Facts and choices about the PSU (power supply unit)
I will cover resetting, testing, diagnosing and upgrading a Desktop PSU.

Power supplies , like TIME, keeps everything from happening all at once. (unlike out of control lightening)
The power supply is actually a SMPS based DC regulator (besides converting AC to DC) inside the PSU are VRM's voltage regulator modules.
Safety first, do not play inside the unit (dismantled) with the power cord still connected to wall A.C.JACKs (wall jacks)
The 339 to 678 volts inside can be lethal if touched, and learn now that the PSU is never actually  off, unless unplugged from the wall.
But the DC side of any PSU are safe to touch,  5 to 12vdc  they are.
See how to be safe here.
One last fact to learn first is that all Capacitors (CAPS) in the PSU store energy even after unplugging the line cord from the wall jack.(AC convenience power outlet)

If the PC is dead or worse dead silent , be sure to reset this GFI button here first. (arrow)

RESETTING ANY PSU:
 (this is mostly a motherboard reset or is done only to make the system safe to remove parts (RAM/PSU/PCI/HDD/SSD) or get the PSU ready to bench test.) 
A dead PSU is the symptom. (this cures sleep mode stuck or I.C. latchup or ESD lockups, steps 1 to 5)

First is Question #1, how does it turn on? (if the CPU fan is dead, the PSU seems dead, but is it?)
These simple steps are covered at all OEM PC's web sites call the hard power reset.
 
  1. Make sure the optional PSU rear  rocker power switch is in the "0" OFF position.  IF NOT present this rocker switch then do step 2 below
  2. Remove the Thick A.C. Line cord at either end  or both ends.
  3. Push the front power button for 10 seconds. ( the fans in the PSU may spin for 1 second and die, this action heard,  are the  CAP.';s discharging)
  4. Release the  power button now.
  5. Wait 5 minutes, touch nothing. (this wait can help you discharge the Cap.s deeper) (and is optional)
  6. At this point you can work on the PC inside and not damage parts removed or installed hot swapping them.
  7. if  only doing the hardpower reset then  reverse steps 1 to 4 and PC now it runs and works perfectly (but may fail again soon if you our PCs drivers are bad and sleep locks up again) Sleep mode can lockup, and is common.
  8. If only doing internal service to the PC you are now safe to work (you safe and no damage to the PC) do not HOT SWAP parts in any PC. (inside parts, ALL)
   If you are only testing the PSU? do the jumper test next.

    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)
    Facts to know first:
    • The PSU does not turn on until the motherboard (Mobo) tells it to do so logically.
    • The PSU and the MOBO both can turn off power, even in 1 second flat at any time, for sure under any overload conditions. Saving the $1000 in chips from damage or worse a FIRE.
    • The PSU is never off, if connected to any wall A.C. power sources, (except this switch off does kill all power inside PSU and PC).
    • PCs can self turn off  for other reasons, not just a bad PSU, overheating is one, and shorted parts inside the PC. (any part mostly can short and cause shutdown or not turn on at all)

    Theory of operation (details for the curious?)
    The PSU is a slave, to the motherboard via the  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 slang Glue logic.(as seen on a real Intel motherboard (MOBO) here.
    Later ICH name 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 front 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....I skip .)
    • 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) and the newer hibrid mode.
    • That  Chip even allows you to define what the front power switch does, (power on/off, sleep or hibernate)  (in Windows click power, click advanced power settings, and read what it can do)

    The first rule of Power managment is stand by  power. (if this is dead the PC and PSU will be dead)
    If +5vsb Stand By, rail goes dead, the MOBO will be dead as will be ICH and all wake on features of this PC.
    One way for +5vSB to fail is a shorted PCI-e card, not just a bad PSU.

    Many PCs have an LED on the MOBO that is marked STANDBY power, and if this is dead?, so be the PC and PSU, (your Mobo manual covers these facts so RTM? (read the manual yet?) (link here is Dell 3020)
    or...
    We use a voltmeter on pin 9 and see +5vdc there. so is good. if not , the mobo can not turn on the PSU. (due to the  ICH chip not having power now)


