Get 1.25V to 3.75V from an adjustable regulator Every PC has a USB (Universal Serial Bus) port that can supply 5V±5% at 500 mA for peripherals. Powered USB hubs also provide this power. You can use a USB port to power an external circuit, which is useful when you have no other dc source available. A USB port has V[SUB]BUS[/SUB], the power pin; a return pin, GND (ground); and two signal pins. If you need just a simple 5V supply, you can tap the power pins from a USB connector, but you should place a 10-μF filter capacitor between the ground and power-supply pins. [TABLE="align: center"] [TR] [TD="colspan: 2, align: center"][/TD] [/TR] [TR] [TD="width: 65"]Figure 1.[/TD] [TD="width: 435"]Resistors R2 and R3 set the adjustable voltage regulator’s output at 1.25 to 3.75V.[/TD] [/TR] [/TABLE] You can, however, use an adjustable voltage regulator to get voltages of 1.25 to 3.75V, a range that many circuits use. The circuit in Figure 1 covers that range. You use R[SUB]3[/SUB] to change that range, as the following equation shows: V[SUB]OUT [/SUB]= 1.25V × (1 + R[SUB]3[/SUB]/R[SUB]2[/SUB]) The 1.25V in the equation occurs because the LM1117-ADJ linear regulator generates 1.25V between the V[SUB]OUT[/SUB] and the ADJ (adjust) pins. Resistor R[SUB]2[/SUB], therefore, has a constant current that passes through resistor R[SUB]3[/SUB]; the I[SUB]ADJ[/SUB] (adjusted current) is generally small enough to ignore. Selecting 100Ω for R[SUB]2[/SUB] sets its current to 12.5 mA. If you use a 200Ω potentiometer for R[SUB]3[/SUB], you get a voltage range of 1.25V when R[SUB]3[/SUB] is 0Ω, causing a short, to 3.75V when R[SUB]3[/SUB] is 200Ω. To prevent circuit damage if the output becomes shorted or when you don’t know the load, you can add a current-limiting circuit that keeps the maximum current at 500 mA. A polyswitch fuse or pair of transistors can easily implement this current-limiter site at the power-supply input line. The filter capacitor shouldn’t exceed 10 μF. That level keeps the inrush current under control in the absence of a current-limiting circuit. Generally, capacitors of 1 to 10 μF work best.