As in the title Power supply from usb port, the power supply circuit using a source from USB port, with an adjustable stabilizer can be obtained voltage from 1.25V to 3.75V. USB port can give power to peripherals of 500 mA at 5 V ± 5%. The USB port can be used to power external circuitry, which is useful when no other constant voltage source.
USB-port consists of supply line V BUS, GND return path and the two signal lines. If you only need 5 V, you can take them directly to a USB connector, you can put a 10 uF filter capacitor between the V BUS and GND.
If you use adjustable voltage regulator, you can get any voltage from 1.25 V to 3.75 V. This voltage is used to power many schemes. This scheme is shown in Figure above. Regulated output voltage trimmer R 3 according to the equation.
V OUT = 1.25 in × (1 + R 3 / R 2 ).
In this equation, “1.25″ – Internal reference voltage source circuit LM1117-ADJ, measured between the terminal V OUT and adj. For this reason, the current through R 2 becomes permanent. Since adj current, resulting from the withdrawal ADJ, ignored, can be considered constant and the current flows through R 3. When R 2 = 100 ohms, this time is 5.12 mA. If you choose a trimmer resistance R 3 equal to 200 ohms, the output voltage adjustment range is from 1.25 at 0 to 3.75 at R 3 = R 3 = 200 ohms.
To protect the Power supply from usb port device from short circuit or overload can be added to the scheme, limiting current of 500 mA. This may be self-healing fuse, or a pair of transistors that can be easily inserted into the input line power.
The filter capacitor must not be greater than 10 microfarads. This value is sufficient to neutralize the current surge. Typically, the Power supply from usb port circuit operates normally when the capacitance of the filter from 1 to 10 uF.