Saturday, May 5, 2012

20" iMac G4 - The Native PSU

20" iMac G4 LCD powered only by Native PSU
The Native PSU is not enclosed.  Once it is plugged in, it is live.  Please use extreme caution with both the Power Source itself and its wires.  Keep any conductive objects away from the PSU and do not handle either the PSU itself or make changes to its wires while it is plugged in.  As always, proceed at your own risk.


C5 receptacle
Courtesy of Wikipedia
From the onset of my 20" All-In-One Sandy Bridge mod, it was clear that the native PSU was not going to fit (from both a physical space perspective and a heat generating perspective).  It was either the optical drive or the native PSU.  I kept the optical drive and used a PICO PSU instead.  This put most of the physical bulk and heat generation on the outside of the mod in the form of a power brick.  However, I certainly understand the draw of using the native PSU. First, a modder likely alread has one, its right there in the iMac, no extra parts need to be purchased (the cost can be substantial as high wattage Pico PSUs cost around $150).  Secondly, there is no power brick, the rear plug would house a receptacle for the rounded, 3 pronged C5 plug of the native iMac G4.  Third, and most importantly is that it was designed to power this display.  Because of this it has a wattage greater than any PICO PSU (190 watts) and has a 24V line for the display, eliminating the need an up converter or separate 24V source.

20" iMac G4 Native PSU
Connected to AC C5 Recepticle
The problem, or so I thought, with the Native PSU has been that it can not be "jumped" in a conventional manner.  "Jumping" is a technique used in conventional ATX PSUs.  Grounding (connecting to a ground wire) a particular pin in the motherboard connector causes the PSU to "turn on" in the absence of a motherboard and the momentary switch that the mobo provides to turn the psu and subsequently the computer and internal devices on.  A few intrepid readers have let me know that they have had success "jumping" by supplying 5V to a particular pin in the motherboard connector.  I had thought of this PSU as a variation of an ATX PSU, with similar properties.  The problem I had envisioned using this solution is I figured a separate 5V source, in the form of a second PSU would be needed to give 5V to the native PSU in order to "turn on or jump" the native PSU.  While this may have allowed for a low wattage second PSU, because you still needed a different PSU, it somewhat defeats the purpose of using the native PSU.  I was mistaken, at least for the 20" PSU.  This is not how the native 20" PSU works at all.  I did initially have a working PSU for the 20" (Side note: The PSU seems to have the highest failure rate of any iMac G4 20" part and has been almost universally dead on every non-operational 20" iMac G4 I have obtained), so I purchased a known operational PSU.  The one I bought is green and like everything with these computers, I have seen variation in the PSUs even between the same size iMac.  So, I can not say for sure that all PSUs will work like this one.

How Does It Work

"Clip" Side
The PSU doesn't really need to be jumped at all.  It is "always on".  Looking at the motherboard connector, one side (that has the motherboard clip on it) has 4 yellow wires, then a blue, then yellow and red that go to the molex, and finally a green wire.  The other side (without the clip) has all Black Wires except for one White Wire.

"Non-Clip" Side
To be honest, this PSU confused the hell out of me and I believed I had a defective PSU initially.  It was the Molex connector here that really threw me off.  To test PSUs, I have a Molex powered fan that also lights up.  I attach this to a molex connector from the PSU.  When I turn on a PSU, I expect to hear the fan and see the LEDs turn on.  This fan (like most) uses 5V, so it really only connects to the 5V lead (Red Molex) and a ground.  No matter what I did, I could not get the fan to turn on.  Of course, I could have supplied the 5V directly to the pin the fan was connected to, but that seemed ridiculous.  In truth, 5V being supplied to this pin is likely what happens with the native PSU/Mobo setup.  It wasn't until I tested the voltage on one of the (non-Molex) Yellow wires that I realized that the PSU was in fact live as soon as it was plugged in and the Yellow lines have +12V DC on them.  This PSU appears to be a variation of a "12V only" PSU (with one notable exception).  12V only PSUs rely on the motherboard for the DC-DC downconversion and most modern ones have 5V and 3.3V rails.  12V only PSUs are usually PSUs for low power boards.  It appears that the PSU supplies the motherboard with 4 12V rails in the form of the 4 yellow(12V) and black(ground) wire pairs.  This is similar to using a PCI-E 8 pin power connector (or 2 P4 power connectors).  The molex power actually emerges FROM the motherboard, 12V directed from the PSU through the motherboard out to the yellow molex and the 5V a result of the motherboard down converting 12V to 5V and sending that through the Red 5V wire.  So when I hooked the fan up to the molex, without a motherboard, I was in essence hooking it up to nothing.  It is pretty obvious that the molex wires come from the connector and not the PSU at all.  Initially however, I thought these wires might interact with adjacent wires within the connector itself, but this is not the case.  That leaves the green/black and white/blue wire pairs.

