Facts and choices about the PSU (power supply unit)
I will cover resetting, testing, diagnosing, fault isolation steps, 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,  3.3v  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)
The PSU logic inside can also lockup (rail shorts?), and can be reset by 2 ways, waiting an hour, after unplugging it from the wall, or the painful way, unplug every single cable from the PSU and AC cord and wait 1hour, (or less) and it will wake up.
Or do the bench tests , many dead PSU and its fan inside dead too,can be woken up.(per the above line)

If the PC is dead or worse dead silent fans , be sure to reset this on  the WALL AC OUTLET  GFI button here first. (arrow) 
      The hard power reset procedure is  on my page by same name here. (cures many things but does not reload windows, ok?)

    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 hybrid 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 small RAIL  is dead?, 
    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.
    this is ATX spec. DC voltages. note the crude 12vdc limits.
    We use a scope to check ripple.  (caused by BAD CAPACITORS (CAPS) inside the PSU , 1999 to 2007 made? the cap plague era?) 
    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.
    All PSU use a SMPS design, switch mode and all make this huge natural noise deep inside, and must be filtered out using  LC filters. or the PC will crash.
    Any old 5year old PSU can have bad output filter CAP.'s in the PSU and will have huge ripple seen on any scope, (even a $50 old scope can see ripple of any kind)
     mV means millivolts or 120mv = 0.120volts noise and p-p means peak to peak.
    Using a scope, 10kHz to 100kHz  chopper noise, & ripple max. 
    My new ThermalTake PSU  with  DSP is rated far less. 0.2% ripple max. 5 times better.

    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.  Buy only ATX with this AUX power cable.

    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 do the unloaded  bench test any PSU made.

    COM = common = ground below.
    First I will show motherboard JACK views: (or the the backside view of the plugs/ wired end of plugs.)  GOOD PSU's have colored wires not all black wires as some cheap makers do.


    The 2 Orange pins are pin 1 and 2.(paired)
    The Below views are with the plug pin side views.  Holding cable connector in hand and looking at the pin mating side only. (the motherboard side is a mirror  of this)

    old relic legacy 20pin ATX  most folks forgot this,  and is best.   
    This next photo set  are the PLUG views not the JACK view below:  The Green wire is the key wire to find first , pin 16 below.        In the future the blue wire may be deleted, as PCs do not need -12vdc today.
             Modern ATX 25pins

    The only thing you should ever connect (testing or test intruments) 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 specification table 10, color spec.  Green is and input and Gray is status output, only.
    The ATX spec connector end,  This is the pin side view not wired end view  Jumper 15 to 16 to force and latch power on.
    -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 the DIE of that chip, making it a 5vdc only chip then!, and 3.3v now or less)
    -12vdc BLUE 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 even the strandards hits of this. )

    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.  This page shows how DELL uses a none ATX PSU and uses only two 12vdc power cables. Seen in my 3020 page link to the left.
    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.
    , 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)
    Each wire and pin are only good for 10amps, max, so 3 pins adds up to 30amps, per connector PCI-e, some GPU have 2 AUX PLUGS.
    When you see to GPU AUX plugs they are not playing games, the card needs 4 to 6 yellow wires to run right. or BSOD you will.
    Always buy a PSU with the 6 pin connector below or with pairs. (for GPU ugrades now or later) Never ever attempt to force the EPS plug in to any GPU card.
    This is the cable connector end view, pin mating side.
    Most folks messing this up are those with NEW GPU card and need 1 or more 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. Count the yellow wires and do not go less. Some PSU have no 8pin cable.
    The worst is here  an almost 300 watt card.(2 x 6 pin power cables do work)
    In most cases,  6x8 cards, using 6x6 cables is ok, do not leave an GPU Aux JACK missing.

    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 PC side is safe inside and out,  but for sure NOT SAFE INSIDE ANY PSU CASE, EVER. ( unless you learn how to do that, working highvoltages lethal)
    • 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 F1 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  LETHAL 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 of criteria for chosing a PSU  are:
    • Top Brand names and quality and long warranty (7 years good)  Corsair, Thermaltake,
    • Enough power to run now , run any game and add any new GPU car later for yes, faster gaming. (FPS, mania)
    • Not  catching fire if one PSU yellow wire shorts to ground or HDD shorts 12vdc to ground due to any hdd /dvd 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)
    • 500watts is enough but in most cases the 700watt is  no deeper and has more quality. (and has better gaming support, for updated GPU cards)
    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 to be safe?)
    • 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.
    • To be safe at home turn off PCs when not home. Be safe, and keep that fire insurance current.