    The PC Case has a front power switch, this does not actually pass 120vac (or 240v) to the power supply, at all, ever. (so is safe to handle at any time)
    That switch is 100% digital    (a momentary closure SPST switch it is, seen on the front of most PC cases or front side wings)
    It has 2 pins one is ground other is Pwr_sw ,  one pin that measures 5vdc if you tired to measure it.
    If you got to thinking the power switch is dead, (bad) you can short the 2 pins here, to see of the PC turns on. (no two motherboards use the same pin-out rules, ok? (unless both are the same mobo)
    The PWR pin below has rule for how long to push the power button, if you hold the button for say 4 seconds, this ends any sleep modes,(intel rules in the ICH)
    The below is random sample of 1 switch and is NOT YOURS.
    The 2 pins here to jump are PWR and GND pins and bingo the PC turns on, if not we test the PSU next.
    If you jumped those 2 pins above, and it failed ,I use a voltmeter (DMM) and see of +5vdc is on the pin above marked PWR. If not then standby power is dead. ( and the real problem is that)
    You push the button and the mobo ( motherboard ) ICH (PCH) chip sees (senses) you do that at the MOBO jack called "Pwr_sw" pin (PWR above)   and the ICH chip, then sends POWER_ON (or PWR_ON) to the PSU to .(pin 16)
    Some PCs the BIOS can stop power on  due to it discovering a dead fan at this instant, ( the fan reads  0  RPM fans, are not allowed in this model PC, and CPU fan dead is a BIG DEAL)
    It may also have a too  hot thermal  sensor reading on the MOBO and fail to power on. (If say the PC just shut  off overheated and is still way to hot to run safely)
    The ICH (PCH) chip   now,  monitors the Power_OK pin on the PSU and the same signal from the MOBO onboard VRM (voltage regulator ) Keep in mind the PSU  can set POWER_OK TO FALSE AND SO CAN THE MOBO VRM'S.
    This can happen in 1 second flat , the PSU turns on then off in 1 second. (In half the 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.
    SHORTS do Happen, in all electronics made. (for sure ESD events or lightening hits damaged)
    Many things can SHORT OUT,  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.
    Any OLD capacitors can short, as many did during the CAP PLAGUE years.
    A short circuit is when a filed part uses (tries) infinite current. ( I = E/R) if R is 0 BOOM smoke happens but on our PC the VRMs detect this and shuts down the PC fast, preventing ,BOOM , SMOKE, SPARKS and FIRE.
    The VRM's use OCP (over current protection) preventing horrors unlimited.
    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)
    But a wise man unplugs all the external devices first (not the power cord) first. to prove there are no shorts on those devices.

    End theory of operation.


    Checks Simple. ( this is how I do the tests, briefly.
    1. Clean the PC, fans (all and all heat sinks and blow out the PSU , or the PC will be dead, or want to self turn off, from overheating. (< see why here)
    2. Check if the RTC coin cell not dead. If this tiny $1 battery goes dead the BIOS and NVRAM go NUTS. (even black screens possilbe)
    3. If PSU internal fan is dead, as you push the  case front POWER switch,  make sure the rear switch is  set to "1" on.
    4. Some PSU internal  fans  do not turn at PC switched on time., (most old ones do, but modern ones turn on only if hot, or only about 20% PSU load so if 500watt PSU , fan comes on at 100watts load)
    5. Many modern PC's have  BIOS page for fan controls and you can make it do what every you want at all, even setup a fan profile, like on my Z270 prime. (my DSP PSU has USB port and application prog, that do what I want)
    6. Make sure the power switch is wired (and didn't fall off)  per your OEM.com service manual free on line. (Google it)
    7.  The MOBO wires to the PSU with the huge 24pin  connector seen below, and must be fully seated. (try to know some have lock levers on there side that actually works and lock so. do not yank cables willy nilly or you will wreck it)
    8. The first goal on any PC is fans ,most fans must spin , for sure CPU and if GPU has one , then it too.  If the PSU fan always spun before "dead cold PC at power on", then it surely must now. (using the wrong fans invites trouble here)
    9. Ok, we can do more tests, we can use a magical tool called  a VOLTMETER , sold at Walmart for $20 ,  and when set to 20vdc range can measure all power pins as you push the power button.
    10. I test pin 9 first STANDBY +5vdc for 5volts first, if dead this pin the PSU is bad. (In ALL cases, if you bench test the PSU and this pin is dead the PSU is BAD ,end of story)
    11. Next after power on and fan PSU  fan works, I then test all DC power line out pins, seen below for spec voltages. (seen below in a photo of side sticker)
    12. If all power lines show good? voltages, then the PC should run ok, ( if power is good DC, and the screen is just dead, see my black screen page. ) or see my dead PC page.
    13. A bench test is easy, jumper (24connector) pins 16>17 (modern 24p 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.
    14. 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. 
    15. A voltmeter can now read all supplies, one line at a time,  3.3v, 5v,12v, +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.
    16. 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 20-40khz, if that is the noise hash you see on the lines out. (the SMPS chopper noise too has  max spec)
    17. My new DPS PSU runs 100kHz faster chopper to gain higher efficiency (but all PSU's made have filters there, to keep that chopper noise  out of the DC bus or  hell will be paid.
    18. One common way to get in trouble, is upgrading to a faster (FPS gamers) GPU and now the PSU is overloaded.
    19. Keep and open mind, that one of the many things connected (any) to the MOBO can be shorted, or RAM shorted, the PSU shuts off with any kind of overload, there are 5 levels of protection. (the tests are here)

