When in the course of transferring my PM 8500 to a new case I discovered the unique nature of legacy Mac power supplies, meaning that they were somewhat different from modern PC power supplies, and also that it seemed likely modern Macs were now using standard ATX power supplies instead of the older type. Understanding power supplies gained some new importance in my life.

Note that PS is often used for power supply.

Apple system are extremenly variable as to the particulars of their PS, even with more recent systems.  A number of newer Macs continue to use some of the older conventions, such as a "high" signal for power supply activation, with differences existing even among the members of the same motherboard family.  

Examples:  The Biege G3 (PowerMacintosh G3) systems used  a standard ATX PSs that went "low", the iMacs which came out later but were based on the same Gossamer motherboard went "high", an the iBook Dual USB (first generatino white iBooks) released MUCH later also went "high".   It is up to grabs what a particular system does but it does seem that laptops tend to use the "high" convention for PS activation, or did for a time.  To confuse things a number of Macs with ADC connectors used non-standard PSs to provide the 28 voltages for the monitors (ADC was an Apple standard the carried monitor power, video, and USB in one cable), but not all ADC equipped Macs use such supples.  The bottom line is always check before assuming any Mac will work easily with a standard ATX supply.
 
At this point I knew nothing about power supplies as a serious power supply concern had never arisen.  Several issues were readily apparent.  The most obvious problem was I had no idea how a power supplies was activated by the computer or whether my old Mac and modern PCs did it the same way, or some completely different way.  It was also clear that I needed to figure out what both an ATX power supply and my old Mac power supply were providing the motherboard (and vice versa) so that I could hopefully match leads.

As a general rule I make efforts to figure things out on my own before turning elsewhere for help or the answer.  I have found that following this pattern allows me to much better understand the particulars of a problem, and therefore makes the solutions of others more clear and useful.  Since I didn't want to blow anthing up I did only a bit of self investigation.

To begin I dissembled the original Mac and an ATX power supply and inspected the circuit boards reading (where possibly) the imprinted text.  Many of the lead attachment points on the circuit board were labeled with useful names, such as GND, +5v, PG, etc...  I also used a multi-tester to map the voltages on the running Beige G3 that was at that time mounted in a standard ATX case with an ATX power supply.  This permitted the creation of a rough table comparing the leads.  I then did some online research immediately finding valuable information along with clear descriptions of the details of adapting ATX power supplies for use with older Macs on the XLR8YourMac site.

My initial investigations and subsequent research answered several questions. 

The first and most fundamental question for my project of discovery was,  'What if I have 8 hot pink leads on the motherboard, but only 6 off the supply, can I just split the leads? or am I SOL?'  The answer was yes (on the lead splitting) as the leads join and are common on the motherboard.   Multiple leads presumably just allow larger currents without requiring huge single wires, and also provide more precise voltages by dividing power demand over multiple leads.

The remaining important details can be summarized as follows:

As I looked at PS from multiple older PCs I found lots of variation. It seems in the early days of  PCI many computer makers, Apple included, continued to use their basic power connectors since their existing pre-PCI motherbords and wiring harnennes already used them.  Often PC makers just added a second power connector  to provided separately for the 3.3v leads.  Although the colors are generally now standard: Yellow 12v, Red 5v, Black Gnd, Orange 3.3v, Green Power On (Hi), the colors where not so standard then: For example the Mac had Blue for +12v and a Dell PS had Blue/White for +3.3v. 

Several other leads were present:  A continuous +5v lead that provided power to the motherboard from the PS even when system was "technically" off to allow for such functions as soft power, scheduled power up, etc.., some other power leads and some and some "sense" leads.  Both the Mac and ATX supply had one or more sense leads (presumably involved in feedback from the motherboard to the PS) but they could apparently be ignored.

It was clear that soft power would require a circuit that would read the +5v signal from the Mac motherboard and use it to ground the green lead on the ATX power supply, but that problem could be ignored and a simple switch used to ground the green lead in order activate the power supply. This allowed for a hard switch controlled power up of the computer.  This would allow me to delay having to solve the soft power problem, or even decide to not use soft power at all.

 
The basic pin out comparing legacy Mac power supplies to ATX supplies is outlined here.

The white lead on the ATX supply (power good) and the white and brown leads on the Mac motherboard could be safely ignored and left unattached.

To activate the ATX power supply obviously the green ATX lead from it cannot be directly attached to the pin for the purple lead on the Mac motherboard.  Either it must be manual grounded via a switch, or connected to the Mac motherboard via a signal inverter circuit, an example of which is discussed in detail in Theory and Reality of Building a Soft-Power Signal Inverter.

Note:  You can activate any standard ATX PS by grounding its power on lead (usually the green lead) easily accomplished by touching it the the PS case or connecting it directly to a black lead.  However most power supplies require something be drawing powers from them (they have to have a load)  before they will activate. The is easy to do by connected a spare (and unneeded) hard drive or a case fan to the supply.  This also gives uselful feedback that the PS did activate.  Also if the PS ever overloads (as in  you touch a red lead to ground) most PSs will automatically deactivate requiring you to diconnect and reconnect the AC cable to reset them.

Connecting it up this way I was able to provide power using an ATX power supply to the Mac motherboard without much difficulty.  I used a switch initially and then added the soft power signal inverter to enable soft power, scheduled startups, remote shutddown, etc. 

The entire saga is discussed in Just in Case.

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Created 10/25/04
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