In applications where there is no danger of any contact with live points on the circuit by the user, a source without power grid isolation may be used. This type of power supply has the advantage of not needing a transformer, which besides being an expensive component, takes up a lot of space. The source suggested in the figure can provide output voltages between 3 and 12 V as determined by the zener diode, under currents up to 100 mA. The transistor shall be provided with a small heat radiator and the input capacitance of 4.7 ?F shall be metallized polyester having at least 250 V working voltage if the grid is 110 V. The circuit may be connected in 220 V using a capacitor of 2.2 ?F with at least 400 V of working voltage. The diodes of the rectifier bridge admit equivalents. This power supply can be used to power calculator, small portable radios and other similar devices.
