If you are connected with electrical or electronic technology then definitely you would need a power supply unit for carrying out the various experiments in the field.
A power supply is basically an electrical equipment which is able to convert AC mains power to DC low level power. For example in a 12 volts adapter which is a power supply unit, the input mains AC at 230 V or 120 V is converted to 12 volts DC, so that it may be used for operating or testing smaller electrical or electronic gadgets.
Please refer the CIRCUIT DIAGRAM to properly undestand howto build a transformerless ower supply circuit
All power supply or DC
One alternative can be acquired through capacitive power supplies which are not only very light but also very cheap to build. Few drawbacks are ofcourse associated with these power supplioes also. These power suppliers cannot provide enough current and may sometims accomapny electric shocks.
However if the circuit is well covered and used for low power applications, nothing can be as useful as these types of power supplies or the capacitive power supplies.