Adjustable Low-Dropout Voltage Regulator Schematic Circuit Diagram
This circuit is based on a design for a low dropout voltage regulator in the book 303 Circuits (published by Elektor in 1998). The author adapted the design to make the output voltage adjustable, and he designed a new PCB for SMDs. However, the power transistor is still an ‘old-fashioned’ type. The following changes were made to the original design:
• The 4.7-V Zener diode is replaced by a TL431 shunt regulator, which has good stability and generates an output voltage that can be varied with a 5-kΩ potentiometer, which allows the output voltage to be adjusted over the range of 5 to 16 V.
• The 390 Ω is replaced by a current mirror consisting of transistors T5 and T6. The operating point of the current mirror is set by R3 and the TL431. The stability of the output voltage depends on the cathode current of the TL431. The data sheet specifies a minimum value of 1 mA for good regulation, so with 1.7 mA at an output voltage of 5 V the margin is more than adequate.
• The output stage consists of two transistors (T1 and T2) in a Darlington configuration. Resistor R9 ensures that the base-emitter voltage of T1 is always sufficient to keep T1 conducting, even at very low output current levels.
• D1 was an AA119 germanium diode; it is replaced by an MELF4148.
• To ensure that the regulator starts up properly, the value of R8 is reduced from 100 kΩ to 3.9 kΩ.
• Output filtering is integrated on the board in the form of C1 and C4. The PCB design  is double-sided. All of the SMDs are fitted on one side, and the BD136 is fitted on the other side (the copper side). If desired, the power transistor may be fitted with a heat sink and insulating washer. However, the low-drop design makes a heat sink largely unnecessary. The small PCB can be fitted in place of a conventional voltage regulator IC, since JP1 is pin-compatible.
Setup is very simple: remove JP2, connect a voltmeter to pin JP2-2, and adjust P1 to obtain the desired voltage at the output. Now replace JP2, and you’re done. Bear in mind that the input voltage must be at least 1 V higher than the output voltage. The transistor types are not critical; any pin-compatible equivalent type can be used. You could even use leaded types instead of SMDs — for example, BC547s for the NPN transistors and a BC557 for T2, which is the only PNP transistor other than the power transistor. The table shows several readings measured with an output voltage of 7.39 V, which was used to drive two white LEDs connected in series.
VIN = 9 V, VIN,min = 8.20 V.