A tale of an SGI Indy, a Sony power supply, and how to keep the fan spinning

Let's not dwell on why I bought an SGI Indy. Anyway.

One of the common things that I've seen is failure due to power supplies or heat death. The irony is:

• The Nidec power supply has dirty power, fails hilariously, but the fan is at least always spinning, and
• The Sony power supply has clean power, less hilarious failures, but the fan only comes on when the unit is hot. Sometimes.

 The fan in the Sony PSU is a 12V fan, and it turns on based on a thermal control line from the Indy. There's been plenty of research into the behaviour of that signal, and I'm not going to go into it here. What I instead want to talk about is a quick way to actually just get the fan constantly spinning, without having the open up and modify the power supply itself.

The TL;DR is this:

• Make a small voltage gate using two diodes - one from the 3.3v power supply rail (via a resistor, I used 100 ohms; (1Kohm was too high) but I may try something smaller like 56 ohms to make sure enough current is flowing) and the other from the Indy board;
• That way the Sony PSU is always fed at least 3.3v into that thermal sensor input.

 

So!

• There will always be a minimum 3.3v voltage into the Sony PSU fan control, which is enough to turn it on; 
• The fan will always spin at a minimum voltage;
• if it DOES get warm enough for the Indy thermal sensor circuitry to feed above 3.3v (+ diode drop) into the thermal control line, it will also increase the fan speed.

It's not too hard - tie two diodes together at the cathode side, cut the brown control wire, feed it into the power supply, and then tie the two anodes in as above.

 

(The grey wire in the image is going from one diode anode to the 3.3v (white) wire in the Indy main PSU connector; there's 100 ohm resistor at the end of said grey wire that's temporarily jammed in for testing.)

With this the Sony PSU fan is always spinning, and your SGI Indy should die less of a heat death.