In power applications it is a step down transformer. In a lower power application it might just be a circuit that converts AC to DC. A rheostat or potentiometer can also be used, but can waste power.
yes you can it because a transformer can change so they can change the voltage Sorry but you need alternating current for a transformer, for dc you would have to interrupt the power supply to make it...
You can make a voltage divider set of resistors and tap off the desired voltage. Some voltage regulators can be fed much higher voltages. The regulator will provide a constant settable voltage with...