Currently, voltage inverters are used almost everywhere and very often replace traditional linear regulators, which for high output currents, gives a significant energy loss. Circuit described here is a simple Step-Down converter (12V to 5V) built based on the popular and inexpensive chip MC34063. The device is designed to work with 12V car installation, and can be used to charge/power GPS or mobile phones equipped with USB slot. In idle state the circuit is completely switched off and in normal operating conditions it can be automatically turn off when there are no power consumption at the output. The converter can be turn on, by a monostable switch, when load is connected to its output .