Why do some devices have external power supplies and some do not?
Laptops, portable CD players, electric pencil sharpeners, ...anything that you plug a transformer into an outlet have external power supplies.
Radios, stereos, blenders, toasters, clock radios, plug straight into the home outlet.
Most electrical devices need the +5V or +12V DC or whatever but some of them have that power supply built in and some do not?
This question is in the General Section. Responses must be helpful and on-topic.