DIY iMac G5 Power Supply Atx external

Hey this is a story about an iMac with a 250 watt powersupply. But now has a 500 watt one that supports sleep mode.

More links to come.

AT to ATX converter is what I used here. There are other ATX-ATX extension cables as well.

This cable makes it so you can edit the power supply rails (or groups of wires) without ruining the power supply’s.

Here is sort of a preliminary diagram that I made to guide my efforts. I might have made a few adjustments that went unnoticed, but as far as I can tell, this is exactly how the 20″ G5 iMac is re-engineered. It is working as generally stable. The next goal is to get a DC-DC upconverter that as I understand it will take say a 12V 3 Amp line and up voltage to 24V 1.5 Amps or so.


53 thoughts on “DIY iMac G5 Power Supply Atx external

  1. Cool. I’ve been trying to do this without success so far, though I’m not sure if I have a different Motherboard problem. Did you take the extra +12V from the other 4 pin connectors? Do those grounds have to be hooked up anywhere? Thanks for all your help.

    • A ground is a ground no matter what, grab one from anywhere that’s not being used. The 12V can be taken from any wire that is yellow generally. The big trick (and maybe your not this far yet) is getting a 24 Volt line (for the LCD inverter, Firewire) from an ATX power supply. What we are looking at is getting an additional 12V->24V 3 amp up-converter. This might bring the total cost in parts to 30 $. There is a -12 V line that some one might try to use with the +12 V line to achieve the 24 Volt difference, THIS IS NOT RECOMMENDED for the backlight inverter. Our G5 20″ prototype machine is really solid right now, except without the voltage up-converter the backlight stutters ever so slightly. 17″ models seem to do ok with the 12V, though would be still putting excessive strain on the inverter board.

      Edit: On grounding, It is good to instead use grounding that is on the same rail as what is coming from the power supply. If you grab a 12V wire, it’s good to utilize it’s ground wire instead. The iMac has a very odd ATX style pinout, and further research is needed for proper grounding use. Online people are suggesting that it “works” to grab a 12V line and bring it in to the connector, though possibly improper.

      • Thanks! I got mine up and running. It’s a 17″ power pc model imac g5 and everything seems to be running fine with 12V connected to the 24V pin. Not really sure what problems I was having before. I was using an extender cable and maybe just had a bad connection happening. I took that out of the equation and it started up.

        The 24V pin (really only 12V) is using a ground from the same rail (going into 11 & 22 on the mac). The ATX 20 pin cable remains the same for the most part. Pulled two 12V wires from other rails to fill in the -12V (blue) and missing pin. Also, pin 11 on the ATX 20 pin cable had two orange wires. A little unclear about this, but found that maybe it is a sense voltage (?). I swapped this out for one of the other single orange 3.3V wires.

  2. I was able to get my 17″ iMac G5 up and running using your diagram. I just used a 20-24 pin ATX adapter (wound up actually using two so I could disconnect it outside of the iMac) and it took no time at all.


  3. I just hooked up a PC 200watt PSU to my 20″ pre-isight power pc model imac g5. the lcd is not working but I am using an external monitor and that is working fine. I’m going to try a bigger power supply. (500watt). not sure everything is wired up correctly as well. used your diagram and the machine boots which is great. I’m thinking that the bigger PSU my have more grunt. Also have to check over how I wired the ATX extender cable.

    Had a few teething problems getting the machine to boot. The machine had sat idle for 10 months and had had the HDD removed. Once connected The machine would chime and then nothing. Did a PMU reset. Took out the ram (2* 512mg) and reinserted each one. REPLACED THE BATTERY (Cr2302) –this last thing seemed to get the machine over the line , that is, to boot. Though it could have been a combination of the other things I did.

    The 3 green LEDs light up.

    took very little time to do the wiring. – aword of caution to others – don’t try this when you have been drinking. the wiring diagram was great. which PIN is for the 24v for the inverter?

    Still don’t have a SATA HDD (I only have IDE’s) so have been using a boot utility start up disk to see what is happening. Ordered a SATA to IDE converter. Will buy a 2gb SATA HDD once I have the machine more functional.

    Reinserting the RAM was difficult as the it needed a fair amount of convincing to get it to sit in the slot correctly. (had quite a few 3-beeps). The trick was to insert the RAM from the bottom first (and close the “latch” thing) and then push the top in (and close the “latch” thing) and then give a further push so that you can’t see any of the pins.

    Overall I’m stoked.

    I’m have 2 other macs. A G4 400mhz and a Emac 700mhz. pretty lo-fi. I don’t game, I just use them for uni work. They do the job. I’ve had the G4 as my regular comp for 8 years. –it’s sturdy and stable.