Looking at the motherboard side
The Green Wire (only on the 20") likely represents the only DC-DC conversion that the PSU does itself.  And its an up conversion from 12V to 24V (Though this could also be generated by a separate transformation from the AC current).  The Black wire that it is paired with is simply a ground wire.  There is no voltage detected when you plug the PSUs AC Cable in.  This is because the motherboard "tells" the PSU to supply the 24V rail only when its on.  The motherboard does this by supplying a +5V current to the White Wire with the Blue Wire acting as the ground in this circuit.

The PSU Pinout:

ATX PSU Pinouts
Naming Convention of ATX PSUs numbers from Right to Left, Top then Bottom, looking at the open pins (motherboard side) of the connector while the clip is positioned on the bottom.  I will keep this same convention for the pin numbers I use for the iMac G4s native PSU.

20" iMac G4 PSU Pinout

The pin numbers are identified on each pin.  I have also divided the 16 pin connector into 4 colored zones.

Yellow Zone (Pins 1-4 & 9 - 12): These 4 yellows and 4 black wires are on as soon as you plug in the PSU.  Each of the 4 Yellows (9 - 12) has +12V DC and the Blacks (1 - 4) are their corresponding grounds.

Blue Zone (Pins 5 & 13): These 2 wires form a powered switch.  By supplying 5V+ to the White Wire (Pin #5) and grounding the Blue Wire (Pin #13) the PSU turns on Pin #16, the Green Wire.

Red Zone (Pins 6,7,14,15): These 4 wires were used by the iMac G4 to power the optical and hard drives via 2 molex connectors.  The 12V for the Yellow and the 5V for the Red and 2 Black Grounds came from the motherboard.  As these are simply a molex cable hooked up to nothing, you can ignore these wires.

Green Zone (Pins 8 & 16): +24V will be supplied by the PSU via the Green Wire (Pin  #16), which is required for the backlights, when 2 conditions are met: The PSU is plugged in and the White/Blue (Blue Zone) Wires are supplying 5V, turning the the "Switch" on.  Pin #8 is the Black Wire Ground for the backlights.

Applying this to the 20" iMac G4 Mod

This will only make sense if you are familiar with my more recent TMDS to DVI iMac G4 mods.  In this case knowledge of the 20" iMac G4 mod in particular is recommended.  Click the link below for details:

All in One 20" iMac G4 Sandy Bridge Mod

and the iMac G4 20" Inverter Pinout:

Updated iMac G4 20" Inverter Pinout

I used a PICO PSU for this mod.  The PICO PSU has 12V rails, 5V rails, and 3.3V rails (the 3.3V rail is not needed for this mod).  In addition, I used a 12V to 24V DC-DC Upconverter to get the 24V rail required for the monitor.  To replace the PICO PSU with the Native PSU you need the PSU to supply 5V, 12V, and 24V.  As the native PSU has 12V working as soon as you plug it in, we have our 12V rail.  As noted above, if you supply 5V to the Native PSU, a 24V rail will be turned on, so 24V is taken care of.  This leaves only the need for a 5V rail.  While 5V is not actually needed by either the LCD or the backlights itself, you would need 5V for any device that is molex or SATA powered.  This includes hard drives, fans, optical drives, and even touch screens.

There are 4 basic ways to get 5V, which one you choose depends largely on what you intend to use in the base.

1) Use a second PSU - a PICO PSU connected to the native PSU

A PICO PSU with rocker switch
Almost all PICO PSUs use 12V DC input (some have wide ranges in DC input).  Power it by hooking a 12V line from the native PSU to the DC power in (the white and black wires pictured that hook up to the connector for an external power brick).  As the native PSU gives you 12V DC already, you do not need a power brick.  You must "jump" the PICO PSU in some fashion, depending on what your intended iMac G4 mod is.  The Jump mechanism can be "always on" with a simple wire or "on/off" with a rocker switch (as pictured) to control power out from the PSU.

If you are using a motherboard in the base that requires an ATX Power connector, this is without a doubt the way to go.  You can plug this into the motherboard connector, without having to wire all sorts of adapters and converters, at a negligible loss of space.  Of course if its connected to an ATX mobo, the motherboard will jump it for you.  

