Any electronic bods here ?
 

Having done searches with Google and through many forums I have come to the conclusion that what I am suggesting here hasnt been done. Please correct me if I am wrong.

High power fans are not able to be connected to mobo headers due to their lack of current handling abilities, usually 0.5A max. So fans then have to be connected to molex connector via adaptor for supply.

The problem with this is although the speed can be monitored by wiring back to the mobo fan connector, it now lacks speed control by software like Speedfan or similar.

Would it not be possible to make a small circuit that adds pass transistor or FET output stage that is controlled by the mobo header but is powered from the molex.

To me this has to work. Can some one draw us a circuit for this?

Thanks

I would think that would work.

A sufficient NPN transistor could be connected as follows: base to the positive lead on the fan header; collector to the +12V on a standard 4-pin molex. emitter to the positive lead on the fan. Then, fan's ground lead connected to the ground lead of the 4-pin molex. RPM signal connected if desired.

Another method may be to use a high-speed optical switch or solid state relay. If you could find one of sufficient characteristics, you could use the positive & ground leads of the fan header to control the switch. -- Paul

Here is one circuit that could be adapted for what you need.

see http://www.bit-tech.net/article/51/

It's Pulse Width Modulated circuit is more efficient that a regular transistor circuit and it's produces less heat.

Best regards

frenchnew