Desktop PSU (power supply unit)bench testing ,etc.
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 lightning)  Time is what causes CAP.s in side the PSU to fail at 10 years old and heat,  (both help each other fail)
I do not cover laptops (too many types exist ) but this is the TOP PSU failure besides all .batteries now bad is this power feed cable/jack
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, See how to be safe here !
But the DC side of any PSU are safe to touch,  3.3v  to 12vdc  they are.

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:  ( PC jargon?)
  • The PSU never really is turned off and safe until the AC line cord is pulled first. ( do not hot swap parts inside any PC (or BOOM happens) if this line cord is still attached to AC power jacks on the wall or AC power strips)
  • 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 fully, 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 at any of 5 classes or overload , voltage, current or excess power demands. (or overheated) the CPU also shuts off if overheated, (and some newer GPU too)

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 is complex but the South bridge chip set runs all power modes. (via OS control)
  • Hibernate mode (a deeper sleep) and the newer hybrid mode same thing, same ways.
  • 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 Southbridge 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.

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 exactly unless from . (no 2 makers of PC use the same pinouts seen here)
The PC case power button connects to at least 2 pins here, PWR/GND the switch closes and forces PWR pin to 0volt (ground voltage) The SOUTH BRIDGE chipset chip monitors this pin !
The 2 pins here to jump are PWR and GND pins and bingo the PC turns on, if not we test the PSU next.
This pin above is only one thing that can turn on any PC just this pin is button power on.
The South Bridge chip also has other power on features, wake ON features,  sleep, hibernate and hybrid modes, and the PC software can run a TaskScheduler, feature that can turn a PC  off and the BIOS can has power on by time and date too.
Power controls are complex but the Southbridge next turns on the PSU by sending 0v signal on pin 16 of the PSU (16 of 24) green wire,  if not the PSU can not turn on, unless hotwired by you.
See these controls here.

End theory of operation.

Checks Simple. ( this is how I do the tests, briefly.  DO not Hot swap parts inside any PC.
  1. Some users of PC have  AC power strip that runs many things on the desk, and if it is bad or its circuit breaker tripped, then reset that now, or test with out this strip.
  2. If all fans in the PC are dead and the DVD tray eject are dead, the PSU may be bad.  (there can be 2 or more fans the CPU fan must spin)
  3. If PSU has a smart fan it will run only if it is overheating or over 20% load, if you know this fact about your PSU (them manual on it covers this).
  4. Check if the RTC coin cell battery for dead.(this can keep a PC from turning on)
  5. If PSU internal fan is dead and ran before, as you push the  case front POWER switch to "ON",  make sure the rear switch is  set to "1" on.
  6. 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)
  7. Make sure the power switch (photo above) is wired (and didn't fall off its pin jack header) on your motherboard (MoBo).
  8.  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)
  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) AC line plugged in of coarse here.
  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, 
  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. (upgrade to a 700+ watt PSU is best)
  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 SIMPLE bench test
the PSU, we jump pins PS_ON# to any ground pin{COM} (24pin).   Jumper is pin 16 to 17.

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.    Circa 1995 relics.
This next photo set  are the PLUG views and  JACK view below: 
The Green wire is the key wire turns on the PSU "PS_ON"   , pin 16 below.  
In the future the blue wire below is deleted -12v, as PCs did not need that for years (as will be +5v and +3.3volt wires in year 2021, soon to be  a 10 pin jack)
         Modern ATX 24pins, KEYED.
Pin 17 is an input only pin and is the pin the ICH chip toggles to ground (0volts) to turn on the PSU.
Pin 8 is output pin from PSU to MoBo and ICH, and is a shared pin, any logic on this pin that  grounds this pin cause the PSU to shut off
 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.  These colors are ATX spec. too, same PSU makers are too cheap to do that, and give you  all black wires. (sadly)
Power good is sometimes called Power OK or PWR_OK.
Valid from 2000 year to 2021.
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. )

Some OEM PCs have custom PSUs, with fully non compliant ATX PSU even tiny sized PSU all non standard and a pain to source. (used on ebay yes)
I converted my Dell 3020 and 9020 to ATX form factor using  $7 adaptor cable. (sold on ebay and amazon both).
I only cover ATX PCs, no queer  non-ATX here.
The new PSU 2021 year are seen here

