O2 Power Supply Basics

Elf

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Feb 4, 2019
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It's time for more Power Supply Basics! This time of the Silicon Graphics / SGI O2. This one had less to figure out than the Indigo 2 and Indy.

If referencing or duplicating this information, I ask that you maintain a prominent link to this original forum post for further updates, as well as a credit by name.

At least of the time of this writing, you won't get any better power supply information than at the Silicon Graphics User Group!

Here is the power supply pinout from the O2 front plane perspective, looking from the back into the recessed cubby hole for the PSU:
O2 power inlet.png
Power is supplied over a 24 pin Molex Mini-Fit BMI connector. This is like the Mini-Fit Jr. commonly found on wire to board solutions, but has guide posts and no clip. A 24 pin Mini-Fit BMI "plug" (machine side) is pictured to the right, with the Mini-Fit Jr. to the left for pin number reference purposes.

The O2 front plane (which connects the motherboard, power supply, and other accessories) has the male pinned connector, while the O2 power supply has the female connector.

+3.3V standby (Aux) power is supplied when the PSU is connected to AC Line power, but has not yet been turned on. In contrast to the Indy and Indigo 2 PSUs, the O2 PSU does not turn itself on by default, and the Run line must be grounded to start.

Pin #Wire Color / SizeFunction
1Red / small+3.43V rail feedback / sense
2Blue / smallRun (Ground to start)
3Red / large+5.1V
4Red / large+5.1V
5Black / large0V
6Black / large0V
7Black / large0V
8Black / large0V
9Black / large0V
10Yellow / large+3.43V
11Yellow / large+3.43V
12Yellow / large+3.43V
13Grey / small0V return feedback / sense (?)
14Brown / smallPower good signal (3.3V)
15Yellow / small+3.3V Standby
16Purple / small+12V
17Orange / small-12V
18Black / large0V
19Black / large0V
20Black / large0V
21Black / large0V
22Yellow / large+3.43V
23Yellow / large+3.43V
24Yellow / large+3.43V
Pin numbers are as per the mechanical drawing. Wire colors are internal to the power supply, as only the connector is exposed when fully assembled.

The unit under test is Sony model APS-90 / part # 8-68-1189-41, SGI part # 060-0022-001, with the following specifications:
RailRating
+3.43V31A
+5.1V10A
+12V3.2A
-12V0.25A
+3.3V Standby (Aux)10mA

Unlike the Indy and Indigo 2 which have most of their capacity on the +5V rail, the O2 is primarily +3.3V driven, with more of its power on the oddly spec'd +3.43V rail. The 3.43V regulation was confirmed. Standby power comes from a separate converter, and is at 3.3V, versus the main power rail.

Power factors were in the 0.6 area; bad as usual! Standby power is about 4 Watts. No precise numbers this time around, although they could be reported if desired.

O2 test wide.jpg O2 test normal.jpg

As you can see the O2 PSU underwent the usual test setup. Initially what gave me some trouble was the nature of the two sense lines. Without Pin 1 (red / small) and Pin 13 (grey / small) connected to the +3.43V rail and 0V return respectively, the power supply goes wildly out of regulation. The +3.43V rail will swing above 5V, and the power supply has a tendency to shut down under any load.

This made it difficult to test until I uncovered the nature of the sense lines by testing the O2 front plane. I am still uncertain why there is a feedback connection for the 0V return, but without this second sense line connected the PSU does not operate properly.

SCR01.PNG

(C1 - Pgood, C2 - +3.43V, C3 - +5.1V)

Pin 14 (power good) is asserted at a 3.3V logic level well after the +3.43V and +5.1V rails have stabilized. Notice the odd ramp up of the +5.1V rail, closely following the +3.43V rail until it gets a bit more of a boost about 150ms after +3.43V has stabilized. Strange! Examining the conversion stages would probably make more sense of this.
 
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Elf

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Feb 4, 2019
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Also worth noting: while testing with the cover off and fan removed, my particular O2 power supply had capacitors that were starting to make bubbling noises. Given the age of these (and many other SGI power supplies), new capacitors may be in order for continued life.
 
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Jacques

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Dec 21, 2019
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Registered just to say thank you for the power supply diagnostics on the various machines! Probably one of the most important research matters to keep these old beasties going.
 
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Jacques

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Dec 21, 2019
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I appreciate it, thank you! :)
Quick question, while my O2 PSU is open for a fan replacement ( putting the original back), I noticed a potentiometer on the daughter board labeled 'Fmin'. Does this regulate the minimum fan voltage? I only ask as my fan spins much slower in the O2 while it's connected to a dedicated 12v supply outside the psu.
 

Elf

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Feb 4, 2019
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I apologize, I wish I could say! I did not try adjusting that or tracing it out. If you feel adventurous, you might mark its current position and try changing it?

The only other thing that occurs to me that "Fmin" could specify is perhaps switching frequency.
 

drmadison

New member
Jun 30, 2020
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I'm curious if the machine will run if "only" 3.3V is supplied over that admittedly odd 3.43V line. And same with 5V over the 5.1V lines. Maybe at least on more limited systems? Would lead to a nice upgrade to a more modern PSU for these machines, assuming you can find one with suitable amps on the 3.3/5V lines. The Corsair RMx line of PSUs goes up to 25A on the 3.3V. But it seems crazy to replace a 170W PSU with something capable of 750W in the same of a slightly improved power factor...

(and LOL, the 1200W Corsair HX1200 WILL push 30A over 3.3V. But overkill much?)
 

Elf

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Feb 4, 2019
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My guess is that the 3.43 / 5.1V is used to compensate for a bit of voltage drop at higher current draw. The logic itself is almost certainly 3.3 and 5V. It might run if only supplying 3.3/5V at the connector, but you might be starving things a bit when there is a lot of draw on those rails. This could lead to instability, or it might just work!

Watch out on the ATX PSUs; with any relatively recent ones there is usually a maximum wattage between the 3.3 and 5V rails as they share a converter.
 

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