMETER , sold at Walmart for $20 whole bucks,  and when set to 20vdc range can measure all power pins as you push the power button.
  7. Keep in mind the 1 second rule.  You must watch power during the 1second rule. (ATX spec, rules are just that, not suggestions) I connect my meter black lead to ground then to 12vdc pin 10
  8. I power on and pin 10  goes to 12vdc, then move the test probe, to all other pins, shown on the side of your PSU. (seen below in a photo of side sticker)
  9. If all power lines show good?, the the PC should run ok, ( but if the mobo regulators are bad , no  it  will not )
  10. A bench test is easy, jumper (24conn) pins16>17 (newer PC)  and then plug in the supply to the wall power line cord. (if PSU has a rocker switch there, turn it on to "1" on.
  11. If the bench tests pass,  then the MOBO has shorts some where (strip it down to find those) or the BIOS is shutting down the PSU for fans dead or overheating. 
  12. A voltmeter can now read all supplies, one line at a time,  3.3v, 5v, +5vsb,  (the newer PSU's have more connectors and pins for power , to GPU video cards and more.  they need to be tested too.)The ATX spec shows colors of wires.
  13. The last check only pro tech.'s can do is use a Scope on the pins and be sure 120hz ripple is in low and not above spec. and at 20khz, if that is the noise hash you see on the lines out. (the SMPS chopper noise too has  max spec)
  14. One common way to get in trouble, is having a gutless 300watt factory PSU then add in a power sucking GFX Geforce GPU PCI card and not the PSU goes dead. (overloaded) why do that, the GFX manual told you not to....!
  15. Keep and open mind, that one of the many things connected to the MOBO can be shorted, or RAM shorted, (all things with power wires, and all PCI-e slots too, remove cards)
5vdc must be 4.75 to 5.25vdc or the logic will not be happy.
Regulation spec, (intel)

We use a scope to check ripple.  (caused by BAD CAPS inside the PSU) ( some more modern PSU can do way better than this,  make sure the SMPS ripple at near or above 40kHz. is not present, or too large.)
All SMPS made on earth are huge noise generators, and have most time 3 banks of huge LC filters,  to snub out all noise, (most) Keep that in mind doing tests.

Using a scope,  120hz or 25k to 40kHz chopper caused, ripple max.  ,mV = millivolts  1mV = 1/1000th volt
All modern 10 year or newer PC use this ATX2 spec. (as shown in the WIKI and here)

Photo Dwg. #1  PSU connections.   To bench test the PSU, jump pins PS_ON to any ground pin.

Buy one, make one, or just use an unfolded paper clip to jump 2 pins.

Almost no person today has PSU with 20 pins, it's so old all are now in the land fill or fully recycled.
old relic ATX                               Modern ATX2 with 24 pins, the 4 pin plug below is commonly used on the GPU card, etc...

The only thing you should ever connect to a pin named Power_OK is a voltmeter, set to 10 or 20vdc range. (the ICH (PCH) chip is what monitors this pin)
 To see if it goes to logic true (logical 1 state) or 5vdc.  (this tells the PC that power is good, if not good, it signals fail (0v) and the PC quickly sets "PS_ON" to the off state, and the PSU goes dead and silent.
THE WIKI covers all this ATX stuff , read here what pin does what?
From WIKI, cut and pasted here. One could read the ATX spec, sure. (it's a standard, unlike silly laptops lack)
-5vdc ended 2002. V1.2 up.  (very old chips that needed -5vdc are now gone forever.) Intel long ago, if a negative supply is needed, in any chip, put a charge pump on board that chip, making it a 5vdc only chip then!, and 3.3v now or less)
-12vdc is mostly only needed for relic RS232 cards, modern cards now have chip with -12v charge pump inside the chip. (I'm sure -12v will be deleted in the future )
A side show of sorts:
Some Alien non ATX PC's use SFF (small form factor )
HP Deskpro 600, PCs use this odd connector linked below... I avoid such PCs.
This  SFF  PC uses a 6 pin Plug,  seen here (hacked to work with standard PSU)
Also a void ALL _in_ONE desktops desktops they too are non-standard, and very hard to diagnose due to the fact (hint: standard parts do not  fit nor work here, only the HDD is standard inside)
Basically the ALL_In_One PC,  is just Laptop missing that huge battery and mobility. ( It is just stripped down PC missing all the good parts and expandiblity and stardards we enjoy now with ATX)

A Random found very old ATX power supply. (old, but shows how it all works ! really nothing beats a schematic, no?) This PSU is simplistic,  with only basic protections.

