I want to build a dimmer for a DC lamp, using a potentiometer as a voltage divider. The lamp is 6V 30W. The power supply I am using is 5VDC, 6 Amp switchmode.

My question is what should be the resistance of the pot? Am I correct in thinking it should be close to that of the bulb (i.e. approx 5/6 ohms)? I want to be able to dim the lamp by up to 50%.

 

The serial potmeter not a good idea: need a really big one. If the PSU adjusting no possible use a small PWM regulator.

 

Try something like this http://www.elecfree.com/electronic/a-simple-pwm-circuit-based-on-the-555-timer/

Note that at 6V, you will need to choose a mosfet with a very low threshold voltage of 5V or less e.g. a "logic level" mosfet.