Ev­ery­thing you need to know about your PSU, and some things you prob­a­bly didn’t


YOUR RIG DRAWS ev­ery watt and volt it uses from one power sup­ply unit (prob­a­bly). All be­ing well, it will sit qui­etly in a cor­ner of your case and keep the magic hap­pen­ing. There are only three rea­sons to buy a new one: your ex­ist­ing one has gone bang or started to smell funny; you are about to em­bark on se­ri­ous up­grad­ing; or you’re build­ing a new rig from scratch.

Its job may be vi­tal, but the PSU is of­ten left un­til last on a com­po­nent list, using the re­main­ing bud­get after you’ve picked out the good stuff. A power sup­ply is a power sup­ply—as long as it’s big enough, you just plug it in, and it works, right?

There’s a bit more to it than that. A good one lasts a long time, stays cool, and en­ables fu­ture ex­pan­sion and sta­ble over­clock­ing. Get it wrong, and you’ll be back or­der­ing an­other, and pos­si­bly other com­po­nents, too, be­cause when a PSU goes bang, it can take other parts along with it.

The cur­rent standard for PSUs is ATX, which sur­faced in 1996, re­plac­ing the an­cient AT. The lat­est it­er­a­tion is ATX 2.31, dat­ing from 2008, which shows how leisurely the devel­op­ment of PSU stan­dards is. That’s not to say PSUs aren’t evolv­ing; the huge ap­petite for power of the graph­ics sub-sys­tem, and the world of over­clock­ing, has put great de­mands on the PSU. The lat­est ones are adding digital con­trol cir­cuits; you can even get ones with Blue­tooth, so you can run them from a phone app. So, how many watts do you re­ally need? And why do prices vary so much?

THE ELEC­TRIC­ITY FROM your wall out­let is com­pletely wrong for your PC. It de­liv­ers a 120V 60Hz al­ter­nat­ing cur­rent (tech­ni­cally it is 120V +/-5 per­cent; the me­dian value is ac­tu­ally 117V). This is great for chunky elec­tric mo­tors or mak­ing toast, but in­im­i­cal for the del­i­cate cir­cuit boards in­side your PC—noth­ing can use this un­mod­i­fied, it’s way too pow­er­ful, and ev­ery­thing wants a di­rect cur­rent in­stead. The PSU’s one and only job is to trans­form this sup­ply into tech-friendly low-volt­age DC va­ri­eties. The orig­i­nal IBM PC’s power sup­ply de­liv­ered 63.5W—things are a lit­tle more tax­ing now.

A PSU is an SMPS—a switched mode power sup­ply. It switches from full power to none and back quickly. The volt­age is reg­u­lated by ad­just­ing the switch­ing ra­tio be­tween full-on and off. The idea has been around since the 1830s, but it took mod­ern tran­sis­tors to put them into ev­ery­day con­sumer elec­tron­ics. An SMPS is smaller, lighter, and more ef­fi­cient than plain old lin­ear de­signs, which waste ex­cess power as heat. Old-timers may re­mem­ber when trans­form­ers were heavy bricks; no longer. The main draw­back is that the switch­ing in­tro­duces high-am­pli­tude, high-fre­quency in­ter­fer­ence, which needs to be fil­tered out.

There are eight main stages in a switched power sup­ply. Th­ese are: in­put fil­ter­ing, pri­mary bridge rec­ti­fier, pri­mary ca­pac­i­tors, pri­mary switch­ing tran­sis­tors, trans­form­ers, se­condary rec­ti­fiers, out­put fil­ter­ing, and fi­nally feed­back and pro­tec­tion cir­cuits. The bridge rec­ti­fier is an ar­range­ment of four diodes that only al­lows elec­tric­ity to pass one way, thus the sine wave of an AC cur­rent is turned into a saw-toothed DC one. This is then switched on and off very quickly, and fed into the main trans­form­ers to be stepped down to the re­quired volt­age. And, yes, we have skirted around some of th­ese stages, be­cause, quite frankly, you don’t need to know, and it may in­volve di­a­grams.

What ev­ery com­po­nent in a PC re­ally wants is a steady volt­age, but all the busi­ness of trans­form­ing and rec­ti­fy­ing adds noise to the sig­nal. En­ter the ca­pac­i­tors. Th­ese are pas­sive de­vices used in PSUs to smooth out the ruf­fles in a cur­rent. They are rapidly charged and dis­charged, but as they re­sist rapid changes in volt­age, the peaks and lows of the sig­nal are never reached. This turns a jagged sig­nal into a much smoother one. You’ll find them both on the pri­mary side of the PSU, deal­ing with the in­com­ing AC cur­rent, and on the se­condary side, smooth­ing the DC cur­rent from the rec­ti­fier.

