| A startup helper for 12V DC fans running at 5V |
A lot of overclockers prefer to run their casefans at 7V or even 5V instead of 12V to
reduce noise. In most of the cases this works well, but as the age of the fans increase,
it is possible that the fan won't start with low voltage when the computer is turned on.
The solution to this problem is to apply the full 12V during startup, and reducing the
voltage as soon as the fan is spinning. A simple circuit which does exactly this is given
below.
Simple fan startup helper circuit.
This circuit works as follows: when the PC is turned on, the capacitor starts to charge.
The current it needs to charge is flowing through the fan, which increases the voltage
over the fan. During the capacitor charging time, the voltage over the fan will drop until
it reaches 4.4V. From that point on the 1N4001 diode connected to the +5V line starts to
conduct, providing the fan power until the system is switched off. The diode switched over
the capacitor provides a bit of usually unnecessary protection.
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