So I wanted to adjust a fan down to 3.3 volt, the fan controller only allows 5-12V. Replacement of R6 was needed:
To understand this circuit, know that any voltage higher than 2.500 volt on the Ref-pin of the Tl431 will lower the zener voltage, and any voltage lower than 2.500 volt will adjust the zener voltage to a higher value:
So what happens if you connect Ref directly to the + of the output? If the voltage on the output is 6V, the Tl431 will adjust its zener voltage down, lowering the voltage on the base of the NPN-transistor lowered, hereby shutting down the transistor. This means that the voltage on the PNP-transistor will be higher because it’s not connected to ground anymore, and because the voltage on the base of a PNP-transistor has to be higher than the voltage on the emitter to allow current to flow, the transistor will close itself.
Now there’s no way to supply current to the output of the circuit, and the voltage will be lowered until the circuit has reached equilibrium at a lower voltage. If we want to get a higher output voltage, we have to lower the feedback coming from the emitter of the PNP-transistor; this is what R6 does. So you lower R6 (a 470 Ohm resistor did the trick) and you get a lower output voltage, which is what I was after: R7 probably has to be adjusted as well to get the maximum output voltage to 12V, but why use a fan controller if you want the fan at >10V?