I'm making a solder fume extractor from an old PC fan (4-pin PWM), driven by a mid-range PIC.
The fan needs 12V @ max 0.28A for power and a 5V PWM @ max 5mA to control the RPM. So I'll be running the PIC at 5V and therefore I will need both 5V and 12V available. I assume the PIC won't consume much power compared to the fan, even though I plan to have an IR proximity sensor too so I can ramp up the fan speed when my hands move towards whatever I'm soldering and then ramp down again when I'm done.
I haven't yet decided whether to use a wall wart or batteries, but would like to know the pros and cons of the available options.
So, as an example, I assume I could power the system with a 5V wall wart and use a DC boost converter to get the 12V for the fan.
Or, I could power the system with a 12V wall wart and use a DC buck converter to get 5V for the PIC etc.
Apart from the cost and availability of parts, what criteria are there for deciding to go one way or the other? This is a one-off personal project, so commercial considerations are less important (although still interesting), I'm thinking there may be practical issues that I'm unaware of (e.g. noise between power rails, efficiency?).
Could someone give me some insight into how these decisions are made?
Best Answer
For this application a buck or boost converter would be overkill. Your best option would likely be to have a 12v source and then take it down to 5V separately with a linear regulator. Cheap, few parts, most likely parts you already have, etc.
Dropping voltage with a linear regulator generates heat based on the current drawn through the regulator and the voltage dropped. This should not be an issue as the PIC will likely not draw much current.
However, a simple "cheat" way to drop the voltage enough that the regulator won't have a large voltage drop across it is to put one or more diodes in series before the regulator and use them to drop the voltage by ~.7v to 1.4v each depending on the diode. Dropping the voltage to 7v for a 5v regulator should be fine and allow enough headroom for the dropout of the regulator. Again, simple design and parts that you might readily find off the shelf or even salvage from old stuff.
Cheers