    • Wow, I like the enthusiasm. Drinking…as for that, I usually stick to Tea or Coke/Vault. My friends keep telling me that I’ll find my taste for beer, but still hasn’t happened at 29, probably never will. I do enjoy a guinness from time to time (yearly).

      As for the 24Volt line, the hope is that your new Power Supply (I used a 400watt I believe) will be able to handle the extra pull on the line, and that the inverter will work alright making the 20″ backlight burn bright, perhaps with some minor dimming (5 / hr or less). If you can section off a rail on the PowerSupply (as I understand it, ha) use that to power the 24V line. (so, yeah, your using a 12V line to power the 24V inverter–which seems to work as long as you keep your iMac cooled and dust-free, brightness not on full)

      • I threw in a 2TG HDD and have been transferring stuff from my old Emac.

        The screen is not working and still use an external monitor. the Imac is face down on my desk currently with its back off.

        The Imac was overheating so I placed a huge semi-industrial fan to blow on the back and that seemed to cool it down enough to work. though it’s annoying having that roar whilst using the machine.

        I wondered if the overheating may have been attributed to there being no HDD temp sensor and have just received one I ordered off ebay.

        I’m going to install that and reassemble the machine.

        The screen is not working with just 12 volts. I was going to hack in a 18volt ac adapter on the 24v to see if that would help. I tried a bigger power supply but I think it may have been faulty as nothing happened.


        • found the problem with the overheating -hadn’t reconnected Fan1. bought a second IMAC that had graphic chip issues- it booted and the screen was garbled. hooked up that powersupply to my Imac and it worked. put the back cover on and went to power-on and nothing happened. trickle power coming through (LED #1 is green). put the PSU back in the 2nd Imac. nothing. just LED#1. So somehow between shutting down my Imac and putting the back cover on the powersupply died. fustration. The second Imac does sort of work with the adjusted PSU (3 green LEDs) though the external monitor nor built in LCD work. have now located a more powerful PSU (freebie) and will hook that up to the my Imac. hopefully that will kick the screen in to life. which would be nice. specs are 2ghz, 2g ram,2 TB HDD. 20″ monitor (I’m going with a “2” theme here.

        • With what you said on your next post,

          Have you tried a new inverter for the screen ? It’s got a longer / squarish grey plastic around it but maybe your original issue.

          With the ATX Power Supply, the screen should at least light. Double check the +/- are right for the LCD Inverter (the 24V/ground) and that they are powered by the same rail. Good to just test it out on the Processor power – wire.

  4. imac G5 17″ – bloc d’alimentation – Netzteil – power supply

    Thank you very much. With a simple ATX power supply (30 $), I can repair my 2005 imac G5 for my children. A really great solution. Better solution than replacing the bad caps (too risky).


  5. I using a 300 watt psu on my imac g5 20″ and got every thing working except the screen , I 12 v going to it. Can any body tell if I move to 500 watt psu with the 12v to the screen port will it work? Or tell me of any other solution? Or how to wire 24 v to it? please?

  6. Have you found a solution for a 20″ isight G5 that has a different PSU and dc/dc board? The diagram above was great for a Gen 1 machine but not applicable to the Gen 3. Please let me know.



  7. I just did this on my 20″ iMac G5 at work. It works perfectly, and my boss thinks I’m a genius. I ordered a Powerbook/iBook G4 charger from Amazon to use for the 24v supply. Note: get the highest amperage you can stand to afford; my 2.65Amp/65W is running warmer than I’d like. I’ve taped it to the imac’s stand to use it as a heat sink.

  8. I have the same 20″ isight model and i have taken a different approach, of using the 5 pin power supply output instead of the 20 pin connector. The power supply board says 12v, so putting a combination of yellow black and red atx ps outputs into the gray black and brown mac wires should do the trick, as long as it is the ps that is the problem. Problem is that a reference document ( shows:
    Grey: +12V
    Grey: +12V
    Black: ground
    Black: ground
    Brown: PFW (battery powered signal between +3 and +6.5 volt to switch power supply on)
    But in actuality the mac uses 2 black, 2 brown, and one gray
    I hooked up the 2 brown to 12v black to gnd and the gray to 5v
    The fans come on but no bong, no picture, so im not sure if im wrong on my voltages or something else is wrong with the unit.

    • What do the caps look like on the iMac’s motherboard? (bulging or flat on top?)

      • The caps on the motherboard look fine, but the PS output goes into the inverter board assembly, so there may be something wrong there. Before i toss the PS board I guess I should put a voltmeter to it, but the fact that I bypass it and something happens leads me to doubt its condition. Ill probably sell this one off for parts, and move on to looking at the 4 G5 1.9 imacs i have, where swapping parts will probably yield at least one good iSight machine. Speaking of iSight, I removed the iSight unit and mounted it on top of my conventional monitor and i works great, if a little slow.