Connectors, plugs and jacks or receptacles:  D.C power connections all. THIS IS ONLY ATX spec here, nothing custom or magic here just SPEC.
, 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 75watts GPU  cards, The EPS is for servers not PC's. "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.  S
Each wire and pin are only good for 10amps, max MOLEX SPEC., 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. or do this
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.
A brillant PSU design some video cards have one 6 pin and one 8 pin AUX jacks both.
I call PCIe plugs AUX power cables/
The PCI-e 6/8 set of cabels (AUX) must not have any pins depopulated all have wires on every pin.
Any GPU card sold over 75watts needs 1 or 2 AUX cables. 6 or 8 pins ,1 or 2, or mixed, and are never optional if seen on the card.
If seen these must be fed lots of 12vdc power, even 20amps total.

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.
Rails or also called DC POWER BUS.
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 AC 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)
keep out and LIVE !

Some top qaulity PSU examples: 
My top of list of criteria for chosing a PSU  are:
  • Fits your PC case and not hit things, if the PSU is too long and hits the card cage it is no good for you, short of cutting out said cage. Fits the case.
  • The PCU power wattage must run everything including for sure a new 300watt GPU card, in the X16 slot.
  • Top Brand names and quality and long warranty (7 years good)  Corsair, Thermaltake,
  • New and better single RAIL 12vdc power rail  even at 60amps rated, now 700watts +
  • Enough power to run any game and add any new GPU card later for yes, faster gaming. (FPS, mania) A.K.A future proofing. (buy for today and say next years needs)
  • 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)  Digital signal processor controlled is now and the future of all PSU.
  • Full protection logic,  5 way ,over/under voltage and over current.
  • Quiet,  a very large quiet fan and only spins  above 20% load and then only as fast as need be. (yes this is good) NOISE reductions are well liked.
  • Has Japanese capacitors or the new solid kind, that last way longer that "C4, CAPS",  China Cloned cheap Crap. (or knock offs from H3LL with nasty bad no good ELECTROLYTE ! )  2010 and older mostly worry.
I do not choose any PSU by pretty looks, no cute lamps, no RGB malarkey,% spec.'s),   a higher efficient is best for cooler running and longer life span.

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/geek do not read this unless you really want to know) Most folks only want spoon fed answers, so if that is you then buy a new PSU and try that.
A real example helps most and only documented on old Intel Mobo.
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, ,do not read below this is for persons wanting to learn more about electronics and for sure power on logic !
The ICH (PCH) south bridge  chipset chip (a.k.a coined "gluelogic"  ) 
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 21  years 1999 to now, all have same pin name.

82801AA I/O Controller Hub (ICH0)  below. (YES OLD,1999  but the 3 pins for power on are still the same and work the same.)
Pwrbtn ,  PwrOK ,Pwr

This is old but has changed little.1999?

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 4+ full seconds is best to end stuck sleep modes)
  • ICHx (0 to 10)  sees this on its PWRBTN# pin, now. (from logical high 5v to logical 0v and is debounced,, this pin is M2 below.
  • ICH chip acting fast, then asserts (logic true) to the PSU-ON pin J3 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,  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.)
  • Keep in mind both the PSU and the Mobo both can signal PWROK false for many reasons, overloads top that list.
  • 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 PSU or the MoBo VRM's ,   both can shut  off the PSU,  PSU sees and overload  or the MOBO sees and overload. (the BIOS can (some) can power on or off the PC by DATES and TIME.
  • The PC can go to sleep, or hiberate in many modes. (EnergyStar Compliance, federal laws)
  •  The hard reset proceedure will reset any stuck sleep mode of any kinds, see how here.
  • Any overload PC power rails can be discovered by doing . ( the strip down test)
  • Do the hard reset to prove the mobo is not in LATCHUP  mode. (caused by ESD, lightning or hotswapping parts inside any PC)
This Great Example below of the  chip provides all Power  features seen in the POWER state logic seen here.

Newer HUBs have more pins marked SLP_Sn (where "n" is S1,S3 or S5, sleep mode command pins.

