DIY Variable Power Supply With DC Motor Controller?
A PWM speed controller varies motor speed by turning the power on and off rapidly with a variable on/off ratio. The output will be a series of 12V pulses, not a smooth DC voltage proportional to the pot position. 

It is OK for brushed DC motors (obviously!), resistive loads such as a heating element or incandescent light bulb, and other devices that can handle PWM (eg. LED strip, Peltier cooler). It will not work properly with devices that need smooth DC power.