Introduction: Take a Look Inside a ATX Computer Power Supply

Picture of Take a Look Inside a ATX Computer Power Supply

Have you ever wanted to see what's inside a computer power supply?
Well here's your chance.

P.S. this is my first Instructable so I hope it's good enough

Caution: Don't do this at home unless you know about the risk of shocking yourself even after turning it off! If you want to take that risk, Always discharge those 1 or 2 large mains filtering capacitors by jumpering the green wire in the 20 or 24 pin ATX plug to a black wire in the same plug while the PSU is unplugged. You should hear a click. Remove the jumper and disassemble.

You might want some rubber gloves to keep your hands from getting electrified

Don't forget to rate if you like it and comment, I accept constructive criticism

Step 1: A Look Inside the Power Supply

Picture of A Look Inside the Power Supply

This particular power supply has a un-switched output meaning the output always has voltage. the rectifier down the bottom of the psu uses 4 diodes to change AC to DC. On the primary side of the board, we see the X-Y caps dominate the space. They serve as the mains filters. Lower down, just to the right of the torroid, are some yellow rectangles. These are caps designed to help the power supply handle surges and inrush current. To the right of the fuse is another inductor (the white thing) used for more mains filtering. It may actually be an isolation transformer. In the middle, we see the heatsinks. attached to them are the mosfets, used to raise the frequency of the AC. This allows the transformers to be smaller, and results in less ripple to be filtered out. On the upper right is the voltage regulating parts. On the heatsinks are more Vregs. On the upper right is the secondary side, with its filter capacitors and inductor.

Step 2: The Output Cables

Picture of The Output Cables

The output cables are the wires that run from the psu to the motherboard, harddrives, cd drives, floppy disk drives and sometimes to your graphics processing unit (GPU). If you wanted to use the power supply outside of the computer for testing temporarily like watercooling systems and peltiers or permanent things like bench-top power supplies, you would need to jumper the green (signal wire, on/off) to the ground in the main power connector to boot the power supply. When the power supply is plugged in, the purple wire in the motherboard connector is LIVE (5v). This is the motherboards standby power source.

Thanks for looking at my first instructable.

Thanks to tech-king and Goodhart for being a collaborator and helping me to update our instructable.


ErwinB19 (author)2017-11-22

Everybody else just warns about the capacitators, but here we have clear, easy instructions how to neutralize the danger, eg. discharge them safely. Bravo!

punkatsub (author)2008-09-04

I have opened many power supplys, well 2, but i have never gotten shocked, i don't know why thought, lol

Yonatan24 (author)punkatsub2015-12-15

That's because the big capacitors were discharged...

slimguy379 (author)punkatsub2008-09-05

i know, its called being lucky thats what...

I've got about 10-20 old ATX's lying around in a cupboard somewhere

Gelfling6 (author)thermoelectric2013-05-27

With great number of power supplies, comes great possibilities. (with apologies to Stan Lee, LOL!)

Gjdj3 (author)slimguy3792008-09-06

Haha, or just being good.

slimguy379 (author)Gjdj32008-09-06

either way hes should be thankful.

punkatsub (author)slimguy3792008-09-10

it's only 250 watts, (whats that purple thing?? ooooo.) *sizzzzle.

Gelfling6 (author)punkatsub2013-05-27

Doesn't matter what wattage.. It's the storage capacity & voltage potential of the filter caps. Granted, they're only 200-400uF, but the working voltage (anywhere from 200 to 400WV. That WV is short for Working Volts!) Now, they may not be charged to the full working voltage, but they still kick you with only 110V. Now, usually, it's not the outer case of the caps, they're usually insulated, but sometimes the regulators on the first heatsink carry some of the potential in DC voltage. (the one closest to the filter caps) Hence the warning on some supplies, that the heatsink may cause shock. 2nd rule of thumb.. If you don't touch it with a volt meter first, Don't touch it with your bare fingers! (and those big high voltage test wands is a bit overkill.)

thermoelectric (author)punkatsub2008-09-10


thermoelectric (author)Gjdj32008-09-06


Gelfling6 (author)punkatsub2013-05-27

Your luck, may have to do with two 100K (brown-black-yellow) resistors across the contacts of the two mains filter capacitors. (and not touching anything while the supply is plugged in.) These are intended to bleed off the charge once you remove the AC (either unplug or if your supply has one, a 'Master" on/off switch near the power input).. Usually, it's still a good rule of thumb (and keeping your thumbs & other fingers) of letting the supply sit for a minute totally disconnected.

