|20" iMac G4 LCD powered only by Native PSU
Courtesy of Wikipedia
|20" iMac G4 Native PSU
|Connected to AC C5 Recepticle
How Does It Work
|Looking at the motherboard side
The PSU Pinout:
|ATX PSU Pinouts
|20" iMac G4 PSU Pinout
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 ModThis 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
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.
3) Also a 12V to 5V downconverter can be used:
|The PSU circuit
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.
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
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
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!!!