    Regulation spec, (intel)  Most PSU  made now do far better and for sure any new DPS PSU.

    We use a scope to check ripple.  (caused by BAD CAPS inside the PSU , 1999 to 2007 made? cap plague era?) (some DMM can measure noise but most can not)
    Most modern PSU can do way better than this,  make sure the SMPS ripple noise ! 40kHz is very typical is not above the values in the table below. (20kHz to 100kHz ranges)
    All SMPS made on earth are huge noise generators, that can land on any RAIL and all have very good  LC filters or far better Pi filters (C+L+C),  to snub/filter out all that nasty noise, a real shop has a scope. (ok?)
    Some old PCU  from the CHINA caused PLAGUE years  1999 and 2007  have bad CAPS and the filters fail.
    In todays context RIPPLE means NOISE and for SURE CHOPPER NOISE.  If too much noise is here PC will BSOD.
    Using a scope, 10kHz to 100kHz  chopper caused, ripple max.  ,mV = millivolts  1mV = 1/1000th volt
    All modern 10 year or newer PC use this ATX2.x +  spec 24 pin molex connector. (as shown in the ATX spec. WIKI verion 2.2 spec and here)
    If your PSU has 20 pin plug it is no good  for you 24pin motherboard, you need a new PSU. with 24pins, forget 20  ,that era is GONE.
    The PSU to buy is ATX spec, 2.2 or higher TODAY.

    Photo Dwg. #1  PSU connections.   To bench test the PSU, we jump pins PS_ON# to any ground pin{COM} (24pin).

    Buy one, make one, or just use an unfolded paper clip to jump 2 pins. You can now bench test any PSU made.


    Almost no person today has PSU with 20 pins, it's so old all are now in the land fill or fully recycled. (RIP 20 pins, end it now) COM = common = ground below.
    old relic ATX                               Modern ATX2 with 24 pins, the 4 pin plug  is only CPU VRM power seen on 4 pin jack on mobo.

    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?
    The wire colors  and names are all in the SPEC ( Spec. means specifications) A.K.A,  ATX SPEC. (slang)
    ATX spec table 10, color spec.
    From WIKI, . One could read the ATX spec, sure. (it's a standard, unlike stinking Laptops)
    -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 modern PC we have a magical -12v charge pump inside the chip of needed. (I'm sure -12v will be deleted in the future )

    Here is a list of OEM PC that are ODD ducks not really ATX SPEC in this PSU REGARD:
    Some Alien non ATX spec. PC's use SFF (small form factor )  with 6 pin power.
    My Dell 3020 MT has a non ATX PSU, but I cured that here.
    HP Deskpro 600, PCs use this odd non ATC spec. connector linked below... 
    This  SFF  PC uses a 6 pin Plug,  seen here (hacked to work with standard PSU)
    Also I  avoid ALL _in_ONE FAKE Desktops  are not ATX spec. (only RAM is and SATA are spec)

    Connectors, plugs and jacks or receptacles:  D.C power connections all.
    ::::
    DO  NOT CONFUSE JACKS
    , or you will blow up the PC. 
    (DO NOT EVER PLUG IN a 12V PLUG  TO 5V JACK OR THE REVERSE) all are keyed but there is one horror that is super bad, and this EPS jack fits some GPU cards and blows it up.
    Last of all is this gem of a drawing,  that shows all main jacks (not SATA not 24 pin shown above)) used on V2.3 up  modern PSU. 
    The EPS, must never be used on any actual PC. ok?
    I call these the AUX  jacks, used for CPU bottom right, all modern MOBO have  ATX-12v jack or the CPU VRM goes dead,
    The 8pins and 6 are for high powered over 75watt GPU  cards, The EPS is for servers not PC. "Server System Infrastructure (SSI) group only"
    If you make the gross error of jamming that 8pin EPS in to any PC mobo or GPU card, you will get lots of smoke, sparks and fire, and  very thin wallet for sure.
    Some Top brand GPU cards (GTX690 worse) have two 8pin jacks that have 3 hot pins each, and the card uses 300watts all by itself.  See other power hog GPUs here, (a list I made)

