Schematic diagram of power supply circuit taken from USB port

Every PC has a USB (Universal Serial Bus) port that can provide 500 mA of 5V ± 5% power to peripherals. Powered USB Hub

The USB port can be used to power external circuits, which is very useful when there is no other DC power supply.

The USB port has a power pin VBUS; a return pin GND (ground); and two signal pins. If only a 5V power supply is needed, you can get it from the USB connector

The power pin is connected to the ground pin, but a 10μF filter capacitor should be placed between the power pin and the ground pin.

However, through an adjustable regulator

, you can also get a voltage of 1.25V to 3.75V, and most circuits use this range. The circuit in Figure 1 covers this range. This range can be changed by R3, as shown in the following formula: VOUT=1.25V×(1+R3/R2). The reason for the 1.25V in the formula is

The LM1117-ADJ generates 1.25V between VOUT and the ADJ (adjust) pin. Therefore, the resistor

R2 has a constant current through resistor R3; IADJ (the regulated current) is usually small enough to be ignored. R2 is chosen to be 100Ω, setting its current to 12.5

If a 200Ω potentiometer is used for R3, the voltage range is 1.25V when R3 is 0Ω and short-circuited, and 3.75V when R3 is 200Ω.

To prevent the output from being damaged by a short circuit or unknown load conditions, a current limiting circuit can be added to limit the maximum current to 500 mA.

This current limiter can be easily implemented at the power input line.

The filter capacitor should not exceed 10 μF. This level can still control the inrush current without a current limiting circuit. Generally speaking, the capacitor should be between 1 μF and 10 μF.