Dremel Junkie

Introduction

While I have not hooked the native PSU up to a mod and there is one wire that I do not understand the purpose of, I have checked all the collages of a working PSU.  I now know that is it similar in principal to the 20" PSU.  As this is all based off of my work with the 20", more information is available in that post:

The 20" iMac G4 Native PSU

17" iMac G4 PSU

vs ATX PSU

ATXPSU

The Native PSU is an always on, 12V only power supply.  Always on meaning that unlike standard ATX PSUs, the moment the AC cable is plugged into an outlet, there is DC power in the pins.  To get an ATX PSU to work without a motherboard, a pin in the motherboard connector needs to be grounded (known as "jumping" the PSU).

12V only means that unlike standard PSUs, only 12VDC and Grounds are supplied to the motherboard.   ATX PSUs transform the 120VAC in the wall to 12V DC and also downconvert the 12V to 5V and 3.3V.  These 3 different DC voltages are supplied to the motherboard in multiple rails via a 20 or 24pin connector.  Because the PSU supplies the iMac's motherboard with only 12V rails, the downconversion to 5V and 3.3V happen on the motherboard itself.

ATX PSUs also tend to supply peripherals directly, Molex, SATA power, 8pin PCIe etc emanate directly from the PSU itself.  As noted, ATX PSUs are not "always on", they have to be "jumped" either manually or by the motherboard.  As a result, the peripherals will not be given power until the motherboard is switched on, despite the fact that the power is supplied directly from the PSU to the peripherals.  Because the iMac's PSU only has 12V, the one molex line with 2 molex connectors (for the cd and hdd) emerge from the motherboard.  Although the molex wires share the same 16pin connector, it comes from the motherboard and does not interact with the PSU at all.  As the peripherals are supplied by the motherboard they will be off when the mobo is off despite the "always on" PSU.

The Connector

"Clip Side"

"Non-Clip" Side - All Grounds

The Pinout:

17" iMac G4 PSU Pinout

My switch theory (Optional Reading):

The 17" Inverter Pinout

The 20" Inverter Pinout - Revised

Option 1 - A computer in the base:

KEEX 6100 with P4 connector to Native PSU

15W 12V to 5V DC downconverter

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.

Background

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.

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

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:

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

Overview

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!!!

When I began this project, my motivation wasn't just to retain the concept of the iMac G4, but to find a way to reuse the LCD screen.  I felt the same way about the iMac G5, and although their Power PC processors had become largely obsolete, the displays were more than adequate.


The 20" LCD was almost identical to the one found in the iMac G5 and in the aluminum 20" Apple Cinema Display (these are all TMDS and swappable).  The expansive 20.1" display had a 1680x1050 resolution as a result of a 16:10 aspect ratio.  The iMac G4 contained one of the first flat screen LCDs on an all-in-one computer at the 20" for the first for that size.  Then a funny thing happened, LCD monitors seemed to stagnate in many respects.  LCDs were remarkably sharp compared to CRT Televisions and as computers used small screens compared to TVs, I suppose many felt the resolution to be adequate.  Innovation continued in the form of LED backlighting and increasing screen sizes, but in some ways, resolution seemed to go backwards.

Courtesy of extreme.pcgames

I loved the 16:10 aspect ratio, you were able to see more of the page without having to scroll.  Having the extra vertical room was better for me in almost everything except for one thing - watching movies, an activity I rarely did on my computer monitor.  The extra horizontal space, look up more room on the desktop and went largely unused for me.  For many sizes there was a decrease in the area as well as a lower resolution.  The less pixels required for the same size made this especially appealing to manufacturers and for the most part 16:9 has become dominant.  For 20" the pixel loss was very significant, the 20.1" 16:10 1680x1050 as compared to the 20" 1600:900.  The result is that I have not seen any modern 16:10 20" LCDs at all, though 22" are common.

Higher contrast ratios, IPS screens, LED backlighting can not be found in a screen that fits within the existing housing of the 20" monitor.  What is more concerning is that tablets and smartphones, in particular the retina display of the iPhone and especially the iPad, have changed the way people view LCDs and pixel density is increasing in desktop displays once again.  Although Apple has generally been ahead of the curve with higher resolutions (began in 2009) on their displays, this is now becoming industry standard.