    Most folks messing this up are those with NEW GPU card and need 8 PIN AUX power, but there PSU does not have that, so they buy or make and adaptor that makes a horrible error.
    Make sure the yellow wires go to only the 12vdc pins on the mobo and to the GPU card, and win.


    Some more facts and safety issues in detail: This section shows clearly how and why you will get shocked or even killed, messing in this EXACT SPOT.

    First off what are RAILS and what are Virtual RAILS? (see real schematics here  too)
    This is about SAFETY (yours)
    • The PSU never turns off right?, true!, unless disconnected from the AC power jack in the wall power outlet. (exception 1 see this rocker here)
    • Near the AC power cord jack if you see a RED slide switch and you turn it to the wrong posistion, boom it explodes is a cloud of smoke. (fire and sparks too)  120 is 120vac, 240 is 240vac , do NOT GUESS.
    • The power button on the front of any PC never turns of +5vSB standby power nor what you see below !!!
    • The RAW power AC/DC is dangerous to humans and very easy LETHAL300to 600 VOLTS DC. (inside USA or outside USA) ,  (if un trained in working near high voltages stay OUT ! of the inside of  any PSU, case)
    • The safe side of the box (outside) is only the low voltage DC side, 3v to 12vdc. Only. 12v is never going to shock you. (even if you drink carbon dust for a week, LOL)
    •  This is a SMPS design. (very efficient and vastly better than any analog power supply made)Switch mode Power supply it is and today even better DSP.
    • To work on any PSU pull the line cord from the wall jack (do this next), wait one hour or more, and when opened up the casing, on a work bench , grounding (case is ground) out all caps using a  chicken stick .
    • The only reason to open any PSU case is to see that fuse blown, and if is blown , you will not repair this PSU. (for sure not with $50 new one in box and labor high)
    Do not touch this AREA. All PSU made look like this, and is HOT all them time plugged in to the wall, (L = line hot, N = Neutral line)

    Some top qaulity PSU examples: 
    My top of list crytiera for chose are:
    • Top Brand names and quality and long warranty (7 years good)
    • Enough power.
    • Not  catching fire if one PSU wire short to ground or HDD shorts 12vdc to ground due to a drive fault. SEE REMOVED below:
    • Fits the case.
    • DPS is good, mostly because it is fast and can have faster better protections and better regulation. (less noise on the DC outputs  at all times)
    • Full protection logic,  over/under voltage and over current.
    • Quiet,  a very large quiet fan and only runs above 20% load and then only as fast as need be. (yes this is good)
    Cons: v2.3  the  240 VA per rail was removed, and not one word from them how to stop a fire if you pull down a 45amp load on 46amp RAIL, on one 18gage 12vdc yellow wire (amazing as that  is....)
    • I would like to see OCP , spec'd out by OEM makers but sadly they avoid this topic,  even when confronted . (and the missing compliance to 240va current limits now abandoned but no statemants as to HOW?)
    • In fact the new single RAIL OCP designs are not addressed at all in the spec, nor to the PSU makers want to talk about it.

    A good analog PSU: Antec EA550 550W ATX12V  (has 2x 12vdc rails at 30 amps each, using funny math ,60amps x 12 is 720watts) 

    Or a Corsair of any kind is good. (power in watts matching your systems needs)
    Or (the next one is an  amazing low price DSP based PSU) The only down side is all wires are 18 AWG not 16 and black. (all black I do not like) nor does it conform to table 10 ATX spec on wire colors,(wow)

    Thermaltake Smart DSP "G 500W" Digital 80+ Bronze Semi-Modular Atx 12V 2.31/Eps 12  ($53) wow !

    I have the DSP PSU above in size G600
    Modern PSU run a very powerful power supervisor chip that works vastly better, and adds overcurrent protection.OCP and over and under voltage protection.