If you are using this for an external monitor with peripherals (ex. dvd drive), you can use the requirement to "jump" this psu to your advantage.  You can put a rocker switch here that will allow you to turn off everything its connected to.  Remember the native PSU will be on as soon as you plug it in, by jumping this psu without a switch, it will also be on.  Thus, drives will be spinning, leds will be glowing etc whenever it is plugged in.  With a switch here you can turn the whole unit LCD and peripherals completely off.  Just remember, no matter what wattage PSU you get, these are not additive, they are connected in serial, so you are still limited by the overall wattage of the native PSU.  Also, you have to make sure that the wattage is adequate for anything you have connected "downstream" of PICO PSU.

The PICO PSU is probably the most flexible option as they are available with P4 connectors. molex connectors. SATA power connectors etc.  If you are not using an ATX motherboard, you could even use the power from the pins intended for the motherboard.  However its expensive and not needed if you use Option #2.  And if you aren't using peripherals requiring 5V - it is way overkill and you should use Option #4.   I'd even recommend hooking up the White PSU wire using #4 and using the 5V rail from the PICO PSU only for peripherals because #4 works the best for sleep/wake.

2) Use a small form factor mobo that uses 12V only for power (ex KEEX ECX boards)

Many small form factor boards including ECX boards use a P4 connector.  This is a 4 pinned connector (2x2) that uses 2 - 12V and 2 - Grounds.  As there are plenty of 12V rails available from the PSU, simply take your motherboard power connector and attach the 12V DC lines to the Yellow wires from the PSU and the Grounds to the Black PSU wires.  In the picture of my KEEX-6100 below, the P4 connector can be seen in the front right corner of the motherboard.  This connector attaches to a 12V rail from the native PSU.
The KEEX-6100
The reason this will work is that this board (as well as several other small form factor boards) works just like the original iMac G4 motherboard in that it requires only 12V in to work and it itself has downconverters on it.  The KEEX-6100 actually has a mini-Molex/SATA power out port, which can be seen in the back, just left of the fan.  This gives a couple molex out, including 5V.  So you have a 5V line from here.  However, as above, I'd still recommend hooking up the White PSU wire using #4 and using the 5V rail here for other peripherals as #4 works the best for sleep/wake.

3) Also a 12V to 5V downconverter can be used:

The PSU circuit
AC Plug into the wall is transformed into 12V DC by the PSU and plugged into the motherboard supplying 4 12V rails to the mobo.  When the power/momentary switch is pressed the motherboard turns itself on, opens gates to downconvert some 12V current to 5V.  Power is sent from from the motherboard to the 12V and 5V rails of the molex wire to supply the drives.  Also 5V is sent in a circuit back to the PSU via the white wire and back via the blue wire.  This turns on a transformer or up converter on the PSU which then sends 24V via the Green wire to the LCD.

This is actually quite brilliant and remarkably simple.  This opens up several intriguing possibilities for future mods.  My first test was to add 5V via white/blue from an AC to molex and confirm 24V on green/black with multimeter.  I did not want to add a second PSU and I knew power was technically already flowing to the 12V rails, I simply mimicked the motherboard using a very small, inexpensive 12V to 5V downconverter.

15W Downconverter

The PSU circuit and how I created it:

I cut free from the motherboard connector these wires: (The grounds are interchangeable, I used the matched pair across for convenience)

12 to 5V DC-DC downconverter hooked up
1 Yellow/Black (NON-MOLEX)
The White/Blue
The Green/Black

The Yellow was connected to 2 wires:
   The alligator wire the comes from the 12V LCD wires in the Gray Cable of the neck (Purple, Yellow, and Orange).  I also connected this to the 12V input of the Downconverter.
The Black was connected to the Gray Cable of the neck/LCDs grounds (Blue, Green, Gray, and Pin 15 of the DVI).  I also connected this to the ground input of the Downconverter

The White was connected to the +5V out of the downconverter and the Blue connected to the ground out of the downconverter.

The Green was connected to the inverter cables Red/Blue (+24V) and the Black to the inverter cables Green/Black (Ground).  Turn in on and there is the video.

