NERD ALERT Pure Electronics Question
I want to use 78xx voltage regulators in my 36 volt lighting. To power integral with heatsink 1 watt fans. Clarification: I purchased new surplus PC obsolete heatsinks with ball bearing fans for less ...
Forum Discussion
GordonThree wrote:Vref for the LM317 is fixed at 1.25V nominal. And, the only power related to it goes into the resistor, which can be a couple of watts at higher current settings.
For this design, keep in mind:
Vref * Iout = Pwaste (heat), heat in the chip and the resistor, watch the wattage rating for R ... under a buck for the chip maybe, but more $ on heatsink?
Not sure what you're calling Vref here, it's a current regulator. Nothing here is referenced to ground. The dissipation depends entirely on the current and the voltage differential across the LM317 needed to maintain that current. The closer the input voltage is to the required output voltage, the less heat. Dropout voltage is about 2.5V @ 1.5A so yeah, about 4 watts minimum dissipation. It will need a heatsink if driving more than about 0.5A with a minimal voltage difference.
