Hi all,
I have a PC here with a motherboard for which the CPU fan port has 4 pins, which assignments are as follow:
+12V
GND
Tach (speed sense)
PWM (speed control)
However, the fan that I'm using only has 3 wires and as a result cannot react to the PWM signal sent by the motherboard to regulate the speed according to temperature.
So my project here would be to create a very simple circuit to take the PWM signal and regulate the +12V, with a minimum voltage applied to the fan in case the PWM time off is too high.
Note that I have found a product that claims to do this "out of the box", but I'd really like to be able to do such a circuit myself, as I find it a nice way to learn the basics here. The product datasheet is visible here:
http://www.paqt.co.uk/docs/PaQ_PWM.pdf
Looking around, I found this post in the forum:
http://forum.allaboutcircuits.com/showthread.php?t=6266
The TI application note attached to it helped me figure out that the PWM signal is of the form +12V = On and GND = Off
So my initial thought was to simply control a MOSFET with the PWM signal, with protection on the gate, obviously. Something like that:
But this does not give me any control on the minimum voltage applied to the fan. So my next idea is to use two bipolar transistors with a minimal gain on the first one. Something like this:
What do guys think of these ideas? What values could you suggest for the resistors and capacitors?
Thanks in advance for your help
Edit: Apparently, I was wrong assuming that the PWM is +12V as I found the following document :
http://www.formfactors.org/developer\specs\REV1_2_Public.pdf
It is specified at +5V, a TTL level then.
But I don't believe it changes my proposed designs
I have a PC here with a motherboard for which the CPU fan port has 4 pins, which assignments are as follow:
+12V
GND
Tach (speed sense)
PWM (speed control)
However, the fan that I'm using only has 3 wires and as a result cannot react to the PWM signal sent by the motherboard to regulate the speed according to temperature.
So my project here would be to create a very simple circuit to take the PWM signal and regulate the +12V, with a minimum voltage applied to the fan in case the PWM time off is too high.
Note that I have found a product that claims to do this "out of the box", but I'd really like to be able to do such a circuit myself, as I find it a nice way to learn the basics here. The product datasheet is visible here:
http://www.paqt.co.uk/docs/PaQ_PWM.pdf
Looking around, I found this post in the forum:
http://forum.allaboutcircuits.com/showthread.php?t=6266
The TI application note attached to it helped me figure out that the PWM signal is of the form +12V = On and GND = Off
So my initial thought was to simply control a MOSFET with the PWM signal, with protection on the gate, obviously. Something like that:
Rich (BB code):
+12
o
|
.-. +12
| | o
| | |
'-' ||-+
| ||->
+---||-+
| |
___ |/ +---o To Fan +
PWM o-|___|--+---| |
| |> ---
.-. | ---
| | o |
| | GND o
'-' GND
|
o
GND
Rich (BB code):
+12
o
|
.-.
.-->| |
| | |
| '-'
| |
'----+ +12
| o
.-. |
| | |<
| | .----|
'-' | |\
___ | |/ |
PWM o-|___|---->|----+--------| +--o To Fan +
| |> |
.-. | ---
| | o ---
| | GND |
'-' o
| GND
o
GNDThanks in advance for your help
Edit: Apparently, I was wrong assuming that the PWM is +12V as I found the following document :
http://www.formfactors.org/developer\specs\REV1_2_Public.pdf
It is specified at +5V, a TTL level then.
But I don't believe it changes my proposed designs