Drawing #2:  (picked only for its simplicity and fits my page easy)  -5vcd Ended back in 2002, so is over 16  years old.
AC input is the AC line cord.  (if you look close you see that there is standby power all the time line power is connected. 
This is a SMPS design.  Q1 and Q2 are the 20khz  Chopper Oscillator. (power on , activates the chopper) IC1 controls power.
This is older than 2002, PSU,  16 years old or older today. (-)5vdc supply rail is now GONE for 16 years.  (it is both cheap, low parts count and simple)
Its clear as day how power on , turns on Q10 and how standby power allows that pin to pull up to 5vdc.
IC2 pin 1 tells the ICH(PCH) chip in your PC , all is good, (and can lie ,note how 3.3v is not monitors, in fact some old supples only monitor +5/+12v rails.)

Newer ATX supplies, have up to 8-10 rails inside, (up to) (newer supplies even have complex , power modulation chips that use only the power it needs, saving power and Carbon Dioxide (green) emissions. (PFC options)
The modern PSU may use SMPS control chip like this from "ON" Semi . NCP-1910 with new over current protection shut down features (OCP)
Even more modern PSU  now use a DSP (super fast and smarter, digital signal processor) this is the future, as it will never catch fire nor waste power or blow up that fast $1000+ gaming motherboard.
The point here is all pins below for power must  be tested by the technician to be sure the PSU is not detecting faults, even faults on supply lines not used,  (-5 and -12v may or may not be monitored and are now obsolete voltages)
Photo: 3   Example of better PSU for gaming systems or any with powerful GPU card (graphic processor unit) PCI cards added.

If all this is Greek?  then you  learned that Electronics is not simple, find a technician to fix it.  I'm retired, now and can help.  (but only here with words and pictures)
Now I show REAL SCHEMATICS so all can learn this, not just with words.
As stated before above the ICH (PCH) chip (glue) Does all the power managment of the PCI (with BIOS controls too) The Chip below, at pin 33, sends the power on command to the PSU, (PS_ON*)
Keep in mind even the BIOS can self turn on the PC, using timer power on page logic. (and wake on lan, or any wake features related to power, even exit sleep mode, or hibernate)
The same chip connects to the power switch at pin marked PWRBTN# ( this is Intel parlance for power button) (the pin actual varies by the package type, and ICH0,1 and up to 10.)
My Z270 chip, pwrbtn pin is BC5 (BGA) as you can see each chip for 18  years 1999 to now, all have same pin name.
82801BA I/O Controller Hub (ICH2)  below.
ICH2 uses W21 ball pin marked PWRBNT.(as one example.) I am never going to show all ICH chips made, here, just one small example.
This  excerpt is from the old motherboard Intel made long ago. 2001? (acccurate detailed schematics for modern motherboards are rare)
 As you can see the intel engineers even call it GLUE themselves, no surprise here. Thanks for Intel D850 actually sharing details this good !  VC820 is here
Below is ICH2 chip Ball (BGA), circa year 2000. Pentium 4. +AGP, old.  It maybe old, but guess what the pins are still there on all Intel chips, doing this job.