The Ja­panese have some­thing of a rep­u­ta­tion here. Years of ex­tremely high pro­duc­tion stan­dards, pu­rity of ma­te­ri­als, and qual­ity con­trol have given them an en­vi­able rep­u­ta­tion. Many spec sheets proudly boast of Ja­panese 105 C rated elec­trolytic ca­pac­i­tors, the ones to look for. Good ca­pac­i­tors are ex­pen­sive, and this is one area where bud­get PSUs cut cor­ners.

A PC re­quires its juice in three strengths: 12V, 5V, and 3.3V. The main 12V is used for the pro­ces­sor and PCIe cards, as well as fans, drive mo­tors, and any­thing else that needs a de­cent amount of power. The 5V sup­ply is used for drive con­trollers, parts of the moth­er­board chipset, in­clud­ing the BIOS and CMOS, and other sub-sys­tems—USB, PCIe con­trol cir­cuitry, and such. The 3.3V rail typ­i­cally runs your RAM, some graph­ics card func­tions, and other con­trol cir­cuitry. The ma­jor­ity of the power re­quire­ment of your PC falls on the 12V sup­ply, and it’s this that needs the most at­ten­tion.

The moth­er­board also has a num­ber of VRMs—volt­age reg­u­la­tion mod­ules— to sup­ply the more del­i­cate com­po­nents. The pro­ces­sor has its own 12V sup­ply, for ex­am­ple, but you couldn’t send that through any pro­ces­sor made since the 1970s. VRMs use switch­ing tran­sis­tors to lower a DC cur­rent. They are mul­ti­phase syn­chro­nous buck con­vert­ers. Note the term “mul­ti­phase.” Each phase means adding a set of tran­sis­tors, which in­creases the sta­bil­ity of the out­put, re­duc­ing rip­ple (we’ll get to that). More phases doesn’t mean more power—that de­pends on the am­per­age rat­ing of the tran­sis­tors. More is def­i­nitely bet­ter, though.


To sup­ply the main volt­ages, the PSU has a num­ber of rails. A rail is a sep­a­rate line of con­nec­tions, which has a set mon­i­tored and lim­ited out­put, to avoid ex­ces­sive heat­ing and dan­ger­ous loads. There are 12V, 5V, and 3.3V rails, plus a small 5V standby rail, and a now vir­tu­ally un­used -12V rail kept for com­pat­i­bil­ity. 2003’s ATX 2.0 spec calls for two 12V rails: one for the pro­ces­sor, and one for ev­ery­thing else. Orig­i­nally, th­ese were rated at 20A each. This is equiv­a­lent to 240W (watts = amps x volts).

Gen­er­ally, a sin­gle 12V source was split into two rails, with the lower volt­age rails

“A PC wants a steady volt­age, but trans­form­ing and rec­ti­fy­ing adds noise to the sig­nal.”

run­ning off the 12V sup­ply, using VRMs to drop the volt­age. Build­ing fully in­de­pen­dent rails is ex­pen­sive. A few spe­cial­ist PSUs boast “real” mul­ti­ple rails—how­ever, al­most all ac­tu­ally run one and split the out­put. The high de­mands of graph­ics cards, all of which fell on a sin­gle rail, soon caused prob­lems, and the power de­mands hit the buf­fers of the ATX spec­i­fi­ca­tions. PSUs started to split out more rails to cope. Th­ese were con­fig­ured (or should have been; there were some early fail­ures) to spread the power-hun­gry PCIe loads. Each rail’s power rat­ing is its limit be­fore cir­cuit pro­tec­tors cut in. It does not mean the PSU can sup­ply that level of power across all the rails; there’s still a max­i­mum 12V it can de­liver, which may be lower. In 2007, ATX 2.3 fi­nally dropped the 20A limit, one which was in­creas­ingly be­ing breached.

En­ter the one big 12V rail de­sign. Gamers have tra­di­tion­ally liked th­ese—you don’t have to worry about spreading loads. Just con­nect ev­ery­thing, and let the hard­ware draw what it needs from a com­mon pool. Job done. Split­ting down is in­her­ently safer, though—if a part fails on a sin­gle rail PSU, it can po­ten­tially get a huge amount of power sent through it, a hefty shock, and pos­si­bly a fire. The sin­gle rail de­sign is in fa­vor th­ese days, es­pe­cially for en­thu­si­ast rigs, although the server room boys still like their safer mul­ti­ple rail de­signs. Some PSUs of­fer both, with the abil­ity to be con­fig­ured as one big rail, or mul­ti­ple lower am­per­age ones. A neat so­lu­tion.