    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  chips , the newer better 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 all about  startup  current flow (amps) 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 (firmware) about 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)   12vdc runs that GPU card so is SUPER CLEAN POWER, ending any BSOD's now.

    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),
    Clean power, lots of power, and no noise, and fast fault detections, and not blown up $1000 CPU/GPU set. 

    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. (YES OLD, BUT a real and rare schematic)
    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. (but intel.com has the data sheet to read on all)
    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. (my DSP PSU can do this is 16/1000th of  a second flat.)
      The PSU all by it self can shut off, if it overheats or its fan is dead, or if it discovers any overloaded RAILS, 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 the CPU fan tachometer and if reads 0 RPM or too slow can turn it self off.
    • The PC can go to sleep, or hiberate or even a shut off timer in the BIOS can turn on/off the PC based on or off using the magic, BIOS internal   calender clock schedule. (so cha
    • ALL PCs come with Energy Star (or other names) USA government mandated sleep modes, (easy as pie to turn off this feature)  (normally you just hit key or wiggle the mouse and speed ends.)
    • One more the 4 second rule on PWRBTN,  if held for 4 seconds, this is the unconditional power off command. (Intel logic this is...)
    •  The hard reset proceedure will reset any stuck sleep mode or any kinds, see how here.
    This chip provides all Power  features seen in the POWER state logic seen here.
    We test the PSU while in the system by doing the forced power reset 
    Here is ICH7 chip, see PWRBT_L (it has 4.8k ohm  pull up to 3.3vdc) This is power button.

    Simply stated, the power button (N.O momentary SPST) switch closes and grounds the ICH chips PWRBT_L pin, the CHIP then sees this or for even 4 to 5 seconds held, the logic in the ICH, then asserts  the power on oline to the PSU  and the PSU
    turns on , and tells the ICH that power is OK.,
    The PSU can in fact turn  itself off., if there is any OVERLOADs of any kind, even overheated. The ICH chip does S0 to S5 power states, this is one of its tiny jobs.
    The OS can set all major chips and cards and ports to go to sleep, or not. (all under you control at any time) (as seen here)
    end power on logic.

    Rapid Desktop PSU Testing to find true cause;  (in PC case tested or  on a work bench) This is not full load bank testing of any PSU , most shops have to such tools.
    (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 PSU (and mobo VRM) will shut off the PSU if there are any kinds of overloads to PSU or any regulator. (this is how fires are prevented, red hot burning wires and even worse) The Isolations tests find this cause.
    • The PSU is bad. (if the bench jumper  tests fail below)
    • The front power button switch   wires  fell off the mobo jack.
    • The MOBO 24 or 4 pin power plugs fell off, either or both .
    • The line cord fell off (AC) or power strip between PC and WALL outlet power is dead.
    • 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? if 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?)
    Some OEM PCs the PSU fan is Mobo speed controlled. (only you OEM knows this or you learned it the hard way)
    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.

    • Warning some goofy OEMs use  non ATX PSU and have no 24pin power plug /cable at all. As seen here me changing that Silly PSU for a real PSU easy.
    • 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)   THE FAMOUS MAGIC JUMPER CLIP TEST. (the Dell funny PSU use this way)
       A paper clip  unbent (never the huge ones, only tiny nor any plastic paper clips) The connector on the left is sold at Ebay;
    • 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 PRO grade shop has a real PSU load bank tester, ( bank is huge powerful resistors or huge dynamic high wattage transistor load bank device) Many shops must make up there own. (10watts to 100 watts at least)
    • I do scope tests for ripple (chopper noise can be excessive, over 1% is bad, my DPS is 0.2% 5 times better
    • Some PSU run its own fan all the time othere run it based on temperatures alone, in  smart way or just thermal switch, (PSU benched, no PC present)
    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  (or even overheated, some can shut down all by themselves)

    The PSU works unplugged bench tested above, but not when connected to the PC fully. ? (this means the MOBO VRM's see faults these VRM's can in fact tell the PSU to shut off , like those 1.8vdc VRMs or lower voltage VRM's)
    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 !
    I mention gaming only because doing so puts a huge load on the PSU.  (and for sure dynamically)!!!
    How to pick a GPU card?
    The card must "fit  and not hit "other things, nor overload the PSU.