Power LOGIC 101: Simple (this is all pure logic inside)  (just enough facts to diagnose dead power or failures of power)
  • A Human pushes the power button, (or your pet monkey, whom ever)
  • ICHx (1 to 10)  sees this on its PWRBTN# pin, now. (from logical high 5v to logical 0v and is debounced, so you push the button just momentarily.)
  • ICH chip acting fast, then asserts (logic true) to the PSU-ON pin of the PSU. (0v = true  here)
  • The PSU powers up, and checks its own outputs, and if all rails are ok, (sends power-ok (good) to ICH, then runs ok, if not it shuts itself off, even in less than 1 second run time.
    The PSU all by it self can shut off, if it overheats or its fan is dead, or if is overloaded in any of 5 ways, OCP, SCP, OVP, UVP, OPP
  • The PSU then monitors the pin Power GOOD pin at all times. (if not good turns itself off, fast) (hint no fires allowed, and no blown up mother boards and friends)
  • If the Motherboards VRMs see dead VRM OUTPUTS dead or WEAK (shorts?) the the mobo ICH chips see this sends power GOOD false and  the PSU sees that act and turns off the PSU, fast.
  • If the CPU overheats at any time, the CPU can slow down or halt (means goes dead) or the BIOS can  see this too at any time and crank all fans full tilt, the fan controller can see 0 RPM ( or a miniumum 200)  and sees that and turns the PSU off.
  • That is pretty much it, but one other thing, the PC can go to sleep, or hiberate or even a timer in the BIOS can turn off the PC based on or off using the magic, BIOS a calender clock schedule.
    The OS has softpower off feature, and sends this to the ACPI portion of the above chip. (and the PSU turns off)  by off we mean STANDBY that most users think is off.(fans go silent)
  • One more the 4 second rule on PWRBTN,  if held for 4 seconds, this is the unconditional power off command. (Intel logic this is...)

Rapid Desktop PSU Testing to find true cause;  (in PC case tested or  on a work bench)
(assumes 120vac line service power reaches the  PSU (wall power) and the PSU fan is not packed solid in lint stalling it and overheating it fast.

A Dead PSU. (or suspected)
You are here because the PSU fan is dead, or the CPU fan is dead, or the Standby +5vdc  LED is dead. or all 3 are dead. A silent PC?  ( my Z600 has 7 fans inside, are all dead?)
Rule #1  do not hot swap  (or hot plug) any DC power cables inside any PC, remove the line cord and let it sit for 15 minutes.

Tools:  a Screwdriver to remove case side, and 1 paper clip unbent foruming  "U" shape.
If all fans are dead, the PSU is bad or (self shut down protecting itself an other electronics from damage) a.k.a.  a silent PC, 100% dead PC.
If one fan works and one other fails, then that dead fan is bad that is dead. (easy no?)''
The PSU can be bad or ANY of the things plugged in to the PSU (colored wires +connectors) can be shorted. 
The PSU can also self shut down if finds that its own regulators went berserk (had a failure).
The PSU can be tested 2 ways, in the PC case or on a bench, easy.(both ways with  the 24 pin plug removed, and all other DC power plugs in side PC .

The best way to test the PSU  is with a DMM. ( a voltmeter of any kind for $20 cost)
If you have a spare PSU, (smart to have as spare? yah) try that, if you lack proper useful tools like a simple DMM.

Preliminary tests:   (do the  power reset test first)
A still dead PSU is present.
This section here is called the Power on bypass test:
  • Remove the PSU main AC to wall , power cord below
  • Push the PCs main front panel power button for 15 seconds, this step discharges all internal Capacitors inside the PC.

