Electronics Need circuitry to replicate logic

I am integrating a miniITX board into my desktop/workstation to offload all of my server applications. This will allow me to shutdown my main rig when it's not being used without interrupting downloads, web/ftp server, aim, etc. The miniITX board will be mounted in the drive bays with a custom mount (it just barely fits into 4 bays vertically) with a 120mm intake fan blowing across it.

What I would like to do is have the 120mm intake be powered by either my fan controller (6-12v) or the miniITX fan header, with the fan controller taking priority. When being run by the miniITX board, I would like it to run at ~6v. It seems simple enough but my electrical knowledge doesn't span into logic. The equivalent programming is below.

Code:

if (A)
{
  input = A;
  output = A;
}
else
{
  input = B;
  output = 6;
}

Where A is the fan controller input and B is the miniITX input.

Any ideas? Thanks in advance!

 

are you using the same power supply or 2 different ones?

 

Two power supplies. A full sized 650W supply, and a picoPSU.

 

i assume that if neither is on you want the fan off...

edit: i dont know, something like this might work... i'd test it before you implement it seeing as I did not simulate it or anything...



oh, and the lamp is the fan.

 

What are the components that look like |E called? I'd like to read up on them to see what they do.

 

Triac?

 

Just use two diodes.
What you want is effectively an OR gate.

 

Fan controller -> diode
Fan header -> zener diode -> diode

Connect the diodes together to the +12v of the fan.
Dunno about the ground tho, tieing the ground lines of two PSUs together may not be great for them?

 

Shouldn't hurt, it's when you put two supplies totally in parallel you can get the fireworks.

 

@dragon: The |E things are Mosfets which are a different type of transistor.

 

yep, p-channel btw.

 

I've been busy for the past few days and didn't realize I had more responses. I remember talking about mosfets in class, but we must've never actually started using them.

Of the available choices which would be the best to use? Getting the components and soldering them onto a project board shouldn't be an issue.

Thanks again, I'm already way further than I would've been able to get on my own.

 

The simple diode gates using 1N4001 for D1, D2.

Code:

12V ----L<------->|----+-----|<----6-12V
     5.1V 1.3W   D1    |     D2
                       |
                      Fan
                       |
                       |
                       0V

The 5.1V zener and diode D1 will reduce the 12V miniITX input down to about 6.2V.

 

Simplist, easiest, and wont even need a board, just some heat-shrink.

 

i'm not sure thats a good idea considering they are different grounds from different supplies...

 

Does a power supply need to be on in order to use it's ground? If not I could just ground the whole thing to the 650W psu. Then I wouldn't be creating a common ground. I would however be crossing currents from the picoPSU to the 650.

 

The pico PSU (presumably, I wouldnt expect it to be isolated) would already share the ground with the main power supply, so you should just use one of those to begin with.