    The RULE of 75Watts max on the one X16 PCI express slot, must be observed.   Some cheap DELL MOBO's have 25/35watt limits on the X16 slot.
    Cards that use more than 75 watts all have what you see below, 1 or 2 AUX jack(s) for  power, and you PSU must  have and support that power cable feed.
    PC what have weak PSU or can not support AUX jacks , buy GTX1050 this card only uses less than 75watts. (even full tilt gaming or running FURMARK GPU bench mark application utility)

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

    <<<< 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 AUX can be on the end of card  so , LOOK before you buy or RTM read the manual on the card. Many cards with 2 fans are too long and hit things.
    The maker of this card wants 6 yellow wires from the card to the PSU, do not use 1 TO 2 "Y" splitter adaptors here, or you will overload the 3 wires on the PSU.  (gaming will BSOD, not just playing about in Windows. ok?)
    Better PSU's have 4 yellow wires per plug for 8 total. ( a 360watt card needs all that) The top grade PSU have  a pair of 6x2 cable plugs, for AUX. ( this ia  best design today this form)
    The 1 or 2 AUX power jacks feeds are NOT OPTIONAL EVER  Same on new GTX1060 on up.
     End GPU overloads PSU, issues.

    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. (watch out some old Dells, have 25/35watt X16 PCI-e power limits printed in words on top of MOBO next to socket it is , if is.
    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. (25PSI or less using a regulator)  The above is also a fire hazard.

    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)  (  the China CAP plague is here to read, and years effected.)
    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.  Do not play inside the PSU like I am doing, I am certified tech and even on 25,000 volt  RADAR stations, (very lethal that)

    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  EVGA  .
    • 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)

    Examples of PRIME GRADE PSU first.
    The new modern age of top quality PSU with modular jacks, seen below easy. And 60amp 12vdc signal rails too.
    Note the 3 grounds are all from a single pad on both end. (3 wires are flexible , not 1, in correct gauge equal effective gauge) EVGA makes PRIME products. (worth every penny)

    Note that again even new single RAIL 12vdc , has only 1 ground, to one point.   so the more ground you connect the less noise you get.

    My (below) huge thermaltake DPS uses these huge power feedthrough connectors far better than wires, short and NO NOISE. This is the bottom of the main PCB.
    On the right is, GROUND and 12vdc power pins huge. gigantic is true. Again a modern Single Rail PSU. DSP class ! Mine.

    And nice 105c caps. not junk. KZE Ultra Low Impedance caps.
    A wholly owned subsidiary of Nippon Chemi-Con, United Chemi-Con (UCC) is North America's largest manufacturer and supplier of aluminum electrolytic capacitors. 1970 on.

    The ATX Spec. size of the PSU is  150mm wide (5.9") by 86mm Tall (3.4") and depth of 140 to 240mm (5.5" to 9.0")  The longer PSU is called EPS sized or extended (more watts possible) 240mm is a huge 1600watt PSU.
    One of the best choices is 700watts that is only 140mm long., and with all and full protections features. OCP, SCP, OVP, UVP, OPP
    The biggest change is ATX12v  and now the newer 12vdc rail rules modified for huge currents on the +12vdc rails.
    ATX mouting holes are spec'd too. Seen bottom left in the photo below.
    The ATX PC Chassis  may have lots of extra room for a  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"(2kw?)

    To gain room for PSU's that are tad longer we see folks delete and remove the DVD cage.  (or that silly PSU clip on the rear of Dells, or buy PSU with modular connectors to gain more room.
    There are many PSU form factors the above is the classic from 1995 up and best, for a full sized desktop with endless expansion abilities.
    ATX, ITX, BTX micro-ATX, Mini-ITX and FLEX-ATX.

    A 700watt PSU will run any modern GPU card made. (even older 200watt cards run in my 790 and 3020)

    2006 Classics:
    Now the obvious,
    to any seasoned electronics  technician , knowing 2 things, about SMPS power supplies (choppers) make huge noise inside the PSU, and this must be filtered out to below 1% noise, (the goal is far less by any wise PSU maker)
    Then #2 the same tech, nows the caps get old and fail, they dry out and fail and the hotter the PSU runs the faster this happens. (CAP life times are about 20 years old ,  on a hot PSU less)
    Here is one PSU, old.  The caps c35 and C36 and L4 form a Pi filter Ω,   this filter will drop chopper noise far below the 1% spec, for noise, far more, But these CAPS dry out and fail. D22 pair are huge pulsating DC at 40kHz.(par) and the like below.
    The noise at D22 is huge ! Only LCLC filers like below can work , nothing less.
    Not withstanding the infamous Capacitor plague.
    Note the  Virtual dual +12vdc rails,  it  is ok , total current is limited to D2 max current and the real OCP show on V1 and V2 pins.  R61 and R62 form OCP over current protection.

    version 10.  8-1-2017   ( 5-19-,2019 last edit)

    Do not play inside with  the AC side of any PSU. OK? (it can be Lethal )