Native PSU and LCD reunited

4) The Best and Easiest Way using the DVI 5V Pin

Really the best and the easiest way to do this especially, if you don't need a 5V rail for anything except for the LCD monitor is to use the 5V from the VEDID.  The is pin #14 on the DVI Connector itself.  5V is sent from this via the DVI source (ex the computer) to Pin #14.  This pin is intended to carry the 5V to the LCD panel via the Hot Pink/Red wire in the iMac's Gray LCD cable.  We are already splicing this 3 times so far.  One goes via the Hot Pink/Red Wire to the LCD as noted.  One goes to a 1Kohm Resistor to DVI Pin #16 (The Hot Plug Detect).  The third goes to a 6.8Kohm resistor then to the Yellow (?Wake from sleep wire) of the inverter cable.  By splicing this again we get 5V which can be sent via the White wire of the native PSU to turn the 24V line of the LCD on.  You can splice this anywhere along the line, I did it at the cable that comes to the Yellow wire of the inverter.  I connected an alligator wire to the White Wire of the PSU and hooked it up to the line going to the connection between that line and the 6.8Kohm resistor on the side before it passes through the resistor.  This seems to work beautifully and results in the screen going completely dark during sleep appropriately.  To be honest, I'm kind of blown away at how elegantly this works.

Now if you see 5V for anything else, you will need one of the above methods.  Meaning, use this for the LCD, but if you want to power a drive in the base DO NOT USE THIS SOURCE YOU WILL DAMAGE IT! You will need some type of downconverter, either a standalone or part of a motherboard as mentioned above.

This has gotten me to rethink LED backlighting and I may try adding the touchscreen in some fashion using the existing CCFL LCD.

It is amazing how the components of this decade old, little machine continue to amaze me.  Whether its the mechanical grace of the neck, the remarkable use of convection cooling, or now even a simple/elegant power supply solution for such a limited space, the iMac G4 is truly an inspired design.  To those working on the 17", I have posted information on that inverter which works in a similar fashion.

Thanks for reading!!!


  1. I'll be honest, this post confused the heck out of me. Would you mind making a video tutorial of how to start with a fully-funtioning 20" iMac G4, and end up with an external monitor with built-in PSU (and possibly a DVD drive?) Using the native PSU is a must, but using the native Superdrive isnt nessisary. But if I have to buy a new drive, I prefer something that is like $20.00 off NewEgg. Thanks!

    1. The iMac G4 17" 1/1.25 Ghz mod uses the exact same pinout as the 20" iMac G4. The TMDS hookups to the DVI are exactly the same. The only difference with the LCD is: The 3 LCD power wires in the gray cable (Orange, Yellow, Purple) connect to 12V in the 20" instead of 5V in the 17".

      The backlights are different. But its still just connecting a few wires (4 for the 17", 5 for the 20") to the different voltages in my guides/pinouts - there is an extra ground in the 20". So there is not much difference for a full video guide, but I have posted step by step tutorials on all of the TMDS to DVI mods I have done.

      In terms of the native PSU, I'm still testing the new way I have it hooked up (Have to make sure the backlights work with sleep/wake, deep sleep, off/on etc). This post was written as I worked on this, as I changed my mind or tried something better I changed it, so it currently is disjointed. I will post more on this that should help and will likely use this method in a newer project in which I may film some parts as a video tutorial.

      In terms of the drive - you should be able to stick with the superdrive. If you are using this as an external monitor only, with the Native PSU, you will need the $5 DC-DC downconverter I mention above. Take a female molex connector with wires (you can even take the molex connector that originally plugged into the superdrive from the PSU). Cut off the 4 wires after a few inches. Solder the Yellow to Molex to a Yellow from the PSU, the Black/Ground next to the yellow to a black/ground from the PSU. Next attach the 12V to 5V converter. Hook up the 12V in to a different Yellow/Black on the PSU and then you will have 5V out. Connect the 5V+ out to the Red molex wire that goes to your drive and the ground out to the ground molex. Lastly, you will need an IDE to USB 2.0 converter that you can find on ebay. This will have to be wired to your computer or you could create a hub on the back to connect a USB.

  2. Thanks for the reply. I think the biggest thing the everyone, including yourself, could benefit from is a nice spring cleaning. This whole blog is great from an information standpoint, but horrible from an organization standpoint. I appreciate that you added the tabs at the top (when I first started reading this it was and there were only like 20 posts) but honestly that was only the first step. I think that if you could compile all of this information in a readily available, organized format, that the whole site will become better. I would be willing to help you organize it if you like. I know some HTML and Blogger is really easy to use. It would be even better if there were different sections of the site. Like "Tutorials" and "Specific Information" "My Goal" etc.

    Just trying to help


    1. I understand, however, my intention is not to provide a specific blueprint on how things are to be done. Instead, I think of this blog as my digital diary and worklog. People are welcome to take whatever information they gain and apply it to their own projects. I will try to make my guides and tutorials more obvious to readers, but there are so many permutations that its impossible to step by step every one. New ideas and approaches keep coming up. Unfortunately, time and money are limiting factors and pushing ahead with new projects is more enjoyable for me than maintaining a database of old ones.