  • Next step is easy, remove this 24pin, jack -plug from the motherboard. (and all other DC power cables to SATA drives, AUX, and GPU cards) all cables on the DC side of PSU (front) are removed.
  • The PSU has many black octopus cables, all are power rails to things. Unplug all of them.
  • Now find a paper clip and unbend it and make it "U" shaped.
  • Pin 16 is Power_ON pin, jump it to 17, ground using a paper clip see the 24 Pins here.( jump green to black wired pins)
   or buy or make one of these.  A paper clip works just as well.
  • Jumper the 2  pins named 16 to 17 seen in the drawings above.(see 24 pin photos)
  • Do not mess with PIN 8 (power-OK) pin. Do not guess at pins or use the wrong pins. Look 2 or 3 times to be sure you got this correct.
  • Plug back in the AC power cord  and the PSU fan now spins.
  • The DMM meter shows all rails read at spec.voltages.
  • A real shop has PSU load bank tester, but you don't so I skip pro grade testing in this step. (and noise tests with a scope, that too you don't have)
PSU's can fail many ways, good/bad, weak , or have noise on outputs (bad caps) and can have the power on pin go dead. (seen them all I have, 43 years on the job experience with supplies of vast types)
The PSU has a  fan,
the fan must blow air , spin and make a fan sound. (if not the PSU IS BAD ,buy a new one now) (yes I've replaced fans on many, or upgraded them to thermal regulation and less noise)
If the fan runs, then the PSU is not totally dead, it can be a good  PSU or partially good. (that is correct , there are shades of gray in bad things, all electroncs fail and a rail can fail , just 1.)
Bad ,good, weak, overheats, intermittant, are all possible in all electronics made.
Were are we,  if the Fan spins now with the paper clip test, the PSU may be good, if the fan is dead the PSU IS TOAST.
The DVD eject button dead (5vdc dead) on all PCU cause said PSU to self shut down, (UVP) But DVD eject tests are smart ! testing, but some PSU have good 5 and 12v but dead 3.3v. and 3.3v not monitor and dirt cheap PSU.
If the fan spins I always use my DMM meter to see if the DC voltage meet spec.
Okay say the PSU passes your testing above.
Unplug AC cord next
Put back the 24 pin connector, and all things you removed before.
Now plug back the AC cord.
Turn on the PC if any rail is dead or the PSU shuts off, start unplugging loads, a load mean any thing that unplugs from the Octopus cables.
Keep an open mind on shorts, and failures here.

Unplug them power cord removed, 1 at time, to learn which one is shorted. (learn that shorts happen, in all electronics made on earth) luck fate or lightening damage happens.

We must then and next , do Isolation tests.

All PSU even the most cheapest and old 250watt PCU will self shut off , if the PSU goes under voltage (UVP) (P means protection)
 A top brand (any) PSU has 5 protections. (cheaper or older PSU have 2 way only protections, 5 /12v only)
The wrost are the old PSU that have no OVP or UVP on 3.3v ! What I do is trash that junk PSU.

In most cases if you hear the PSU run for 1 second or less, the PSU is overloaded.
OCP, SCP, OVP, UVP, OPP protections x5)example.

The PSU works unplugged above, but not when connected to the PC fully. ?
This means we must do the fault  isolation tests next.

The PC has many modules inside, and know that any of them can short and if they do the PSU goes to an OVERLOAD state (detected) and shuts it self off, preventing FIRE and smoke and worse cascade damages.
The steps are easy, easy first then harder.
We remove the power cables to all things (modules we say) inside  the PC case (not PSU) Hard-disk drives (HDD) CD/DVD drive, all PCI-express slot cards, out.
There is no floppy drive to unplug those died years ago, no PATA drives either now.
All USB  ports unplugged, all PS2 device same,(those are relic jacks those are purple or green on the rear of PC)
USB ports are current regulated, and will not short out any PSU, but in the dark ages of USB some crummy makers of PC did  this wrong, so unplug USB ports now. (its easy and fast so now do that)
Unplug the Monitor VGA/DVI/HDMI cables. (monitors do not short and overload any PC but we do this just to be sure, in case you have non standard parts in your PC ,yes some GPU cards are custom to the PC makers wishes, all bad actors there....)
We remove all RAM (DDR) from the RAM bank and try each 1 by 1 only in bank 0 slot. (your mobo manual shows how to run 1 stick of DDR ram in all cases and varies by ever make of mobo, RTM and win.)

