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. John edit: Possibly a combination of both circuits? Two 12V zeners between the +40V and the LM7812 input could lower the voltage difference on the 7812.