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Voltage regulator for cooling fan

  1. Mar 28, 2010 #1
    Hey all,

    I am working on a 0-40V variable power supply right now, and while the power supply is working I am wondering if it is in danger of overheating. I am using a LM317 with a 0-5k potentiometer between adjust and ground, and a 165ohm resistor between output and adjust. Input is regulated from the wall to 40V.

    My question is, that since the voltage across the LM317 varies from about 1.25-38.75 inversely with the output voltage, is a fan necessary (I am using a heatsink) to prevent overheating? I know it features internal overheat protection but I would rather avoid having to trust it.

    My next question is, if a fan is required, would a 12V fan I pulled from an old computer work?
    Which of these two circuits would waste the least power?

    Circuit 1: Input voltage of 40V, with zeners in series up to 28V. Cooling fan in series takes the other 12V.

    Circuit 2: LM7812 +12V regulator. Seems like the best idea, but the input voltage is +40V and so it seems like it might waste a lot of power.

    Thanks, and sorry for long post.


    Possibly a combination of both circuits? Two 12V zeners between the +40V and the LM7812 input could lower the voltage difference on the 7812.
    Last edited: Mar 28, 2010
  2. jcsd
  3. Mar 28, 2010 #2
    I use a couple of 12V pc fans to cool both a laser and the LM317 attached to a CPU heat sink. I drilled a hole in it and mounted the laser and then attached the LM317 metal tab to it with a self tapping screw. The two 12V fans were mounted either end so the air is blown through the heat sink. Works very well. If you can get hold of a CPU heat-sink then you will have a stable platform to mount the fans on to. Good luck. Opps nearly forgot, remember the circuit depends on the current you will be drawing, first sounds more straight forward.
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