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  line to the PSU  and the PSU
turns on , and tells the ICH that power is OK.,
The 82801 pwrbtn pin has its own pull up resistor inside the chip." 9K 3K"   (means 6 to 12,000 ohms) this means to any tech, all I need to do just hot jumper the 2 pins to turn on the Mobo to test power on.
Of the users holds the Power button over 4 seconds all sleep modes are force cancelled..
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 in power settings (CP)  RTC below  is the clock chip and the RTC coin cell, that runs the southbridge (when no power is on tap)
Do not read about sleep modes as your head will explode. (one way to avoid this is  turn off sleep modes in Control Panel, + Power settings, now)
Not all PCs support all states, including Hybrid state.
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.
in situ testing: (PSU inplace but disconnected) or later just bench test it.
I just unplug all PSU internal PC colored PSU cables, to all things, 2 cables to mobo, all SATA power. (done inside the PC all this or removed an bench tested)
Lastly I  then grab the 24pin cable, the one with the green wire ,
I jumper pin 16 to 17 with a paper clip (unbent , normal sized and not painted clips just shinny a steel clip)
Next attach the AC line cord to a hot and good AC power outlet (wall power)
The PSU fan may turn on now for sure older PSU.
I lastly check each pin on the 24 pin plug for the currect voltage. With my DMM meter.(set to VOLTs on the DIAL)
If these tests pass muster (spec) then the PSU is 90% known good, 10% might fail real load tests. sure.  (end fast test first)
When someone says SPEC, ( a tech or engineer) we mean factory ATX specifcations, spec. for short. (easy no?) and we check for NOISE too.

OK all tests pass so far. (if not? a BAD PSU it is)
I put all the above things back to normal PC assembly.  (pull the AC cord and undo (reverse) all steps above dismantling. (I remantle)
If testing the new 2021+  new PSU called ATX12vo  To test this PSU ground pin1 , jump pin 1 poweron, to pin2 ground. (PSU the turns on) 10pin large connector new PSU only.

Why is my PC dead. (good monitor used here and tested on other PC)
The PC is silent the CPU  fan must spin and does not this hints at bad CPU fan or bad PSU.
The DVD drive tray does not eject , tells me 5vdc wiress are dead (RED)
The HDD can be heard to spin using a home made stethoscope. (or use kitchen towel empty tube to ear and other end to HDD casing, with PC turned on.
GPU CARD fans (if present) on the GPU most times spin full time or faster hotter but is a clue too.
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 PSU spec for overload is 1second or less that means if the overload is there full time (like shorted modules) the PSU Flat refuses to turn on. (this magic prevents that FIRE Thing and tons of smoke)
The power switch on PC usually have a LED blink codes, not just off black, on green. (blink codes are usually ,
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 read you service manual 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 of any kind just inside the PSU).
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 simple 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 PC ) The CPU fan must spin or you lost power from PSU or the PC is in DC power overload status (failed)

typical ATX PSU. with large fan (better)

This is the long story (after short above)
  • Warning some goofy OEMs use  non ATX PSU and have no 24pin power plug /cable at all, or use mini sized non standard ATX cases.  And
  • 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.
   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 a 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 others run it based on temperatures alone or above a load wattage level, in  a smart way or just thermal switch.
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 clean power and super good regulation and lower noise, (all good all that)
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)
If still failed do this.

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 )

I have load banks here in my shop that can at least do 100watts per pin.  (about) a good resistor to have is just one wanted, is  1ohm 100watt resistor. (12/1=12amps) (5/1=5amps) or have 2 resistors use 0.5 ohms for 5v rails.  This is not expensive at all. $5
Connect this to each rail to ground 1 by 1  to see if PSU stays on.  one at a time. is  very good level 2 test of any PSU. Called load bank tested. 16awg wire.

SHOW  and TELL GUTS time , do not do this at home, until you read this warning.
This is one old tiny wimpy power 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)
The funny glus stuff seen below is to prevent singing noise the chopper has harmonics some folks and dogs can hear (better PSU makers do this)

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 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, 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.  Those wires are like 6AWG gage wire.  NOW WE ARE TALKING !@
The new modern age of top quality PSU with modular jacks, seen below easy. And new 60amp 12vdc single rail  too.  GO single 12vdc RAIL it is better for sure gaming GPU power hog cards.
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)
EVGA brand prime. (one of best made here) This is only my opinion and nobody pays me to type or hype ever.

