# 4 pin (PWM) to 3 pin (DC) fan adapter

#### obones

Joined Jan 23, 2008
1
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:
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
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:

Rich (BB code):
                    +12
o
|
.-.
.-->| |
|   | |
|   '-'
|    |
'----+                +12
|                 o
.-.                |
| |              |<
| |         .----|
'-'         |    |\
___           |        |/       |
PWM o-|___|---->|----+--------|        +--o To Fan +
|        |>       |
.-.         |     ---
| |         o     ---
| |        GND     |
'-'                o
|                GND
o
GND
What do guys think of these ideas? What values could you suggest for the resistors and capacitors?

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