Beginner needs help with DC/DC regulator project

[toblin]

Hello everybody, I'm a swedish EE student. Since school is mostly theoretical I've decided to do some projects on my own. However due to lack of experience there are many things I don't know how to do. So here follows a summary of my current project and a series of questions that I would like answered so that I can learn how to do stuff properly.

As my first project I wanted to do a fan controller for my PC. Converting 12V DC to somewhere between 3 and 12V and also change it during operation is the main task. However, the fans need a higher voltage to spin up than what they need for operation. So i want the controller to put out about 12V at startup and then decrease to the set voltage.

Google is my friend and after a short while I found a schematic that seems to do exactly what I want. The schematic can be found here:http://www.overclockers.com/forums/showpost.php?p=5270425&postcount=77

The thing I don't understand in this circuit is transistor and the surrounding components which I assume is the kickstarting part. How does one calculate what happens when the power turns on for a circuit like this? And a small other thing is that I have no knowlege about diodes so do the diodes in this have any other purpose than protection?

Despite not really knowing if and why the circuit works I've designed and orderd a custom PCB and hopefully I'll get that in a couple of weeks. But one thing I do realize is that with a linear voltage regulator and a few amps it will get HOT! So started thinking about changing the regulator for a switching regulator. So I started looking in to the theory about them and stuff and then started looking at the LM2678 switching regulator since it looked like a very basic regulator (datasheet: http://www.ti.com/lit/ds/snvs029i/snvs029i.pdf ). But looking through the datasheets there are a lot of info about choosing the right inductor and caps to a specified output voltage but since I want to change the voltage during operation, how do I pick what components I need? And also, can I use the same type of feedback as with the linear regulator and will the kickstarting parts work with this regulator?

Thanks in advance for taking the time to help me 

[jakeypoo]

Well that's a lot of questions. I would tell you you're going about all of this wrong, but if you already ordered your pcbs....    lets try to answer your questions [EDIT: nope couldn't do it]. Or at least get you asking the right ones.

The thing I don't understand in this circuit is transistor and the surrounding components which I assume is the kickstarting part. How does one calculate what happens when the power turns on for a circuit like this? And a small other thing is that I have no knowledge about diodes so do the diodes in this have any other purpose than protection?

You should really figure these things out before you go out and start building a circuit. Go read the LT datasheet (http://cds.linear.com/docs/en/datasheet/108345fg.pdf ). The diodes are flyback diodes, so yes, they are for protection.

Yes it will get hot! That heat is wasted energy! Your fan runs on 12V, why use a linear regulator to bring it down? Which brings me to my next point: 

This is a silly way to do fan control, go read about how BLDCs work, and what is inside a PC fan: http://www.atmel.ca/Images/doc8005.pdf

If you want to build a PC fan controller, you should stick to 4-wire fans that have a tach output and PWM input. Then you can easily control them, and it's a fun project.

Lastly, don't take electronics advice from PC overclocking forums.

[toblin]

Well that's a lot of questions. I would tell you you're going about all of this wrong, but if you already ordered your pcbs....    lets try to answer your questions [EDIT: nope couldn't do it]. Or at least get you asking the right ones.

Well I think making a PCB is really fun so i did that before really figuring out the curcuit. And as long as it can't destroy anything I can't replace, I don't mind a couple of failiures. But yes, I realize this is the wrong approach.

Yes it will get hot! That heat is wasted energy! Your fan runs on 12V, why use a linear regulator to bring it down? Which brings me to my next point: 

This is a silly way to do fan control, go read about how BLDCs work, and what is inside a PC fan: http://www.atmel.ca/Images/doc8005.pdf

If you want to build a PC fan controller, you should stick to 4-wire fans that have a tach output and PWM input. Then you can easily control them, and it's a fun project.

I already have the fans that I want to control and they are 3-wire fans. These won't be replaced, mainly for easthetic reasons. And I want them to run quieter since the cooling for my PC is really overdimensioned at the moment. And I also wanted the controller to be just plug-and-play, no programing required.

Lastly, don't take electronics advice from PC overclocking forums.

Yeah I'll keep that in mind for later projects.