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 ! My Thermaltake DSP here,  and with huge strong power pads (feeds)

And nice 105c caps. not junk. KZE Ultra Low Impedance caps.  This is one fine PSU.  (spend over $50 and win) $100 is more par.
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?)
Make sure deep does not hit your card cage or DVD drive, use a ruler. 140 means (140/25.4= inches = 5.5"inches)
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.

Never buy a BTX PC. they are trash, with backwards mobo and nothing fits , upgrades impossible.

A 700watt PSU will run any modern GPU card made.   I also have a PC with the fine Corsair CX750 , semi modular.
From Newegg?

Real PSU Examples  Classic real schematics to learn how they work: ( near year 1995 to 2006)  ( See my slide show  of old PSU schematics here)

(Back to the future)? Will they (EPA) ever ever make up their minds? Seems not!  What's next? liquid Nitrogen tanks and $5000 PSU with Super conductors?
Warning 12vdc only PSU are becoming law,(calif. derps (Nazi) panic so easy? they started this
What happens is the PSU gets cheaper (2xVRM lost) way way less copper cables and mobo gets more expensive, with 2 more VRMs added back to the MoBo.     (VRM=  voltage regulator modules)
The Energy Nazi panic and save tiny I x R (=Power) losses on those 3.3v/5v now gone cable wires,  the savings are miscule a moden PC uses only tiny currents on those to rail. (unlike year 2000 PC's) . They split hairs.
Breaking news, the new PSU will have only 10 pins for new PSU's 2021,  The new ATX12Vo spec (say Voltage only!)
Legacy PSU will be sold for decades and or with 12vd to 3/5vdc converters, to use new PSU on OLD PCs. 
Some new mobo will be only m.2 and not support any kind of (like me x2) 3.5"inch 10TB HDD.  zero support. (no 5vdc SATA power cables)
MATH 101 and why Energy Star federal and Calif, Nazi's JOKERS. (I have fancy tools here, scopes, and AC and DC mag Probes and DC amp inductive meter, in my lab so what does the PC do?

USB needs 5vdc and up to 1amp per port, so 6 ports can be 6amps just those, for sure charging  everyones cell phones on one PC. (or not using over the counter charge stations)
There are chips on the mobo that use 5vdc and 1.8v and sound chips, and SSD SATA drives run on 5vdc alone. (I measured mine. Red 5v wire then Orange 3.3v wire bundles using my inductive meters.)
Facts + Math (ohms laws) = TRUTH , EPA MYTH BUSTED: (EnergyStar jokers) This began in California so lay blame there and for Fed knee jerks same. (derpish)

My DELL XPS8600 

I measured my 3.3v and 5vdc currents using my lab instruments (and 18AWG wire is .013ohm for 2 feet divided by 4 wires is 0.00325 ohms as a set. Lets call that .003ohms (omg lets go to war on that)
Lets do a table. for clarity.  (current is the bundle currents, be that Red or Orange)
Rail  name.
AWG Ohms/foot -2ft
4 wires in parallel
current flow
P= I x R (power loss)
LOSS  (cable loss total 2ft.) 1 LED is  0.02watts
5vdc  RED set
#18- .013
0.00325 ohms
P=.0625 x .00325
.0002 or  0.2 miliwatts (1/100th of 1 LED.)
3.3vdc  Orange set
#18- .013
0.00325  "
P= 1x1 x .00325
 .003watts 4 miliwatts  1/6th of 1 LED wasted. power

The only value or real gain is less wire, less copper waste, that is it. (EPA nazi's at work again doing ZERO).

Intel says this on ATX12vo
To test this PSU ground pin , jump pin 1 poweron, to pin2 ground.
PSU makers will love this mostly, cheaper cost to make, less failures , less costs on warranty repairs, less wire, all a huge win but lost profits selling LESS HARDWARE. (think about  that) (my guess is they will sell more features to make up the loss profits)

version 11.  8-1-2017   (9-1-,2020 last edit added new aTX12vo
  Do not play inside with  the AC inside of any PSU Casing. OK? ( Lethal means NO SECOND CHANCE) Death is fast.   I like CORSAIR From Newegg?