If the PSU now turns on and all fans blow air, now, then what you just removed is BAD , it is a shorted device (module) or the PSU was running but say 3.3v rail is dead or reads 2v. weak. but not now, you cured the overload there.
Other tests are possible. (learn that cheaper PSU do not monitor all power rails for good and bad) so...
Advance technician testing, we can even test mobo regulators if you learn that skill. (if BIOS runs, my AUSU lets me see all VRM outputs in the BIOS monitor page) but in most case  here , the BIOS is now dead.
DC RAIL TESTING IS EASY: (if you own a $20 DMM)
  1. Lets say the PSU fan now spin ad blow air, next is to test all power rails with the DMM meter.
  2. The voltmeter is set to the 20vdc range, and connected black lead to ground.(COM, or common black wires , pin 24 is easy to reach for ground to DMM lead)
  3. The red lead of DMM meter touched 1 by 1 to each supply output pin rail. Seen here.
  4. If  voltage tests  fail, then the PSU is bad.  (or that rail line is shorted)  The crude spec is 10% error max. (or better here)
  5. If all tests here, fail to discover the cause, yes, the motherboard can be bad, the MOBO has some low voltage (1.8v) regulators present (power) that can go bad and the MOBO is smart enough to detect that and then sets power on to OFF.
  6. The reason it does that , is to prevent that nice $300, 8 core Core i7-7700K processor from blowing to kingdom come , (that is the reason for all protections seen on my page here) I fact could blow everything in the PC to H3LL. (kiss off $1000)
  7. PSU bad yes, then  buy a better PSU
The wiki spec on ATX2 shows the maximum errors allowed for voltage  The wire colors are standard.

 If this far? and still failing try a new PSU rated at 500watts or more and v2.3 versions. $39 at "Thermaltake (tm)"
If still dead, the mobo is history.(I've even removed the CPU last step but I have never seen any CPU short yet, but can have bent pins due to bad service)
end isolation testing.

GAMING/CAD 101:  Fast Video cards, GPU's Graphic Processors.    (or video rendering and editing worksations too)
The RULE of 75Watts max on the X16 PCI express slots, must be observed. Unless, you upgrade to this NEWer  Pascal GPU card.( any series 10 card))
The exception to this , like my  GeForce GTX 1050   graphics card by NVIDIA, launched in October 2016.
Built on the Samsung's 14 nm process, and based on the GP107 graphics processor. The Pascal GPU with 3.3 billion transistors.
The wiki does nice neat table here.
Now with TDP (total power disapation max) of 75w or less !  ( and now Laptops can do gaming 10x better too)
Not only did they shrink the GPU DIE they shrunk the card greatly 5.70" x 4.38", and now sells as , fits all PCs easy. (no need to ask they say) "Gordon Moores law" still wins.
That needs no 6 pin AUX power cable, nor a PSU with it , nor over 300 watts PSU to run the card.
I highly recommend this card, the 1050 at least.  (it is super fast and super low current (power watts) card.  Magic-like is the Moores effect, and changes forever our choices, of cards. 
You get to save big money on  larger sized PSU.  (about a $50 saving for most folks)

Now a Blast from the PAST ! (soon to be dirt cheap used or new, as folks learn to buy the above series 10 cards)
NEXT UP:   (The old power hungry GPU cards made for many years)

I bet most GPU cards like the below, sent back to the vender (sad no?) are good and the builder can't read 1 line of text in the GPU cards manual,  (AMPS needed minimum) 
My other PC uses the Nvidia GTX650-Ti.  (the power hog)

<<<< See it in DM , device manager?
The RED arrows below are Heavy wires Yellow/Black  (if he CARD shorts, the PSU shuts down)
Lets say you want to game or do some fancy engineering Gaming or CAD/CAM?CAE  or complex Computer modeling?
The below caard sucks 30amp at 12vdc off that RED ARROW jack seen below.  (if you see the jack you can bet that your PSU can not handle this power, in most cases)
If you make the error of not having PSU with this 6 pin plugged in the below jack the PC crashes. (it will crash as soon as you do fast graphics, or gaming )
if you run  low wattage PSU same deal for the same reasons.
The reason it crashes is  that you overloaded the PCI-express cards 75watt limit on power pins of said connector slot.

