How does my Desktop Power Supply Unit ,work? (the PSU )

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 15 minutes, and if smart discharge caps with a chicken stick like seen here. (but use a 5K 5watt resistor instead (best for 170 to 300vdc or A.C)
Never trust any built in bleeder resistor in any High voltage box (device or radio RF amp. or PSU) ever. We shunt the caps with a chicken stick. 

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 PSU, 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.)
What is a rail?, well one side is a ground wire and the other is the other is the output of Diode pair like D18 top seen here +12v,  if you had two  more diodes and one more secondary output you now have two +12v rails. But why do that?
Consider this L1 output below, if it is rated at 20 amps, total, then adding more wires to that pin (as many do) does not increase total Amp. spec, at all, no it does not, but adding 2nd RAIL does. (with  new transformer first and new secondary +diodes,etc)
Even the TEGAN below has one +12v rail with 80Amp total,  the 20 Amp limits are just the wire size gauge limits. (note how you fail using 20amps times 6 at 120amps, the 80amps overloads now.

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 and OLD  ICH2 chip Ball (BGA)
ICH2 82801BA, circa year 2000.
Pentium 4. +AGP motherboard by Intel , the older chips are more easy to see pins and these power features, (PS_On, and PWR_good or Power OK &
PWRBTN# BGA pin W21))

Power LOGIC 101: Simple (this is all pure logic inside, and ACPI)  (just enough facts to diagnose dead power or failures of power)
  • A Human pushes the power button, (even for 10 full seconds is best to end sleep mode)
  • ICHx (1 to 10)  sees this on its PWRBTN# pin, now. (from logical high 5v to logical 0v and is debounced,, this pin is T3 on the ICH10 ballgrid array BGA, the ICH2 is pin
  • 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 and sends power GOOD false (fails) and  the PSU sees that acts and turns off the PSU, fast.
  • If the CPU overheats at any time, the CPU will 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...)
This chip provides all features seen in the POWER state logic seen here.
We test the PSU while in the system by doing the forced power reset by holding the power button in , for 10+ seconds, this force G3 state , fully off mode. G3 is 100% off. Then turn the PC on.
Here is ICH7 chip, see PWRBT_L (it has 4.8k ohm  pull up to 3.3vdc) This is power button.

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 5-15 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 PSU 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 PSU 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.

ISOLATION TESTS ARE EASY: ( My instructor said "Remove your shorts!"  and all the girls in class slapped him)

See my strip-down power isolation testing here, for both Laptops classic, or Desktops real (not fake All-in-One non ATX standardized )

GAMING/CAD 101:  Fast Video cards, GPU's Graphic Processors.    (or video rendering and editing worksations too)  If this card overloads the PSU, bingo wrong PSU or  AUX jack never connected !
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.
The lowest power fast card is GTX1030 at 30 watts,
That needs no 6 pin AUX power cable
The worst card made by Nvidia (and many others  is the GTX690 at 300watts,  a real power hog and needs the AUX power cable direct to the PSU .
The wiki shows the true power needs in the right column TDP
I highly recommend this card, the GTX-1050 (75watts max) (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   not needing to buy a larger sized PSU.  (about a $50 saving for most folks)
Most of the high performance cards sold have this power hungy Jack on top.
I bet most GPU cards like the below, sent back to the vender  due top failure to read the manual , the cable needed and PSU power needs.
The Nvidia GTX650-Ti.  (the power hog about 2 times what PCI slots can source) 130watts (not as bad as the 690) 

<<<< See it in DM , device manager?
The RED arrows below are Heavy wires Yellow/Black  (if he CARD shorts, the PSU shuts down)
If  you see this jack ontop of the card, (sadly most sellers do not show the top view) then it for  sure exceeds 75watts power draw. 
The reason it crashes is  that you overloaded the PCI-express cards slots 75watt limit on power pins of said connector slot.
Some manuals show , needs 20amps on 12vdc rails some bigger power hogs claim 30amps draw from PSU.( this means the PSU needs to exceed this total on the 12vdc rails)

The PSU was named  "ATX12V v2.2" and above versions. V2.3 is best.
The 4 pin PSU Molex(tm)at the 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,  needs 2times more current and power.
 No express slot on earth can source that power ever.  (75watts is  max power on PCI-e (express! is the limit)
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 GTX 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 PSU? 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))

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?