Saga of Sunon fan in Acer PC

As regular readers might remember, when IBM’s PC division became Lenovo I got worried, and bought a few Acer machines to see if they were any good. Their backup was dreadful, so I stuck with Lenovo. As is the way of things, one of the samples ended up as my main PC and has been purring along ever since. Until this morning.

It showed all the symptoms one would expect of a dead PSU. That is to say, the mains lead was live but the PC wasn’t.

Luckily the PSU is pretty standard and I had a spare on the shelf, but while I had the case open I gave the fans a twirl. I didn’t expect to find a problem as the machine ran silently, but to my amazement the CPU fan was ceased. Completely solid. I couldn’t shift it.

I removed the heat sink to get the fan off, and saw to my disgust that it was a special with a built-in thermistor and a fourth wire on the cable.

Computer case fans generally have two or three wires. If it’s just two it’s simple, a +ve and GND. The third wire is a spin sensor: usually yellow wires give a pulse as the fan completes a revolution and this the motherboard (or fan controller) can sense the actual fan speed. A white third wire generally indicates either spinning or ceased up completely. The type you need depends on the complexity of your control system.

The fourth wire, if there is one, tends to be for controlling the fan speed. There are basically two ways to vary the speed of the fan – vary the voltage or modulate it. Varying the voltage can be a bit tricky: dropping a voltage generally means converting it to heat somewhere along the line, and this is best avoided. Pulse Width Modulation, on the other hand, is great. You keep the voltage the same but you turn it on and off. If it’s off for 50% of the time and on for 50% of the time you’re only getting half the power to the fan, so it’ll turn half as fast (gross simplification, but you get the idea). The pulses, of course, have to be fast. Switched mode power supplies work using the same principle.

Naturally I had a box of fans, but none of them supported pulse-width modulation. I pretty much knew that before I looked. Never mind, I though – I’ll run a standard fan at a fixed speed and be done with it. Foiled again! This fan is 20mm thick whereas every other 80mm fan is 25mm thick. And the extra 5mm matters, because it won’t fit on the heat sink otherwise.

The fan in question is a Sunon FMD1208PKV1-A. Decoding this shows that it’s a FMD series, 12V, 8cm, 20mm thick and so on but doesn’t say whether it’s a ‘special’ – that could be what the –A is all about as Sunon do make special versions for OEMs.

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It’s actually quite a fancy fan – maglev bearing and other leading-edge refinements. According to its data sheet it can shift more air with thinner blades than most of the competition. Hmm. It’s not like the case is so cramped that 5mm had to be shaved off the thickness of the fan!

A quick trawl revealed that RS Components (Radio Spares to my generation) stocked the beast, for about €30. Sorry, RS, but I don’t do Euros and certainly don’t have 30 of them to spend on a fan.

Com-Com, who recon they carry spares for most servers, advertised the part but you have to call them for a price. So I did. They don’t actually have any, but could get a box of 100 if I really wanted them that badly. Well they are nice fans and 100 would keep me going a long time so I enquired as to the price – £1500. They’re nice fans, but not that nice. The bloke there suggested I call Acer to see if they can sell me a spare. Calling Acer “Customer Services” is bad for my blood pressure, and has never resulted in anything good – basically some fool reading from a script that appears to have inadvertently been designed put you off buying anything for Acer ever again.

Next I tried running the box without a fan, as it clearly hadn’t had one for a while. The CPU temperature was hovering around 70C, which is a bit hot. In-spec, possibly, but not the best way to ensure it has a long and healthy life.

So I had a closer look. The fan can’t fit far enough down the heat-sink due to its thickness – it fouls the fixing posts. However, it’s held on to the heat-sink with a pair of off-set brackets, and these are reversible. If you swap them over the fan will just about clear the fixing posts once they’re screwed down. That’s the key…

Remove the heat-sink, fan and brackets. You then have to replace the heat-sink and screw it down – the screws will be inaccessible with the fan in place. Then re-fit the brackets the wrong way around (swap them over). It’s a fiddle, but you can screw the fan onto the brackets with the heat-sink in position – well the outer two anyway. The inner two could probably be managed if you were keener than I was.

A standard three-pin fan plug will fit the 4-pin connector on the motherboard; just push it over the appropriate three pins and avoid the one the blue wire would have been connected to.

Although the fan is now offset by nearly an inch, the CPU is really cool – about 25C. The down side is that it’s always running full-blast and it’s a tad noisy. Does anyone know where I can get a FMD1208PKV1-A with a PWM wire cheap?