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

Discussion in 'The Projects Forum' started by obones, Jan 23, 2008.

1. ### obones Thread Starter New Member

Jan 23, 2008
1
0
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:

Code ( (Unknown Language)):
1.
2.                   +12
3.                    o
4.                    |
5.                   .-.    +12
6.                   | |     o
7.                   | |     |
8.                   '-'  ||-+
9.                    |   ||->
10.                    +---||-+
11.                    |      |
12.        ___       |/       +---o To Fan +
13. PWM o-|___|--+---|        |
14.              |   |>      ---
15.             .-.    |     ---
16.             | |    o      |
17.             | |   GND     o
18.             '-'          GND
19.              |
20.              o
21.             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:

Code ( (Unknown Language)):
1.
2.                     +12
3.                      o
4.                      |
5.                     .-.
6.                 .-->| |
7.                 |   | |
8.                 |   '-'
9.                 |    |
10.                 '----+                +12
11.                      |                 o
12.                     .-.                |
13.                     | |              |<
14.                     | |         .----|
15.                     '-'         |    |\
16.        ___           |        |/       |
17. PWM o-|___|---->|----+--------|        +--o To Fan +
18.                      |        |>       |
19.                     .-.         |     ---
20.                     | |         o     ---
21.                     | |        GND     |
22.                     '-'                o
23.                      |                GND
24.                      o
25.                     GND
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