    DSP ?:
    Instead of an analog supervior  chip , the DSP , Digital signal processor (new tech) is far faster and smarter, vastly so... and will change all PSU's from today forward,
    Called by some complete digital controls. (or DSP controlled yah, a real fancy PSU. See how this amazing DSP techology will change all this fast. at TI.com
    See this high end Thermaltake presentation.
    They also have a new line of DSP with less power not just 1200watts but as low as 500w, ,   in 2019 seen  here for $60


    The DSP has 1 huge advantage that no analog supervisor chip can do ?
    The Supervisor chips (old tech) are detuned for power on surges, when you turn on any PSU all CAP.s (means large electrolytic capacitors ) all must charge up all cap's at once ! and when they do current is max.!  a 32amps surge is common.
    The old datasheet on my WT7525 chip tells how how to do that, at the chips pins (tuning) tells of 1uF and mine are  4 each 22uF (22 times more slower, power overload trip points,!)
    DSP magic, love  it.:
    But with vastly smarter (firmware) DSP, the brain knows startup  currents conditions (power on) are not the  normal run time currents, so is modal in operation very unlike its predecessor above 7525.
    The DSP sees a PC is running now (not booting up surge), and then can have new current limits on the 12vdc RAIL. (and very very fast turn off times) As low as 16mS turn off times. (one cycle of 60Hz on the AC line  that fast)
    In fact the DSP may even do "soft power on "features, that ends all power on surges for the life of the PC. (amazing no?) and then only needs to worry live running normal or abnormal dynamic surges.(or static shorts)
    DSP is  the future and is NOW. "Toughpower" is one model I like.  500w to 1200w choices, 
    Here is what a top testing company said about the  DSP above.  Techpowerup, on 0.2% 12vdc load regulation (5 amps to max) wow. (one clean PSU) 

    I do not choose any PSU by looks, no cute lamps, no RGB malarkey, what I want is  this in order.   (nor the Green  efficiency % spec.'s),

    End PSU power and related topics.
    Begin how does POWER ON actually work.(the actual electronics)

    POWER ON: To make ACPI work this is what exists.  (more Greek do not read this unless really want to know)
    Simply said the power switch closes (you finger pushed it) and the 2 contacts inside (a digital switch) close and grounds one 1 on the motherboard,  the below Intel ICH chip sees that (logic) and takes over the job of turning on all PC's.
    Next are details, if boring this ,do not read below.
    The ICH (PCH) chip (gluelogic or South Bridge chip) 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.
     


    end power on logic.

    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)  deal silent for sure unlike all the years owned  it all fans spun up.
    The top reasons are: (short list)
    • The PC  has  a shorted module (any ) inside. (parts do short, and shorts use infinite current ( I = E/R   if say 12vdc is E and R = 0 what is I? yes infinity, divide by Zero on  computer or  hand caclulator is yes, infinity or ERROR) so the PSU shuts off.
    • The PSU is bad. (if the bench  tests fail below)
    • The front power button switch   wire fell off the mobo jack.
    • The MOBO 24 or 4 pin power plugs fell off, either or both .
    • The line cord fell off (AC)
    • The wall jack GFI is tripped. or any reason at all for the wall outlet to be dead for any reaons, whatsoever.



    You are here because the PSU fan is dead? it used to spin!, 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 do this.

    Tools:  a Screwdriver to remove case side, and 1 paper clip unbent foruming  "U" shape.
    A voltmeter is always nice to have. (and dirt cheap, unlike the old days)
    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.
    The power switch on PC many have LED blink codes, not just off black, on green.
    The 2 common colors are green blink or amber blink,  (one means SLEEP MODE NOW (stuck) and other means very serious power problems exists (RTM rear you service maual on all blink and all beep codes , they are unique per maker and model PC)
    The PSU can be bad or ANY of the things plugged in to the PSU (colored wires +connectors) can be shorted.    The fault isolation tests are here (stripdown)
    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 naked and the 4pin plug too, 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. (dead most time means SILENT ) If the PSU fan is dead and CPU then it is a power problem , the CPU fan never stops (PC on) nor is it asked to buy the BIOS. ever.
    But the BIOS can see the CPU overheat and turn the PC off in 1 second flat. (at best that fast) Many BIOS sees CPU fan at 0 RPM AND SETS POWER_op PIN TO FALSE And the PCU turns off NOW.
    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 4 pin yellow and black cable to the MOBO, remove this too. (the PSU has many cables (red/yellow/black) and all are disconncted for this test.)
    • 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 pin 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. (if the 0/1 switch on the rear is in the "1" position.
    • The DMM meter shows all rails read at spec.voltages.(seen in tables above)
    • 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)
    Over current protection is found only on modern PSU and on the newest PSU are very smart and powerful.
    This limits can trip powered on or at any time and in 1 second or less. (spec)
    what are these names, coded,  Oxx means over, Uxx means under, Sxx is SCP short circuit protection really just  one more name for OCP)
     
    OCP, SCP, OVP, UVP, OPP 

    The PSU works unplugged bench tested above, but not when connected to the PC fully. ? (this means the MOBO VRM's see faults)
    This means we must do the fault  isolation tests next. (we do the strip down test 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  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 !
    How to pick a GPU card?