Gelfling6 (author)punkatsub2013-05-07

If you opened them after they've sat for a few minutes (one minute is more than enough.) that means a set of resistors, across the pins of the filter caps, are doing their job, and bleeding down the charge. That doesn't mean they can't fail. I've seen a few supplies, with 100K 1/2-watt resistors soldered to the board across both 200uF/240WV caps. they pretty much act as a slow-rate short. But.... I've seen this a few times.. I converted two supplies.. One, has 2 switches.. the one coming from the mains lines, and the one for the Pwr-On wire to GND. IF I turned off the Pwr-On switch, but left the mains switch on, the caps would still be charged. (because they're still powering the standby +5V Logic control voltage. (which gets toggled when we press the power button on a PC, or something makes the PC wake-up.) But, If I THEN turn off the mains switch, within 4 seconds, I flip the Pwr-On switch back on, there will still be trace voltage enough to power whatever is connected. (cooling fan, project, ect.) which proves, it takes time for that mains voltage to bleed off those filter caps.

thermoelectric (author)punkatsub2008-09-04

remember to rate if u like

thermoelectric (author)punkatsub2008-09-04

I haven't either and i have opened and closed about 30-40 (a couple multiple times)

captaincutback (author)2015-09-05

Thanks for explaining how to enable the power supply on the bench

russ_hensel (author)2015-01-09

Just a note to let you know I have added this instructable to the collection:
Encyclopedia of ATX to Bench Power Supply Conversion
Take a look at about 70 different approaches to this project.

Gelfling6 (author)2013-05-07

I've seen plenty of warning labels, which even say the heatsinks are potentially electrified, or dangerous. As Lee said, You probably got clobbered with 150-170V-DC.. Be glad it was to only one hand.. If it was from one hand through the other, you would probably have not read his reply.. that's plenty enough to stop the heart.. Most Xenon strobe lights, boost voltage to about 200-230VDC, and it takes a 2KV external exciter charge to set off the flash through the xenon gas. I'm not criticizing you, I've done it myself.. Dismantled a digital camera, which had a problem.. Guess who placed his thumb right across both pins of the flash capacitor? the mains filter caps, usually have a working voltage range from 200 to 300V. Remember, I I mentioned before about it stopping the heart? Ever watch one of those medical drama shows on TV, where they say charging to 300? they're actually charging a capacitor to close to 700-volts, but it only takes about 50 to knock you on you keester. so 300-Joules, (which the defibrillator is being charged) is about 700-Volts +/- (I still can't find any direct conversion between Joules to Volts, but usually defibrillators have up to a 1000WV capacitor... 'As Stan Lee would say, 'Nuff Said!'.) You're still messing with 150-170.. (unless you're in another country other than the USA, which might be 230-260V line potential. Ever seen that little switch above the line plug? Yep!) You're still looking at two little canisters with enough potential to kill, if not seriously injure.

skaar (author)2010-12-28

so, theoretically, if i desolder the rectifier, add some heavy wire, block up the ac-in with a heavy binding post, it should be just a dc-dc supply. what makes me wonder, is if anything uses the ac as some kind of timer or clock signal, i'd hate to completely ruin a supply cause i don't destroy it in the exact right way.

i may have an instructable in this... i have the rest of the system figured, the 4 truck batteries will be a killer, just getting them home on the bus.

thermoelectric (author)skaar2010-12-28

Yep. If you bypassed the rectifier, you'd have just a DC-DC converter. Probably better to buy one, though, as (depending on where you are), you'll need something akin to 170VDC, or somewhat around 300VDC. (that's what the AC would've put into the PSU after it was rectified.).

*does some math* If you had a PSU wanting 170VDC, you'd need a bit over 14 12V batteries. I could imagine how fun that would be to take home on the bus.

Good luck!

skaar (author)thermoelectric2011-01-02

i don't think so, doesn't make sense, the transformers, to lower the voltage, likely aren't after the rectifiers, cause transformers are usually piss poor at lowering the voltage to something the motherboard can use. if the fets burned open, it'd be a full 125v to the board, or the person holding the wires, so it's not likely the transformers are chokes. which means, there's gotta be transformers before the rectifiers to lower the voltage, making it more manageable with regular logic chips n stuff. the rectifier chip also looks like a low voltage device(same old fashioned crappy ones that rat shack has sold for 30 years).

so, there has to be rectified dc at some low voltage, otherwise the switching section would only be on a few percent of the time. i figure it's likely to be at most 15vrms into the rectifier.

course, if it really is designed to switch rectified line voltage, that'd be nutty...

thermoelectric (author)skaar2011-01-07

Yeah, but this type of power supply is a switch mode power supply, which means the mains is first rectified, then switched at a high frequency to drive the transformers, which steps the voltage down to something the motherboard can use. (higher frequency reduces the size of the transformer, and makes it more efficient, generally)

There is rectified DC before and after the transformers.

