Slowing fans down to make them quieter, startup circuit
Some DC powered vent and cooling fans can be annoyingly noisy, and slowing them down sometimes helps. Assuming that the fan still does enough cooling, you can run the fan on lower voltage. But beyond a certain point, the fan won't get started below some voltage. But if you gently finger the blades it will stay spinning. One way to "kick start" the fan is to have it see a higher DC supply voltage upon system power-up, and have it drop to a quieter operating voltage. My example is a small fan that is rated for 12V and half an amp. And I used a 200 ohm resistor in series of a 12V power source to drop the voltage and thus operating speed to a reasonably quiet and still effective cooling level. But it won't start like this. So I added a 1000uF cap across the resistor. This cap is initially discharged at power-up, and the fan will see for a short while a full 12V supply, then the cap charges up, and then the fan sees a lower supply voltage. This short period of higher DC voltage is enough to get the fan spinning, and once it is spinning, it keeps spinning at a lower rate at the lower voltage. And be less noisy.