    The RULE of 75Watts max on the one X16 PCI express slot, must be observed.  

    The Nvidia GTX650-Ti.  (the power hog uses about 2 times what PCI slots can source) 130watts (not as bad as the 690 at 300 watts) 

    <<<< See it in DM , device manager? Tells you what you have inside.

    The correct PSU that has the plugs below (6pin) was named  "ATX12V v2.2" and above versions.  .
    The 4 pin PSU Molex(tm)at the MOBO Jack  runs the CPU  VRM regulators,  but  the 6 pins Molex runs the below card and neither are optional    ,over 75watts GPU (express slot max)
    Now Imagine 2 cards in SLI mode,  needs 2 times 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)
    Never ever plug in a EPS cable to this card, or BOOM.
    When you see this connector, think new PSU.  (if your PSU is below 400watts)
     
    The 1 or 2 AUX power jacks feeds are NOT OPTIONAL EVER Before Pascal chips.
    The GTX-1050 uses very little power now 65w or less, and is  far better than the above power hog.



    GPU cards from H3LL? (101):
    Anyone who runs expensive $60 "AAA" 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 total spec, max. is combined on all RAILS, and only amps available matter per Rail.  (best for 12vdc GPU power is 20 amps minimum) that is 240 watts, but there are a few 300watt GPU cards (GTX690 is one,  25 amps it needs)
    Learn one thing, here, if the GPU card overloads the x16 PCI-express slot or OMG oveloads the CPU, power regulators. for any reason at all , the PC will CRASH, or BSOD.
    If you use a proper sized PSU, the GPU will NOT CRASH (99% crash free, 1% are bad games not  updated)
    If all this sounds complicated, buy a GTX1050 and be happy at 75watts max.
    End GPU overloading.

    Overheating?  Next is  lint packing up PSU/GPU/CPU fans ,or the heat sinks, clean them. Over heating is a problem.

    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.(we use PSI shop air regulator set to 5 to 10 PSI) never at 175PSI ever)
    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 OUTDOORS,  using Can-O-air  ,or shop air.

    SHOW  and TELL GUTS time , do not do this at home, until you read this warning.
    This is one wimpy supply.
     (bent open for easy viewing) from wiki pages. Do not bend yours like I just did.
    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.
    I found out all +12v wires go to (soldered) to a single pad proving it is a one RAIL 12v PSU. (many PSU do that trick and do not have 2,3,4 rails on 12vdc.
    This PSU has only one 12vdc rail and no overcurrent protection . (it is not a GPU card ready)

    Old PSU will have lots of parts inside, if I see lots of TO-90, tiny transistors (descrete) that is relic junk PSU

    2004 to 2008 technology PSU were not so bad with great protections, and less parts.
    Now newer PSU with a much batter IC below, Supervisor chip, at black arrow.
    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.
    • Many folks what a new quiet PSU, so this is next , thermal regulated, PSU fan. (all ?) are with 120mm diameter FAN inside, and much more quiet today.
    • 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. Platinum is best. (but costly)
    • 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 , never and worse limits choices in a vary bad way.
    • Buy top name brands, like: (there are many)
    • Like Corsair  and Thermaltake.   (check out Newegg.com 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 Alibaba.com for $10  (non ISO-9000 certified co.) and no FCC part B certs.
    • Make sure it matches your form factor, ATXv2.2 up below, 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 ATX Spec. size of the PSU is  150mm wide (5.9") by 86mm Tall (3.4") and depth of 140 to 230mm (5.5" to 9.0")  The longer PSU is called EPS sized or extended (more watts possible)
    The ATX case may have lots of extra room for a very longer PSU, or with shorter DVD drive more or none at all, or the case with the PSU on the bottom can have crazy extra lengths. even 12"


    version 8.  8-1-2017   ( 5-19-,2019 last edit)

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