How can I divide a variable voltage in half for a fan (variable load)?
You can use a current boosted op-amp design with a voltage divider on the input. something like:







simulate this circuit &ndash; Schematic created using CircuitLab

The op-amp is powered directly from the fan supply, and always biased at half the input voltage. The BJT acts as a current boost for the op-amp, and will of course drop the same power as the fan, so you will need a transistor capable of taking at least as much current as required by the fan, and probably need to mount it on a heat sink.