'Tis designed to switch line voltages, if I can find a power supply, I'll grab the part numbers on the switching devices, they'll more than likely be rated for 400V or so.

skaar (author)thermoelectric2011-02-07

oh yeah, that makes more sense, though, if the switching increases the frequency... dc could be inverted to ac high voltage, or, the second rectifier stage could be bypassed, and feed dc into it there...

Yep, the MOSFET used in this power supply is rated for 800V at 7A continuous..

It's part number is 2SK2765, if you want to check it out.

thermoelectric (author)2009-10-14

Was it powered on? I've done that before, but never to the extent of ablue finger :wink:

Usually the heatsinks aren't insulated from the transistors/diodes, soyou might have gotten a low voltave shock or a high voltage shock at whoknows what frequency.

Mario1 (author)thermoelectric2009-12-31

Well... it was powered on ... :D
I touched the heatsink, yelled for like 4 seconds smoke came on one of the capacitors and the PSU died...
My heart started bumping like a 12" subwoofer i didn't felt my finger and i started feeling immortal (like when you try amphetamine for the 1'st time...).

ChewyCoyote (author)Mario12010-01-07

Puts hair on your chest, after you've fried it off that is

thermoelectric (author)Mario12010-01-02

Haha, Try not to do that again, I doubt it is too good for you, lol.

Hycro (author)thermoelectric2009-11-18

I have an old switching PSU (non-ATX, it's the kind where the power switch on the front of the computer shut off the mains power to the supply) and on the heatsinks inside, it specifically says that they are NOT isolated, and that touching one and the chassis, or touching two of them, will give you a very nasty if not deadly shock...

thermoelectric (author)Hycro2009-11-18

Hehe, I've done that before, It is usually the earth or ground or something, because it has ceased to kill me, yet.

Hycro (author)thermoelectric2009-11-18

Yea, I think you can short it to the earth ground...that's how ground fault interrupters work...*I think* 'cause I've accidentally connected a live wire to the ground wire, and it tripped the GFI on our outdoor outlet...

thermoelectric (author)Hycro2009-11-18

Yeah, Not in power supplies, but on outlets, if any current (~20mA) from one of the live conductors leaks to ground, or earth, it trips the GFI.

Hycro (author)thermoelectric2009-11-18

Oh, ok. But even still, in the power supply, one like an ATX supply, you can get shocked when you touch a live heatsink and the grounded (as in through the ground prong on the plug) chassis...I've had it happen, though every electrical shock I've had that's been under 200V hasn't really been more than a tingling "buzz" feeling...

thermoelectric (author)Hycro2009-11-18


Lee Wilkerson (author)2009-12-27

You fellows have no sense of adventure. I find that electrifying!
But seriously, those heatsinks of which you speak have voltage in the range of 150 - 170 VDC.

Not always, If you live in a place that uses 240V, you could be getting whacked with up to 300VDC, lol. Good to keep your heart in good shape XD.

jacobclinester (author)2009-04-28

i found out that on a AT power supply with a switch you should never mix the prongs around on the switch because i did and it flipped the breaker to the house when i turned it on

That means that AC outlet is wired correctly! B)

Which is usually a good thing, lol.

Oh, Yeah. That would have happened because you would have had the prongs from the powerpoint connected to either side of the switch and pressing the switch would have short-circuited the whole circuit....

bioerror (author)2009-01-30

Here's a tip!!! If you have a PSU that does not work anymore, why not take the circuit board out and de-solder the componants to use for other projects. Also you can use the left over case as a project box (yes its metal but a metal cutting bit for a dremel will be useful to cut holes in it for plugs and such.

mr. clean (author)bioerror2009-10-12

DUDE thats exactly what i did with mine i used the box to make a thermoelectric heater/cooler with a peltier unit i think ill post a slide show of what it looks like, unfortunately i forgot about making an ible for it :(

thermoelectric (author)bioerror2009-04-19

And I use the other parts for a lot of other things too...

thermoelectric (author)bioerror2009-01-30

Yeah, You can do that, I mostly use the capacitors, I've got a 1000uF bank from only computer PSU caps....

ercle (author)2009-06-16

i shocked myself on one of thoe things once :(

thermoelectric (author)ercle2009-06-16

Ouch, I've been lucky enough to have ones that are discharged...

Hydroelectric (author)2009-06-13

Good Job Thermoelectric. I like the way you go indepth with the explaining about what to do with it. I've looked at your other instructables and have also liked the way you've done them. I would like to see a new instructable soon.

joinaqd (author)2009-01-29

nice instructable...This question is nothing related to this instructable,but any1 over here knows if its possible to connect a transistor to a transformer to run it with DC electricity?

thermoelectric (author)joinaqd2009-01-29

Yeah, you can, You might want to contact 11010010110 here he can help...

About This Instructable




Bio: I am a high school student in Cairns, Queensland. Most of the time I am either at school, sleeping, doing stuff on my computer, making ... More »
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