I'm planning a project where I will be driving a small brushless DC "cooling fan".
[This is so important that I should put the "HELP!!!" and URGENT!!! in the title, as this is for air-flow control for my outdoor cooker (UDS -- Ugly Drum Smoker). I built a 16-gallon.]
I (in all probability) need very little "push" on the air flow. I plan to adapt the business end of the fan through couplings and into one of the four ~3/4"/19mm air holes at the base of the smoker that allows combustion air to enter. (A normal cook at 220-230 degrees F has 1.5 holes open.)
I've found the fans as low as US$1.08 each on eBay from China, shipping included. Anyway, absolute cost isn't critical.
I've been looking at 25mm and 40mm units, two wire, 5V and 12V. The rating is like 100mA give-or-take.
Q1: I should be able to vary the drive voltage to control speed, right?
So, if I use AVR8 PWM, what filter do I use between the AVR and the fan to get a rough approximation of DC voltage? Or if I use a fast PWM (tell me what "fast" is --1kHz? 10kHz?) can it just be done directly?
Q2: As the 5V fan is rated at e.g. 100mA but I only want to drive it at say 1/10 speed, what will be the power draw?
Q3: I was thinking of just using a pair of AVR pins from the same port with current-limiting resistors (which then serve as part of the RC filter?), and driving direct. Bad idea?
Q4: Or do I use a MOSFET? If I do, then do I want fast PWM or slow PWM?
As I anticipate that very little extra push will be needed from the fan, I was thinking of doing very coarse bang-bang control. Like "on for 1 second out of 20" for lowest setting; 2 seconds of 20; etc. Good or bad idea?
I might do this part first for feasibility. The plan is to integrate a thermistor or PT100 probe for temperature setpoint and closed-loop control.
Awful lot of work for a UDS, eh?