Problem with fan swap on old generic PSU

geekbanter · Post by **geekbanter** » Fri May 19, 2006 9:29 pm

I busy rebuilding one of my older machines for my grandfather to play flight sims on. The system has an old 300-400W PSU and the fan (80mm) had a worn bearing. I opened the PSU case for two reasons 1) to replace the fan, and 2) cut a hole for a duct that will suck air through the Thermaltake SilentBoost and exhaust out through the PSU. (Have to, the cheap $14 mATX didn't have an exhaust!)

The fan iself is wired to the PSU, I clipped the wires, removed the fan, and taped the wires up out of the way. I then installed a YL 80mm I had lying around. The problem is the PSU won't boot without the fan circuit being closed.

So my question is: how do I get the PSU to power on without the fan connected? I didn't think it would be a good idea to simply short the wires, so do I use a resistor or something? I currently have the fan wired into the circuit, but I would like to hook up a fanmate between it and the motherboard in the future.

Thanks in advance, I'm sure my grandpa will be happy when he gets his new box with the amazing power of an AMD XP 1400+ and...OOOH!... a GeForce 2 card!

I'll likewise be happy when I take his old PC (400Mhz eMachine) and make it into a NAS for my network.

jaganath · Post by **jaganath** » Sat May 20, 2006 1:44 am

I'd have thought soldering a resistor in there would "fool" the PSU into thinking it is powering a fan, although I don't know if a DC fan truly acts as a resistive load. The key question is what value of resistor do you get.

justblair · Post by **justblair** » Sat May 20, 2006 6:30 am

You could do a lot of maths to work that out, or you could do what I tend to do.

Take a variable resistor, turn the dial till you get the behaviour you wish. Then Either leave it in place, or measure it with a voltmeter and replace with a resister to that value.

Gojira-X · Post by **Gojira-X** » Sat May 20, 2006 9:23 am

When i did a fan swap in a my old Mercury/Kobian 300W PSU i had the same problem i.e the fan wires were soldered to the pcb.
I did almost exactly the same as you and chopped off the old fan, but that is were the similarity ends.

I bought a FEMALE molex connector (Maplin 85p) and crimped that to the ends of the fan wires. That enabled me to plus in my choice of replacement fan in (I-Chia 80mm fan ~25dB at 12V).

If you are clever (and good at soldering) you can solder a variable resistor on the positive (red) wire so that you can increase or decrease the flow/noise of the fan you are using.

Mind you, if you are that good at soldering, you might as well unsolder the old fan wires and solder in a fan socket taken from a cheap fan extension cable so that you can plug a fanmate in and route stick the fan mate to the outside of the case.

I did somat similar to that when I originally did a fan swap on my Xelo Gf4mx440, before I broke the fan connector that I had transfered on to a 40mm Bi-Sonic fan.

As you can see from my sig, I dont run the 300W psu anymore, mainly because it was too unstable on the 3V lines, so I swapped it for a Mercury 400W psu from my mums old computer.

geekbanter · Post by **geekbanter** » Sat May 20, 2006 12:38 pm

I went running off to my box of assorted parts, "goodie goodie goodie!" I was sure I had a variable resistor, except I haven't the foggiest idea where my box of parts is.

I guess I'll have to buy one online, since I'm pretty sure my local Radio Shack has been turned into basically a cell provider with RC cars, bleh, but it doesn't hurt to see what they have in stock.

I hope it works, thanks guys.

GHz · Post by **GHz** » Sat May 20, 2006 2:12 pm

Just buy a zener diode and solder the fan leads where the original fan was attached. Be sure to use shrinkwrap or electrical tape to prevent shorting.