  9. Just wanted to leave this tip here for those who may not have a Molex or ATX pin extractor and who (like me) didn’t want to spend upwards of $20 on one.

    I was able to use a single staple to release the prongs inside each pin and remove them from the plastic connector housing. Simply flatten a staple, then bend it in half. A staple is strong enough to handle a little pressure without bending / breaking, but thin enough to fit both ends in the pin housing and release both prongs simultaneously. I was able to remove four pins in about thirty seconds by simply inserting the staple and gently tapping it.

    I hope this helps someone who didn’t want to spend money on a tool to get this job done!

    Great blog and tips, thanks for sharing!

    • Awesome tip, this beats cutting / resoldering wires. Great tip and thanks for the big blog support! 😛

    • What is unclear 🙂 ?

      Are you curious about which power supply adapter to get ?

      Are you curious about which wires to change around?

  10. Great tutorial by the way!! I have a 20″ iMac G5 1.8 and have just completed all the steps necessary for the tax conversion. I hacked of the connector for the old apple psu and followed your instructions. I now have the problem of no picture. Everything else is perfect. Any ideas what I can do. I am a noob to all this mind 🙂



    • 1. No picture or just no backlight ?
      2. Additionally are you using an iMac power supply or a PC Power Supply ? (This tutorial is for a PC Power Supply for use on an iMac, which is generally better for the heat inside an iMac G5 !)

      If you can shine a flashlight through and see the menu bar / the dock, then it might be a case of the inverter not supporting the configuration. This causes the backlight to go out.

      • Hi

        Thanks for replying. I am using an old atx and followed your excellent tutorial to the Letter. You can see desktop by shinning a torch on there. I will be connecting up an external monitor as soon as I receive the mini avg cable. Not sure if it is because the inverter is not getting the 24v it needs or if the backlight has gone. Any advice?

        • Ok. External monitor works great. Hooked up to tv and everything there is fine. Leads me to believe that it is the inverter board. I have replaced the inverter board and now on start up it flashes white once (very quickly) then back to black. Torch test shows desktop but back to square one again 😦 could this be a light issue with LCD? Or does it need the 24v?

        • i was thinking of getting a laptop 24v power brick and splicing that in for the LCD back light. the 24v and Ground are the pins on the end of the plug.

          This would mean 2 power supplies but they would both be small.

          My regular G5 (see above -Nov 2010) just died with 3 beeps on start up (ram issue).

          My back up G5 (which needs the GPU reballing) takes 5 minutes to boot but then is fine except slightly garbled graphics. I might try the power brick slice as soon as my spare power power adapter cable thing arrives.

          I wondered if it is low wattage that fails to fire up the inverter? my current set up only uses a 200watt psu. would a 500+watt psu give it that extra power? hmm.

        • Hi altomic

          I was considering splicing my current PowerBook brick to see if that works. I am using a 230v psu at present don’t fancy having to purchase anything else lol. Could just buy one that works lol. Will let you know how I get on!!

  11. Hello all
    Great blog, I came across it because I have a 17 inch imac lying dead on the garage for 3 years, and I’m willing to give your tips a try.
    Our local reseller wanted to replace motherboard and PSU (would cost 100’s) and having that little LED lit on the motherboard, made me think of a PSU problem only,
    You’ve done an excelent job, and I’ll post the outcome of my try.

  12. Thanks for this post! I pulled out a power supply from a Dell Optiplex that had a harddrive die on me…everything seems to work…but no monitor. I know it comes up because I hit the volume buttons and it makes the noise! I have the g5 PowerPC 20″.

    I may just go ahead and get the external monitor adapter. Anyone happen to find another way to the the 24v to the monitor?

    Thanks again for the post!

  13. I have an iMac G5 20″ iSight (EMC: 2082) I am trying to replace it using an ATX power supply. The power supply for the iMac had 2 brown, 2 black, and 1 gray. I tried what Rick did with the 2 brown going to 12V, 2 Black going to ground, and the Gray going to 5V but this did not work for me. I am not getting any LEDs turning on on the logic board and am not sure what to do at this point. I am not sure if the logic board is functional or not. How could I possibly test it? I am fairly good with electronics and have been fixing PC’s for about 10 years. This is the first iMac that I have tried to repair but so far with no luck. I have tried finding articles that are related to this particular model but so far with no luck.

    • Hi palak

      I also have a IMac G5 20″ i also did the same wiring with a ATX 20pin connector you did
      +12V yellow, brown
      +12V yellow, brown
      GND Black, black
      GND Black, black
      +5V Red. gray

      But the same here it boots up for a second with the sound and no screen then it shuts down ?

      Who can help, Greetings from the Netherlands

  14. Just wanted to say thanks vm to supercollidercomputers.

    Was able to get my 20 inch iMac G5 back running using this method.