And now back to those rip­ples. Th­ese are tiny, rapid fluc­tu­a­tions in the volt­age. They are a con­se­quence of the AC to DC con­ver­sion, a har­monic dis­tor­tion at the switch­ing fre­quency, and are ex­pressed as a de­vi­a­tion in mV (mil­li­volts). The ATX spec­i­fi­ca­tions lay down al­low­able lev­els: 120mV on the 12V sup­ply, and 50mV on the oth­ers. Ex­ces­sive rip­ple can cause dam­age to hard­ware. For­tu­nately, not the burning and smok­ing kind, but it does stress elec­trolytic ca­pac­i­tors in par­tic­u­lar, low­er­ing their life­times.

Good rip­ple sup­pres­sion means bet­ter over­clock­ing po­ten­tial. Faster clock speeds mean less time for the VRM to reach the re­quired volt­age; a rip­ple at the wrong mo­ment of a clock cy­cle can mean it never reaches the re­quired op­er­at­ing volt­age, and it be­comes “un­sta­ble.” If you are in­tent on some se­ri­ous over­clock­ing, look for a PSU with good rip­ple sup­pres­sion on the 12V rail. A lot of progress has been made here re­cently, and high-end PSUs can man­age down to 30mV; the very best boast 10mV.


De­cent PSUs are re­li­able, but like ev­ery­thing, they don’t last for ever. The main rea­son for PSU fail­ure is the fan. If this goes, your PSU rapidly over­heats un­der load. A good PSU has a good fan—it should be as good as or bet­ter than your pro­ces­sor cooler and case fans. There’s no point in a PSU with a life of 100,000 hours be­ing cooled by a sleeve-bear­ing fan with a life of 50,000. Sin­gle-fan PSUs al­ways have an ex­haust fan, suck­ing air from in­side the case, across the PSU’s heatsinks, and out the case. Look for de­signs that can run with­out a fan at low loads, known as Eco mode. Some don’t need the fan un­til loads hit 40 per­cent—well above idle.

Ca­pac­i­tors have a lim­ited life­span, too, which is af­fected by how hot they get. They can leak, which gets nasty. At this point,

you may get some ca­pac­i­tor squeal—this is gas vent­ing through a pin­hole in the cas­ing, mak­ing a whis­tle. If you can hear this, fail­ure is im­mi­nent. Less wor­ry­ing, and more likely as a source of an an­noy­ing sound, is coil whine. This is when in­duc­tion coils start to vi­brate un­der load; this varies in pitch and in­ten­sity with the cur­rent. If th­ese coils are in­side your PSU, there is not a lot you can do. They may “bed in” after a while; they may not. A good PSU will have tested and damped its coils prop­erly.

It may be tempt­ing to use an old ATX power sup­ply you hap­pen to have hang­ing around, do­ing noth­ing, in a new sys­tem—not rec­om­mended. It may tech­ni­cally fall within ATX spec­i­fi­ca­tions, but th­ese are pretty loose, and a lot has changed in the last few years, par­tic­u­larly the dis­tri­bu­tion of power re­quire­ments, which has moved off the 5V and 3.3V rails and on to the 12V one. A per­fectly good, but old, ATX sup­ply makes a poor part­ner for mod­ern rigs. If it still has a 20A 12V rail, for ex­am­ple, you’ll strug­gle if you load the PCIe sys­tem.

Power is de­liv­ered to your sys­tem through a set of ca­bles with stan­dard­ized con­nec­tors for each job. The wires are color-coded (or used to be; it ap­pears to be go­ing out of fash­ion): The +12V power line is yel­low, the +5V is red, and the +3.3V is or­ange. Time was a PSU came with a mess of ca­bles dan­gling from it, each hard­wired with a con­nec­tor. This is un­ruly, be­cause any con­nec­tors you don’t re­quire are left dan­gling about in­side, and you may have to start using adapters and split­ters to get ev­ery­thing pow­ered.


Bet­ter PSUs now have ei­ther a partly or com­pletely mod­u­lar ca­ble sys­tem. The PSU has blocks of sock­ets, and it comes with a set of sep­a­rate ca­bles, of­ten snazzy rounded, flat­tened, or braided ones. You only need to in­stall the ca­bles you need. This looks good, im­proves air­flow and is much neater.

