22. Need 28VDC 4Amp PS... (Bill Sudbrink)
Date: Wed, 23 Jan 2008 16:52:23 -0500
From: "Bill Sudbrink" <wh.sudbrink at verizon.net>
I'm fixing a PowerMac G4 for my Dad's wife.
The original PS is dead.
It was a pretty standard ATX supply, the only difference is that it
supplies 28VDC at 4 Amps on two extra pins. Rather than spending a
lot of money on a new supply (more than $250 from the few sources that
will guarantee the PS) or a not insignificant amount on a used PS that
could go at any time, I want to put in a new standard ATX supply and
"tuck in" a small 28VDC PS to drive the two extra pins. The supply
is always on, so I can power it from a pass through on the ATX supply.
Does anyone have (or know where to get) a small supply?
The 28V serves a couple of (mostly silly) purposes. Primarily it is there
to supply 28V through the ADC video port so that a connected Apple
display, which uses the (proprietary) ADC connector, can draw all of its
power from the host Mac through the video cable at 28V. That is why the
amperage rating is so high. I'm rather hazy on what the other purpose of
the 28V supply is, but I'm under the impression that the machine will not
start up without it.
If you are not connecting an ADC based monitor, your power supply can have
a much lower capacity.
Which brings me to this: <http://www.io.com/~trag/28v_vreg.pdf>
I did not design this, but it is a handy little circuit which boosts the
5V trickle to 28V to supply the needs which the Mac power supply has,
other than powering an ADC monitor. I would not recommend connecting an
ADC monitor to a Mac with this adaptation. The National Semi 2577 on
which it is based is not rated to deliver 3A.
You may wish to browse NS's website a bit. They have a variety of voltage
boost chips available and IIRC the 2586 has a higher power rating.
The ADC connector has been a curse to many Mac users. In addition to
requiring the off-kilter power supply, most of the non-iMac/non-Mini Macs
sold had dual video connectors but one of them was that stinkin' ADC
connector. So if the user wanted two monitors he either had to buy an
expensive (but fairly nice) Apple display or an expensive adapter.
Furthermore, Apple routed the 28V supply to the video card through a pin
which is now used on 8X AGP video cards. So one cannot install an 8X AGP
card in "28V" Macs without neutralizing the 28V connection in some way.
The analog video signals are present on the ADC connector, so it is
possible to adapt through to plain old analog VGA, but the unusual form
factor of the ADC connector means that as soon as Apple stopped hawking
ADC, products, such as adapters, with the ADC connector stopped
production. ADC to VGA adapters are only available on the used market
now and rarely. There appears to be an infinite supply of Formac brand
ADC to DVI adapters available, especially in Europe. However, for some
reason the Formac (and most others) adapter does not pass through all of
the analog signals. So going ADC-DVI and then DVI-VGA will not work
unless one modifies the ADC-DVI adapter.
I opened the thing (Formac adapter) up and it contains a small circuit
board to which all the incoming analog signals are connected. It boggles
me that they did not bother to connect them at the other end of the
adapter.
Jeff Walther