hi all,

i have a little problem: i have a 12V fan, but most powerful than a standard fan: it takes 1mA when it works.

Now i need a circuit to change fun speed from the motherboard, but the moderboard can't give 1mA.

I've tought to an analog optocoupler.. as an HCNR200 ... or using a darlington transistor, but seems not so easy to design a working circuit...
Anyone have some working circuit to use ?

Posted on 2004-04-26 07:01:54 by Bit7
Yellow wire and ground. Insert some series resistance in the 12V supply line. If you know power consuption of fan than easy to calculate. P=E squared over R. Nothing fancy. Make sure resister is propper wattage. If you don't have resistor wattage than what ever value you come up with you can acheive in either parallel or serial as long as wattage ratings add up to power. In other words if you need 100 ohms at one half watt then use two series 50 ohm 1/2 wat or two parralled 1/2 watt 200 ohms.
Posted on 2004-04-26 14:18:21 by mrgone
thx mrgone :) but i also need the speed regulation that come out from the motherboard fan connector :(
Posted on 2004-04-27 01:22:36 by Bit7
Read this article first:http://www.analog.com/library/analogDialogue/archives/34-04/fan/index.html

The circuit in fig 7B should do the trick for you (of course, the "-" connection of the fan is grounded). It essentially regulates the fan voltage to be proportional to the output from the motherboard. The proportionality constant is given by the ratio of R2 to R1, so you can adjust one of them to get the correct airflow from your fan.

This linear circuit is easy to build and should work satisfactorily (unless you want to try the PWM technique).
The opamp can be LM358, but if you power the fan from +12V, then the opamp should be powered from +12V as well, otherwise it may not decrease the fan speed enough. Use a decoupling cap for the circuit (10-100uF), and maybe a separate one for the opamp.
Posted on 2004-04-27 11:29:36 by VVV
infinite thanks VVV, you're great :)
Posted on 2004-04-27 17:36:17 by Bit7