PSUs only draw the power they are asked for, so over-spec­i­fy­ing won’t cost you, other than the ini­tial out­lay. The chances of you putting too much strain on the 5V and 3.3V rails is next to none. Th­ese com­po­nents sim­ply don’t draw much power, and any de­cent PSU al­lows more than enough. It’s the 12V side you need to match with your hard­ware. So, where is the 12V power go­ing? There are two de­vices that soak this up, and the cul­prits are the pro­ces­sor and the graph­ics card. Com­pared to th­ese, ev­ery­thing else is a light­weight.

Pro­ces­sors have a TDP fig­ure, ex­pressed in watts. This is the ther­mal de­sign power, and is the max­i­mum heat a pro­ces­sor gen­er­ates un­der a heavy load. It can be ex­ceeded in cer­tain con­di­tions, and isn’t the same as elec­tri­cal power con­sump­tion—

“If your bud­get has scope, go for a bet­ter qual­ity PSU in pref­er­ence to a big­ger one.”

how­ever, it is a pretty good in­di­ca­tion of what you will need. For ex­am­ple, a Ryzen 1700X has a TDP of 95W. Un­der very heavy test loads, it ac­tu­ally needs feed­ing about 105W. An i7-6900K has a high TDP of 140W, and needs about 148W at its ab­so­lute worst. Th­ese fig­ures are enough to run the rest of the sys­tem put to­gether. The most ex­treme desk­top chip is AMD’s Thread­rip­per, which has a TDP of 180W.

The pro­ces­sor isn’t the big one, though— that’s the graphic card. The tech­nol­ogy has re­ally bloomed over the last few years, but so has power con­sump­tion. Orig­i­nally, the PCIe slot was enough; it car­ries 75W, taken from the moth­er­board’s 12V feed. This was soon sup­ple­mented by a six-pin PCIe power con­nec­tor, which added more 12V power lines di­rect from the PSU, and 75W. A sec­ond six or eight-pin con­nec­tor added up to an­other 75 or 150W, bring­ing our to­tal to a po­ten­tial draw from one graph­ics card to 300W. This pretty much matches the rest of the sys­tem, in­clud­ing pro­ces­sor, added to­gether. Some cards are go­ing be­yond the PCIe 3.0 spec­i­fi­ca­tions by using two eight­pin PCIe con­nec­tors, for a max­i­mum of 375W. PCIe 4.0 is ex­pected to stan­dard­ize this, or some­thing sim­i­lar, shortly.

The novice mis­take is to spend your bud­get on a big wattage sup­ply that you don’t re­ally need. One matched to your com­po­nents, with a mod­er­ate amount of breath­ing room, is enough. If your bud­get has scope, go for a bet­ter qual­ity PSU in pref­er­ence to a big­ger one. Look for bet­ter rip­ple sup­pres­sion, high-qual­ity ca­pac­i­tors, fan and en­ergy ef­fi­ciency, sophisticated cir­cuit pro­tec­tion, and such, rather than just get­ting the big­gest head­line num­ber of watts. It’s about sup­ply­ing the power re­li­ably, ef­fi­ciently, and smoothly for the whole life of your rig. Get it right, and you won’t be buy­ing an­other for some time.

The $ 450 EVGA Su­per­NOVA: 1,600W of power—far too much, of course. Un­less you re­ally do want four graph­ics cards.

Cor­sair’s flag­ship AXi PSUs were the first to use digital con­trol cir­cuits, en­abling you to re­motely mon­i­tor and con­trol them.

The 6+2 PCIe 12V power con­nec­tors. Th­ese feed the ap­petites of GPUs. If your PSU has two of th­ese, and two six-pin ver­sions, it was de­signed to run two graph­ics cards.

With mod­u­lar cabling, you only in­stall the ca­ble you need. Bet­ter PSUs are fully mod­u­lar, mid-range ones are of­ten partly so; both re­duce the col­ored spaghetti look.

Of course, there are LEDs. Ther­mal­take’s $120 850W Tough­power Grand boasts a 14 RGB LED fan—plus some de­cent specs.

Cool­erMaster makes much of its fans, as you might ex­pect. This com­pact V650 comes in at about $180. Poky enough for a very de­mand­ing sin­gle graph­ics card box.

A ba­sic $ 30 PSU, here from EVGA. This still de­liv­ers 400W in to­tal, with 360W through a sin­gle 30A 12V rail.

Some nice Ja­panese elec­trolytic ca­pac­i­tors help to smooth out the cur­rent.

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