The PSU was named  "ATX12V v2.2" and above versions. V2.3 is best.
The 4 pin PSU Molex(tm)at MOBO Jack  runs the CPU but  the 6 pins Molex runs the below card ,over 75watts (express max)
Now Imagine 2 cards in SLI mode, wow. 60  amps....  ( 2 time 30Amps is 60) or 720 watts total for SLI)
 No express slot on earth can source that power ever.  (75watts is  max power on PCI-e (express! is the limit)
Lets call these legacy GPU cards, AUX CARDS.
The 1 or 2 AUX power jacks feeds are NOT OPTIONAL EVER Before Pascal chips.
The GTX-1050 uses very little power now, and is  far better than the above power hog, my GTX650 is now retired to the attic. (I show this GPU as it makes a worst case for overloading any stock weak PSU)

The best PSU story: "Power Supply Unit"
First the list of bad PSU's
  • Most new $15 PSU are all junk  (some as low as $3 bought direct from China, on funky )
  • Weak  250/300watt supplies.?  300watts is common (and quality they are) on any low-end PC sold by Dell or HP. This   may work ok with the new GTX series 10s above, but will not work on the power hogs above. (AUX Cards)
  • Used PSU? sure if the PC is worthless , risks are not a problem.
  • No Name PSUs  like "some ting wong:" brand, or Guangzhou or Dongguan? endless strange names. Try brand "THERMAL TAKE." that  are not junk.
A good PSU is at least 500watts, or 700watt, better or above if running a power hungry GTX graphics card  that sucks 100watts or more all by itself.
The best PSU have a
"Full Electrical Protection OCP, SCP, OVP, UVP, OPP" over/under voltage and over power protections are best, in all cases. $50 is not expensive. (it's not just about power)
Pay at least  $50  or you will get a very poorly designed PSU.  (new prices) sells great PSUs look there first. (click link above to see it)
Many dirt cheap knockoffs PSU are made by reverse engineering very very old PSU designs. (and are pure junk)
Modern PSU have  huge smart SMPS chip  inside, (and way less or none at all tiny TO-92 sized transistors.
The very powerful 12vdc RAILS here are what matters most.  
Ohms laws are simple math , Power is watts,  P= V times Amps (algebra Ohm laws)  Power equals Volts times Ampere draw.  So 30v x 12v = 360Watt true power or dynamic power.
In most cases for sure a PC with 3 hard drives and huge GTX card,  you need lots of 12vdc AMPS.

A powerful desktop can use 300watts of power an surges up to 500watts.  (but at idle very low) The PSU must handle ANY LOAD, even fast gaming.
The Dynamic loads must be be handled and noise (electronic) made from such an overload.
IMO : 500watts is minimum. (if using  power hog GPU card)

Better is: ( this is just my opinion, I get no cash for saying so)
A Thermaltake PSU 500w or more is also a good deal. $35 at
 Make sure the amps are good,  some makers of PSU lie about the watts (sold as no name PSU from China direct),  read the label first. 
The Below PSU , see those numbers in the +12v1/2 column, this must be correct, 384w will run the 360w card. (but will not run any SLI card setup like this)
Or this nice Coolmaster. 

GPU cards from H3LL? (101):
Anyone who runs expensive $60 games, knows all about this issue:
I see endless posts by folks saying bad things about a PC, Windows, and many things, all the while violating the  GPU card official manual spec., they can't read? or can not understand, that AMP'S matter.
Try to know watts is combined on all RAILS, and only amps available matter per Rail. See the above photo for PSU with a really honest sticker, there.  384w just for 12v and Video cards.
Adding any OLD GFX card to a PC with 250watt PSU, is just silly, It is a wasted effort and endless grief watching said PC crash endlessly BSOD's , or even goes DEAD. 
But again this card is better.  GTX--1050
It is sad to see vast buyers of MOBO/GPU and PSU rejecting what they bought when in fact is the the buyers fault. (cockpit problems) . (learn to buy The PSU  first, to match what your GPU and MOBO needs first)
So is it silly to try to run 360watt GPU card from only a spec. limited 75watt, express slots.  (hopeless that) unless you have GPU card AUX power plugs and a matching PSU, AUX cable  to drive it. ATX12 v2.3.
Learn that the GPU card is not magically smart and knows to just turn itself off if you forget the 1 or 2 AUX cables or the right sized PSU.  (this is your job and doing a power budget)
We see folks doing this and then burning up there nice motherboard (the 75w rule is not  guide lines, its a hard cold limit, you must keep  under control)
In some cases the power traces on the Motherboard blow up and smoke lets out  of the PC. (just for forgetting the AUX plug.)
What matters most is the 12vdc rail .  The yellow wires are all 12vdc. 
The PSU has  huge power regulator inside to do 12vdc.
My PSU has run crash free for 5 year now. Not one hiccup.
  if you use a proper PSU, the GPU will NOT CRASH (99% crash free, 1% are bad games,not updated) no lint packing up GPU nor CPU fans ,nor heat sinks, clean them.