A good example is the evolution of the "smaller" sized iMac.  The 20" iMac G4 was released in 2003 and it was the largest size available and continued to be so into the iMac G5s and into the intel era right until the end of the white polycarbonite iMacs, where the 24" size appeared making the 20" the middle size.  The aluminum iMacs kept the 20"16:10 size with the larger 24".  With the appearance of the unibody aluminum iMacs apple went to the 16:9 aspect ratio.  However, they did not stick with the 20" size in 16:9 and the 1600x900 resolution.  Nor did they upgrade to the 22" 16:10 size and its resolution 1680x1050 (identical to the 20.1" iMac's resolution - thus actually less dense).  They went to a 21.5" 16:9 LCD that is true high resolution: 1920x1080 (1080p = HD).  There is now even talk of High DPI LCD Screens that go beyond HD.

The iMac G4 has 2 features that make great to use - its ergonomic arm and its LCD.  The ergonomic arm is especially useful with a touchscreen.  In my touchscreen variations, I have loved using this design and incorporating a touch screen remains my highest priority no matter which direction I go in.  However, as the LCD becomes more dated, I'm considering trying a radical redesign for my "Pro" Mod.  As of now these are the possibilities as I see them:

1) The 20" iMac G4 mod with some improvements:
- A better cooling system and processor/RAM upgrade
- Native PSU
- Touchscreen
- Possible iSight Cam

Four Possible Touchscreens

I discussed this in my Touchscreen post and I have obtained a 3M 16:10 20" capacitive screen that seems to work with OS X.  It has a significant thickness to it and can not fit inside the housing as is.
The easiest solution is to place it in front of the display or cut the front bezel (the bezel is about the same thickness as the capacitive touch overlay).  This should work and if done properly may not look bad.  I am worried about the weight and ruining the elegant look with a touchscreen jutting out somewhat.  Also,  placement of the controller and wiring down the neck would have to be worked out.

2) LED backlighting
I am currently working on this method and will discuss it in my next post.  But, the general idea is to upgrade the lcd with led backlighting which will allow me to use a thinner diffuser.  This may allow me to fit the touchscreen inside the LCD housing and will help alleviate the additional weight.

The iMac's Diffuser

Unfortunately 16:10 20" LED monitors do not exist, thus, I will have to alter a larger diffuser and backlight in order to fit in the housing.  I can't just add LEDs because the entire purpose is to use a thinner diffuser allowing for the touchscreen to fit.  In theory this method would allow the 20" iMac to have a touchscreen added without any notable external change.

3) New, Larger LCD with Custom Housing
- Adding a 21.5" screen would be ideal, high resolution screens with LED backlighting and IPS would truly be an upgrade.  Also, 21.5" LCDs with optical touch screens do exist and some of these can be multitouch OSX compatible.  But this wouldn't fit in the current LCD housing.  While it may be possible to enlarge the front housing, the back would have to be done custom.  While 3D printing and prototyping services are available, there are certain drawbacks to using the original design.
- The weight would be very different.  Also, the screen would likely use LVDS, so fitting the controller board so that DVI/HDMI would go through the existing TMDS wires in the neck would be advantageous.  Thus, the existing shape may not be ideal.
-  As the 21.5" screen is notably wider, it may start to look awkward.  Elimination of the surrounding clear plastic and wide bezel would help the proportions.
-  This would be very expensive and difficult to reproduce.

4) Redesign using a different LCD Housing:
- I have secured a different housing to the iMac before and made it into a successful mod.  So it can be done even with non VESA complaint monitors.

Apple Cinema Display Mod Housing Secured

- Obviously keeping the dome is critical.  It is what gives the iMac G4 its identity and allows the LCD to be pulled forward and rotated in a way that a base alone would not allow.
- The LCD housing would have to look modern, have an apple style, be able to fit a 21.5" LCD and have a weight and width similar to the original G4's housing.  While I know nothing about the weight issue, the obvious candidate is the housing from the 21.5" iMac.
- There are many technical details that would make this impossible.  But here is a very quick mock-up:



As I have the 20" touchscreen and have already begun, I am going to pursue the original design and housing with the touchscreen and LED backlight, but I may pursue the 21.5" based iMac in the future.  I would love to get opinions and suggestions, so please comment below.  Thank for reading.