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Homemade case fan controller...advice? Also, your crazy case mod stories.

mcdermottmcdermott Registered User regular
So, here's my idea, in a nutshell:

Use a micro controller reading the tach on the fan, then based on the current RPM adjusting a digital (8-bit) signal to a DAC, which in turn adjusts a voltage (up to 12V), which in turn goes into a voltage follower feeding a single-transistor current booster (to provide enough current to the fan, which the DAC won't do). The current booster feeds back to the voltage follower (ensuring the proper output voltage to the fan) and then the fan feeds back the new RPM to the microcontroller, closing the loop.

So basically:

Fan RPM signal -> Microcontroller -> DAC -> Current Booster -> Fan RPM signal

The idea is to use the reset button on my computer (which I don't use) to toggle between a few pre-set fan levels (low/med/high, maybe up to five levels) to control one or more of my (non-PWM) case fans. Total cost for parts looks to be around $10. I've got half the code for the MCU written already (yay for keeping old lab projects).

Good idea? Bad idea? What are the chances this will either set my entire computer on fire or generate more heat than my fan even dissipates?

I think it seems like a fun project in my spare time, and I have access to all the test equipment I need before I place it in the system.

Note: I am an electrical engineer, so I kinda sorta know what I'm doing. Just looking for second/third opinions.


Also, feel free to talk about any other crazy case mods you've attempted, and the triumph or disaster that ensued.

mcdermott on
Spoiler:

Posts

  • ecco the dolphinecco the dolphin Registered User regular
    edited September 2009
    Looks fine!

    Just a few thoughts:

    I'd consider using a PNP transistor as the main power transistor (it's easier to get closer to the rail than an NPN).

    Instead of a DAC (unless you have one built into your micro), I might consider doing a PWM into a simple RC filter. You're going to buffer/amplify anyway, so why not use a low pass filter instead?

    Penny Arcade Developers at PADev.net.
  • mcdermottmcdermott Registered User regular
    edited September 2009
    Looks fine!

    Just a few thoughts:

    I'd consider using a PNP transistor as the main power transistor (it's easier to get closer to the rail than an NPN).

    Instead of a DAC (unless you have one built into your micro), I might consider doing a PWM into a simple RC filter. You're going to buffer/amplify anyway, so why not use a low pass filter instead?

    Thought about that, actually (and already have code written for it). I guess I was just concerned about any unnecessary oscillation, probably for no good reason. That, and an 8-bit parallel-input DAC costs like $1. Lastly, because the MCUs I have sitting around only have two timer channels...and the PWM would take one. Whereas if I use a DAC, I can use the GPIO bits to run them, and use two input channels to read the RPM in on two fans...basically it makes the whole thing scalable. Hell, if I don't care about accurate RPMs (and maybe just voltage levels instead, so I could use the ADC to monitor) I could easily run up to four or so just on these 16-pin MCUs I've got.

    And I've never really worked with a DAC before, so I thought it could be interesting. <img class=" title=":wink:" class="bbcode_smiley" />

    I'll note the PNP advice, though.

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