    I bought a cheap CIT 550 watt atx psu from amazon and used exactly the method here including the 12V for the screen.

    No flickering so far – only been a couple of hours though.

    if I have a problem with screen flicker, then i’ll try an old dell laptop psu i found in the loft which is 20V. I’ll just hack the connector off of that and solder it into the ATX cable. Feel pretty sure that’ll be close enough with only 4V of difference, since it seems to work well enough with 12V difference right now!

    Is there any chance of damaging the ATX psu or the screen inverter by running it from the 12V line – i’d appreciate any thoughts.


  15. I have an iMac G5 20”. The original power supply is dead! I’m trying to get it work with an ATX power supply and did as discribed above, but when I connect it led# 1 lights and when presing the power button in the mob led# 2 lights and then goes off. What have I done wrong? Maybe I haven’t connected the wires right. Please any comments to help me

    • Hi there! I did some corrections on the ATX power supply connections, took a dell laptop adapter with 24 volt 2.2 amp and attached to the ATX power supply (2in1) and finally to my very surprise, it works perfectly without screen flickering. Now I need to install the hard disk and finally the mac os x. Thank you to everyone!!!

    • Various things can go wrong with these iMacs. The motherboard chips can become unsoldered from the board, the power supply can be damaged, the caps on the motherboard need to be replaced. An iMac with more ‘ghosts’ involved is more likely with complications. Very important to recheck your work !

    • I have not yet ! There hasn’t been a lot of demand lately to improve this idea. As G5’s get slower and slower, the first gen iMac Intel machines become more delectable ! Have you been exploring a bunch ?

      • no I never needed to as it worked fine with 12V. The PSU has just given up after 3 years and so I’m on the lookout for a new one. PD happy with the extra 3 years Ive got so far – so thanks again for the OP

  16. my second imac has become quite unstable following the hot air gun GPU reball.

    after reballing I didn’t screw everything back together e.g. gpu heatsink to the gpu. So sometimes while watching movies the machine will freeze or kernel panic. it may be that I need to screw the gpu heatsink to the gpu as it was out of the factory.

    I just got my dumpster found dell dimension 3100 hackintoshed. running 10.5.6 iDeneb. hooked up my 2tb from the Imac as a second HDD and it reads!!! I was a little concerned that as the hackintosh HDD uses MBR formating whilst the Imac HDD was GUID that somehow it wouldn’t work. but it was fine

    anyway, with a seemingly stable mac/hackintosh it gives me an opportunity to experiment more with the Imac and using a 2nd power source to fire up the screen of the Imac. similar to what Helpneeded did.

  17. I added the wires to a 485 volt power supply as per the above diagram and led#1 is solid and led#2 simply flashs as I try a power up, does anyone have any suggestions?

  18. Update have power on and off no sound no screen I am assuming its the 24v thing, was wondering if anyone had a solution/suggestion

    • Hey Paul,

      Is the computer doing the same thing it was doing before ? If you are getting LED lights, you probably have it working and hooked up right, but check the diagram before, (sorry it’s the not best) and make sure all connections are solid (use an Ohm tester or voltage meter w/ a beep for continuity) Also, if you are in Kansas, I have parts for a working G5 iMac which you can have all of them.

  19. Hi there,
    Excellent Article.
    I have a 2006 EMC 2111 24″ Apple 2.33MHz iMac (working). However, I wish to use it as a monitor (now that I cannot install Yosemite onto it)

    All I need from the power supply is the 12V and 24V pinouts but don’t know if they are the same as the smaller screen PSU displayed in your diagram.

    I have purchased a Video controller card, which will effectively turn it into a really cool 24″ 1920×1200 monitor

    Any help appreciated before I take the plunge !



    • This part of the article is sort of a cheat, and it banks on the size of the screen.

      What can be done is you can get a DC-DC 12 to 24 V converter. That was the advice given to me. The 24″ will require more strain on a power supply if you just use the 12 Volt, I believe in testing the screen was way too dim. Also, you could just go for an AC 24V adapter and just split the AC behind the screen. In this design, you’d want the switch to turn off the AC to all parts so you aren’t wasting electricity when it’s not on.

  20. Wow !

    Thanks for the super fast reply 🙂
    There are two reasons I wanted to use the original power supply:-
    1. Wattage/amp will be correct
    2. So that it will fit inside the shell of the iMac as not to disrupt the asthetics.

    I will planning on putting the HDMI input etc where the RAM is currently based, to keep any physical hacks to a minimum (even the 240v mains cable would remain in tact.

    When you say a bit of a cheat, if I can find a Ground contact, it should be pretty simple to work out where the 12v and 24v +’s will be with a multimeter.

    Any idea where I can find out ?, just seems a shame to reinvent what is already there.


    TK 😀

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