Is this your PCU? and the fan stalled and packed in filth,  (all PSU do this, if not cleaned every year) We just blow it out.
really? You expect this to be ok? It stalls the fan and makes all parts overheat and die. That dirt blocks air flow too)
Always clean parts OUTSIDE using Can-O-air or 30PSI shop air.

I use this  wimpy supply below in my SAS hard drive cage ( no video there , no mobo there) so what you use, must match your needs, in all cases.
 (bent open for easy viewing) from wiki pages. Do not bend yours like I just did.  (I removed wires only for one rare application)
Learn that some PSU run a thermostat fan (speeds based on temperature only) On newer ones the main board if it see way too much heat shuts down the PSU,  "self preservation" and very wise that action.

More chips ok?, for PFC controls (95%) efficient, or OCP, SCP, OVP, UVP, OPP protections are all worth having more chips or a huge one, I see now in the mighty year 2018 some makers of PSU have wised up and use a HUGE $10 or less ARM chip.
The ARM chip does it all, getting rid of other chips inside. (or DSP)
Called by some complete digital controls. (or DSP controlled or this) yah, a real fancy PSU. See how this amazing DSP techology will change all this fast. at
See this high end Thermaltake presentation.

The below is  low end new product that replaces the old above photo of very old PSU.:
Many top brands say using real Japanese capacitors (a smart move)  ( yah, many sold in China by SOM TING WONG  are pure crap, read the wiki horror on that)
Look for more larger chips and less descrete tranistors, on all modern PSU.  The huge transistor are needed, like the ones on that heat sink.
Some PSU can exceed 95% efficiency... (nice) A PSU that runs cooler lasts way longer ! A fact.

If you buy a new PSU, get one with modern ACTIVE PFC (power factor correction ) chips inside for up to 95% efficiency and way less heat.  (yes it costs more to get quality) Quality is never free.
The future is DSP.  for sure, only now we wait for lower end products to adopt DSP (I can't wait, to seem them), and save money they will. (more for less)  (I own a DSP SDR RADIO RECEIVER and is amazing)
Digital Signal Processors (DSP) are the wave of the future, and glowing now like mad.  (all a good thing ) the DSP even lowers noise. (electric and  sonic noise both and less fan noise.)
Here is a hit list of the  good things. (and wise match to your  system)
  • Get one with enough power (WATTS is power), ohms law volts times amps = power (watts)  500watts at least, less watts invites (begs for) cheap junk or very old designs.
  • Full and complete OCP, SCP, OVP, UVP, OPP protections
  • PFC  option (a chip set feature for power factor controls) this also cuts down heat inside PSU, making it last longer.
  • The large fan, only get those with the huge 120mm fan makes less noise, it's not actually better, CFM is what matters for better, for cooling, CFM is cubic feet per minute, air flow, that matters most, but 120mm are best for low SOUND (noise)
  • In the same vein, a thermal controlled fan is very good, as that too cuts down noise. (you will see it speed up as you begin gaming) ( a smarter PSU will even turn it self off it IT , overheats.)
  • Get one with modular connectors if you want a neater looking PC inside or are paranoid the extra cables will lower case air flows. (it is  more expensive and now you  get 8+ more connectors that can fail.(complexity for kicks?)
  • Do not buy and pay extra for silly  blue (or RGB) spinning lamp LED's  in the fan blades. (gee) cute yes, useful , no.
  • Buy top name brands, like:
  • Like Corsair  and Thermaltake.   (check out for top brands, not fleabay trash)  Even go wild get $200 DSP based PSU Axi series? from Corsair. (if you are gamer do this) 
  • Do not buy products from companies you can not even pronounce their name. Yueqing Qili Electrical Co. or Dongguan Yuhong Electronics Co. , like seen on for $10  (non ISO-9000 certified co.) and no FCC part B certs.
  • Make sure it matches your form factor, ATX2 on my page, but there are many more now. Even mini ITX. (from mini to nano and mobile)
Here is a  list of ATX variants:

