DIY cooling questions: Volts, ohms, series, parallel.

Discussion in 'Home Theater Projects' started by Jeff Meininger, Mar 27, 2003.

  1. Jeff Meininger

    Jeff Meininger Second Unit

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    I have a rather interesting idea for shutting up the loud fan in my Harman Kardon AVR-525 receiver. I want to use one or two ultra-quiet PC case cooling fans to constantly suck air out of the top of the receiver's case.

    PC cooling fans are designed to run on 12V DC, but it's popular to run them on 7V DC to slow them down a bit. I have a universal AC/DC adapter that can supply either of these voltages.

    If I use one fan, it seems that I could just set the adapter to 7.5 volts and connect it.

    But how would I configure two fans? The only things I know about wiring and electronics I've learned in the realm of DIY audio. So I know that wiring two 8 ohm speakers in parallel will give you a 4 ohm load, etc. But now I'm talking about cooling fans. I have no idea about the resistance properties of these things, and I have no idea how my little wall brick would handle fans wired in series vs. fans wired in parallel. I also have no idea if either scenario affects the voltage delivered to each fan. Will fans wired in series from a 12V source each receive 6V?

    Anyone have any advice for me on this?
    THANKS!
     
  2. Brian-K-Owens

    Brian-K-Owens Stunt Coordinator

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    If you wire them in series, it will require double the voltage vs. a single fan, but the current will be the same. If you wire them in parallel to the source, the voltage will be constant across them, and the current will be doubled.

    Example:

    A single fan requires 12 volts and 5 amps of current.

    If you wire 2 fans in series, you need to apply 24 volts so they will each get 12 volts. The current will be 5 amps.

    If you wire the 2 fans in parallel to 12 volts, they will each get 12 volts, but it will take 10 amps from the power source.

    Brian
     
  3. Dave Milne

    Dave Milne Supporting Actor

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    Jeff,
    You're right... two identical fans wired in series and connected to 12V would each receive 6V. I'm not completely sure a series connection will work, though. DC motors usually have a commutator which briefly interrupts the path. You can always try it and find out.

    I would recommend wiring them in parallel. With a 7V source, each fan would receive 7V. Just make sure the current capability of your adapter is higher than the sum of the current loads of the two fans.

    Personally I would also investigate whether or not the two PC fans at 7V will provide adequate cooling. I would hate to see you fry your receiver in your quest for a little less fan noise. My guess is that HK didn't install a deliberately noisy fan... and Austin isn't exactly cold in the summer. But if you have lots of clearance around the receiver, and your room is tightly climate-controlled, you can probably fudge a bit with fan flow. Perhaps you can compare CFM ratings (if you can get these as a function of input voltage). I would imagine two identical PC fans in push-pull would be roughly double the CFM of one. Or perhaps you can actually measure and compare internal temperatures on your receiver with the old fan and the new.

    Finally, realize that modifying the existing fan will probably void your warranty. Maybe the existing fan with some sort of acoustic treatment near the exit would solve your problem without introducing these other risks.
     
  4. Jeff Meininger

    Jeff Meininger Second Unit

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    Thanks for the help, guys! Hopefully I'll be able to find the current specs for the fans and adapter I'll be using.

    I'm not planning on modifying the receiver by replacing or defeating the built-in fan, I'm just trying to prevent the fan from coming on by providing active/forced cooling. The receiver's fan comes on about once every 10 minutes or so, presumably due to heat build-up. It's loud and distracting enough that guests watching a movie at my place for the first time turn their eyes away from the screen to find the source of the noise the first few times it kicks on. That sucks.
     
  5. Rick Guynn

    Rick Guynn Second Unit

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    Two case fans running at 7V is more than enough to cool a receiver. I am using two fans running at 6V to cool my equipment case with no difficulties. I run them in parallel with a selectable-voltage power supply plugged into the switched outlets on the prepro.

    RG
     

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