    2. Where can I purchase a 12V to 5V downconverter here in the U.S.?

    3. The easiest place to get things like this is from a Chinese retailer via ebay. Just do a search for 12v to 5v dc dc converter and you should have a selection. You could make your own down converter but at 3-10 dollars, I don't think it's worth it. Also remember that you may not even need a down converter if you are using a Keex board. Many have onboard down converters to 5V

  3. Nothing write like a good weekend project! (it's a 20 incher) thanks JBerg!

    1. Awesome!! Looks great. Let me know if you have any questions. With an optical drive and the native PSU, you may want to consider keeping the fan, though it's probably not critical. My iMac G4 that used the cinema display controller was similar to what you are planning. I added a rocker switch on the back o tun the drive on and off. It's nice not to hear the drive noise when you are not using it. Also definitely hook up the native inverter as I describe in my post - with the 4 way split of the dvi 5v. However - for 5v for the drive do not use this source. Use the downconverter with a 12v line. Good luck and keep me updated on your progress.

  4. I have run into a problem. My inverter cable has no yellow wire. Should it? Also, all of the arrows in the infographic are rather confusing. Mind labling them better?

    1. The infographic is just an overview, there are too many connections to make it simple. It's really just to give a general blueprint, you should follow the written step by step instructions. If you go to the parts list on the 17" iMac TMDS to DVI mod (the 4th tab above) I have links to 8 sources of this connector.

      However, I think you at have a bigger issue. I am basing this on the 2 pics you sent. You may want to measure your screen because I see some inconsistencies. The 20" has 7 inverter wires, you have 6 (yes there should be a yellow). It's difficult to see but the 20" base has metal inserts on the sides of the fan at the top of the dome to give it extra weight. Again, I can't be sure from your pic, but I don't think they are there in your dome. Third, on your PSU cable there seems to be a yellow wire at the end, not a green. This likely represents 12v not 24v which the 20" needs. I have no picture of the LCD, but the parts you are displaying seem to be from a 17" iMac g4, not a 20". Sorry, but you should measure your screen diagonally to confirm this.

  5. Also, van you link to me where I can buy the DVI port? I can't find it. Thanks!

  6. I just measured the screen, and you have no idea how incredably enraged I am right now! You're right, it's a 17". It never occired to me to measure it, so I just assumed it was a 20". and based on the listing, it should have.been a 20. I just left some nasty feedback. I will make sure I get my money back for this! $250 is too much for a 17"

    1. Sorry to hear that. Good Luck - hope you get a refund.

    2. I have been going back and forth with the seller all day and finally decided to open up an eBay Buyer Protection case. It's one thing to make a mistake, it's another to use false advertising!!! Anyway, I'll do my best. I may just have to use the 17". We'll see.

  7. I found a iMac g5 that I would like to convert into a monitor would option four work on my project ? I've never done a project like this so I'm being patient and trying to learn as much as I can before I start my project. Any advice would be helpful thanks.

    1. I believe so, the LCDs seem to be interchangeable. I am not exactly sure how the PSU on he iMac g5 works, however.

    2. Thanks for the advice I have been able to get the screen to power on, using the first four wires on the inverter wired to the psu.I'm going to continue and let you know how it goes.

  8. Hi,

    I'm hoping to use the original G4 15" 700mhz power supply to power the screen and inverter.

    Can you offer any help?


  9. Hey jberg! Thank you for this amazing blog with all this information!
    Do you know how the upconversion on the native G4 20 PSU works? Might it be possible to "kill" that upconversion and have a pure and mighty 12V PSU? What are the real specs of the G4 20 PSU? The most powerful PSU I can find specs of is the 17 inch (1GHz) PSU with 13.4A on +12V. The 20 inch version should be even more powerful.
    Thank you for your reply.

  10. Hi, I am trying to get 5V from pin #14. I spliced the wire before the resistor. But unfortunately, everytime I used this spliced 5V for my USB audio device or extra fan (5v), the LCD's blacklight flashes and does not turn on. My purpose is whenever computer turns on, my audio USB can use this 5V. Could you please help me with this? Thanks,Quyen Tran

  11. Hello ! thanks for the tutorial, is very easy to follow. Unfortunately, didn't work for me. My Macbook pro recognize the Screen as an iMac Screen, and also the iMac screen turned on the the backlight. But there is no imagen, just a black with backlight screen. I checked the correct connection of all the cables and everything is OK. I saw the diagrams several times before attempting the mod. The iMac was working before, so Im was sure all should worked. Please let me know if you have any ideas of what can I don. Thanks !