Beginner needs help with DC/DC regulator project - Page 1
Forum section: electronics / beginners

author: toblin
author_rank: Newbie
timestamp: July 31, 2014, 09:42:30 pm
content: 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=77The 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

author: jakeypoo
author_rank: Regular Contributor
timestamp: August 01, 2014, 02:19:21 pm
content: 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.QuoteYou 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.pdfIf 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.

author: toblin
author_rank: Newbie
timestamp: August 02, 2014, 11:51:50 am
content: Quote from: jakeypoo on August 01, 2014, 02:19:21 pmWell 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.Quote from: jakeypoo on August 01, 2014, 02:19:21 pmI 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.Quote from: jakeypoo on August 01, 2014, 02:19:21 pmYeah I'll keep that in mind for later projects.