|MadSci Network: Engineering|
In a perfect transistor without losses, the power (which is calculated as p = V * I, p = Power, V = Voltage, I = Current) stays constant between input and output. This means if you want higher current, you need to step down the voltage. For example, a 20V-1A line can be stepped down to 10V-2A with a transformer having twice as many windings at the input side as at the output.
A real transformer, of course, has losses, which means the output power will always be lower than the input. The difference is converted into heat and radiation.
Please also note that it is not up to the transformer to decide how much current goes through a given resistor (e.g. a light bulb). At half the voltage, only half the current will go through the resistor, so increasing the current through a resistor is only possible by increasing the voltage.
Try the links in the MadSci Library for more information on Engineering.