EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: ruku on April 18, 2011, 06:18:35 am
-
So I'm looking for a practical project to build, and a bench power supply seems to be like a sane thing. I started out with a LM317, went to a LM338, then to a few transistor based ideas, and then an ATX... ...and back all over again.
I could go whole hog on the design, but I really liked the idea of a single transistor power supply. Its simple enough that I don't get bogged down, but complex enough that I have to learn a few things.
Based off of this design, http://tuxgraphics.org/electronics/201005/bench-power-supply-v3.shtml (http://tuxgraphics.org/electronics/201005/bench-power-supply-v3.shtml) The TL;DR here is he's using a BJT power transistor as a voltage follower, and using an AVR microcontroller to generate the buffer levels. Since the BJT's base terminal has a low impedance (and thus requires amplification), I was thinking of using a MOSFET power transistor. My understanding is they have high input impedance, which means a voltage buffer would be more simple.
I'd like to be able to source 0 - 12 V at around 2 amps max (...25 Watt transistor?) from an ATX power supply. I've heard they can be noisy, so I figured stack the 12 V and a 5 V line together and feed it through my regulator, then use a cleaned up 5 V line to run the uC and supporting hardware. Maybe use a relay to switch between the 12/5 line and the 5/3.3 line so not so much power is wasted...
ANYWAYS. I come to my point. I'm fairly set on my single transistor design, unless there's a significantly simpler and better way of doing this. However, I haven't decided on my transistor (BJT, MOSFET, JFET?) or its characteristics. I'm also not entirely sure how to analyze my circuit... there seem to be a few ways to lay out a voltage buffer, and I'm unable to decide which is the best. Any help would be muchly appreciated!
-
...I've heard they can be noisy, so I figured stack the 12 V and a 5 V line together and feed it through my regulator...
If by stack, you mean connect the 12V and 5V supplies in series to get a 17 volt supply, it won't work. They have a common ground.
-
Do all the rails in a pc power supply share a ground our are they isolated.
-
PC PSUs are all Common Ground from what i have seen.
Regards