Making a switchmode power supply adjustable.

Something for the tinkerers but take care, anything switchmode can be quite dangerous.
Being just a mere bloke who mucks about a bit I found out about a lovely little shunt regulator called a TL431 the other day, it seems all switchmode power supplies use these and they're effectively an adjustable zener diode. I did a bit of reading and theorising and decided to tackle a power supply. The TL431 is the TO-92 cased semiconductor shown as U2 on this board and the voltage output is set by the voltage divider made by the resistors R7 and R8. It looked very convenient that they printed a box and drilled holes for VR1.

So I removed the R7 3k9 resistor and plonked in a 5k trimpot, then magically it worked.