Dimmer or Potentiometer for LEDs

[Chris]

worth saying but dont try to control the primary side voltage, if you are using a transformer the reduced primary voltage will draw more current which will eventually cause damage. If it is a switch mode power supply, a lot of these are wide input constant output, my laptop can take a supply voltage 100-240V and still provide 19.5V so altering the input wont really effect it.

Finally there is the issue with working with mains voltage and the increased risks

The little PWM units are great and can be used to drive most things including DC trains.