end  true and real  standard desktops. (true  DT, not All-in-One desktops  (a new oxymoron))

LAPTOP (LT) power structures are complex.  (the actual PSU is hidden deep deep inside all LT)
One way to think about any Laptop is to Imagine most of the parts above, in the PSU are  moved to the Laptop MOBO.
Because that is what they did and added 1 or 2 battery packs and a battery charger chip.  (to test any laptop, first take out the huge batteries ,  usually just 1,  and see if the LT runs now.? Batteries can short, and overload regulators.
The LT  also has a power controller chip, to control all 6 or more modes of power.
All this can fail. Just like Desktops.
The typical LT uses no more than 65watts (tiny)

The Laptop has a large number of power regulators. 
To isolate shorts in any PC even the LT we...
We remove all modules on any LT to find modules of any kind  that are shorted, and overloading any of the many regulators. (large battery out first , as they just love to short and overload power regulators)
Normally and best , power comes from the 19vdc power pack (aka: the AC pack) best is to test with only this supplying power , main huge battery out.  Keeping AC line hot, also makes the batteries last longer (LIFE SPAN)
That 19vdc must make it onto the mobo actual pins and not have broken power jack at the DC plug at the MOBO raw edge (PCB).

Drawing LT#1:  Here is my Laptop state drawing,  2009 made PC.  There are 6 states, of operation, for sure normal, sleep and hibernate.
If all batteries are dead or removed the RTC coin cell must be good or the PC will never turn on.  (there are up to 4 power sources here, COIN, Main1, MAIN2 battery and AC power pack 19v) 1 of the 4 must be good.
If the coin cell is dead, all bets off.  BIOS goes NUTS, all batteries main are pulled and AC line pack is removed., (the coin cell  keeps NVRAM alive , with zero power from all 4 sources the PC is now dead.)
S0 is full on power state (I guess S3 is sleep and S4-S4/AC is hibernate?)

See MY  power page here. If you have  schematic like me, matching your PC not mine, we can test each regulated supply seen here.
If one supply rail reads wrong (10% out of spec or more), we remove modules to find out if one is shorted.
Never condemn any MOBO until you prove all modules connected. are not shorted. (a new coin cell is always first to change out. the words RTC above mean real time clock.(time of day and date))
The regulators on all MOBO all shut down overloaded, in all cases.  (that is why there are NO FUSES)
Lacking a real schematic means all you can do is measure voltages, (key voltages) and pull modules.

THE CMOS is messed up?: 
That tiny battery called RTC COIN cell runs the NVram and the RTC (real time clock chip) both go dead, if the battery fails, or drops below 2.9v, (3.3 is new typical value)
This causes the BIOS  brains to go NUTS and TIME of DAY and Date, and if Windows sees time wrong, oh boy,  it goes nuts too and then "Certificate errors from H3LL"
NVRAM , is a tiny RAM that must  not forget things (NV = Non-volatile), if it does you will play hell. (endless pain, wild symptoms , things that make no sense at all, why let that happen ever? for $2)
If the PC is over 5 years old and you don't have a  $20 DMM meter to test it, just replace it.( Walmart has them in the WATCH department, take yours with you and match it up)
On some laptop the coin cell has PigTails, and is very hard to find them or make one (as I do). Best is to search on Ebay for your pig tailed battery.

This page I made shows how the RTC and NVRAM WORK  and what to do if the battery dies. (RTC COIN CELL)

version 7.  8-1-2017   (July,28,2018 last edit)

Do not play in the AC side of any PSU. OK?