Hi Stephane.
Between the mbed and your two fans you may use 1 NMOSFET + 1 schottky diode. The NMOSFET must have a gate to source threshold voltage (Vgs_th) lower than 3.3 V (so that it can be turned on by the 3.3 V coming out of your mbed), a Vds_max higher than 12 V, and a Ids_max higher than approx 1 A. For instance, this one:
http://uk.farnell.com/fairchild-semiconductor/fds8884/mosfet-n-so-8/dp/1504995
and it is quite cheap.
The schottky diode must sustain 12 V of reverse voltage, and also 1 A of current. For instance, this one:
http://uk.farnell.com/nxp/pmeg3020ej-115/schottky-rect-30v-2a-sod323f/dp/1757772
Connect:
1) The two fans in parallel.
2) The ground of the mbed to the ground of your +12 V supply.
3) One node of the group of two fans to the drain (D) of the NMOSFET.
4) The other node of the group of two fans to your +12 V supply.
5) The source (S) of the NMOSFET to your ground.
6) The gate (G) of the NMOSFET to your pwmOut output from the mbed.
7) The schottky diode in anti-parallel with the group of the two fans, so that the cathode of the diode touches +12 V, and the anode touches the drain of the NMOSFET. It will act as a freewheeling diode, letting the current through the inductance inside the fans to extinguish through it, after you have turned off the NMOSFET.
That's it. Modulate the duty ratio of pwmOut as desired. The fans will produce an air flow (not linearly) proportional to that duty ratio.
Best.
Hello,
I have a bunch of electronic devices (internet DSL access, firewall, wifi router, GuruPlug, NAS...) in a cupboard in my office and as the weather is very warm, some devices are halting because of heating issues and I do not want to leave the door open...
I want to install two 8cm fans in the cupboard to get rid of warm air. I know how to mesure temperature with a one wire thermo with the mbed and I want to start the fans when Temperature is too high. I want to use a pwmOut to drive the fans from 4V to 12V.
pwmOut is 0 to 3.3.V...
I just do not know what king of electronic equipment I should use between the 2 fans and the mbed...
Anyone could give me some advice on this very easy problem ;-) ?
Thanks in advance.