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[on:]

Hi.. check the pics first

1) Did I put the pwm ground in the right place? ..& in general can grounds of differing voltage be merged? eg -12v & -5v ..

2) How to create 2 or more voltage rails from a single voltage source?

3) I see the recommended operating voltage for the Arduino is 7-12V ..is there any reason to use 12V over say 10V ..my Arduino/fan project will be running 24/7/365 ..I'm thinking lowering the voltage from max might prolong the life of the regulator ..or is that nonsense

[Reply #1 on:]

You should swap the position the fan and transister.
Silicon N-channel wants to work by sinking current to ground, vs sourcing current from a positive supply.

The fan spec doesn't say how much current it needs at startup.
Don't know if that will smoke this transister or not.
http://www.digchip.com/datasheets/parts/datasheet/406/2SK3065-pdf.php

The 47K will keep the MOSFET gate from going very low (into the off state) when the NPN transister is turned on. You can get rid of it - the 22K above the collecter will provide the all current limit you need (about 0.5mA). Might even consider making that a 10K.

All grounds must be connected. In this case, the 12V supply GND in the barrel connecter is the arduino GND as well, so they are connected.

[Reply #2 on:]

The fan spec doesn't say how much current it needs at startup.
Don't know if that will smoke this transister or not.

couldn't find a spec sheet just this from Antec:

Note: The minimum voltage to start the fan is 5V. We recommend our users to set the fan speed to High if you choose to connect the fan to a fan control device or to the Fan-Only connector found on some of Antec’s power supplies. A fan-controlled-device regulates the fan speed by varying the voltage to it. The voltage may start as low as 4.5 V to 5V. Connecting a TriCool set on Medium or Low to a fan-control device may result in the fan not being able to start. The already lowered voltage from the fan control device will be further reduced by the TriCool circuitry below 5V.
http://www.antec.com/pdf/manuals/tricool_install.html

not sure if I calc this right but: 5V/1.18w ..is a start-up current of 236ma ..from what I can make out from the data sheet the max continuous current of the 2SK3065 is 2A which would we well within the range ..again not really sure
http://www.farnell.com/datasheets/612773.pdf

You should swap the position the fan and transister.
Silicon N-channel wants to work by sinking current to ground, vs sourcing current from a positive supply.

The 47K will keep the MOSFET gate from going very low (into the off state) when the NPN transister is turned on. You can get rid of it - the 22K above the collecter will provide the all current limit you need (about 0.5mA). Might even consider making that a 10K.

errm ..my electronics knowledge is a bit limited, I grabbed grumpy-mikes schematic from this thread, :
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1240301018

should I change it as you suggest?

[Reply #3 on:]

should I change it as you suggest?

If you do you will only get on off control. The filter that precedes it generates a smooth DC voltage and the FET runs in linear mode.
I used this circuit on a very expensive piece of equipment for maximum reliability of the fans. By expensive I mean it sold for $25000

[Reply #4 on:]

I used this circuit on a very expensive piece of equipment for maximum reliability of the fans. By expensive I mean it sold for $25000

thanks, for confirming ....that sounds exactly what I'm looking for