Background and Design Inspiration
The foundation of this circuit draws from a low-dropout voltage regulator featured in the book “303 Circuits”. Additionally, a new printed circuit board (PCB) was crafted specifically for surface-mount devices (SMDs). Despite these updates, the power transistor utilized in this circuit remains a conventional, older model.
Modifications to the Original Design
Several alterations were introduced to the initial 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.
Setting up this circuit is an uncomplicated process: start by removing JP2, then proceed to connect a voltmeter to pin JP2-2. Adjust P1 until the desired output voltage is achieved. After this adjustment, reinsert JP2 to complete the setup. Keep in mind that the input voltage should be at least 1 V higher than the desired output voltage.
Flexibility in Transistor Selection
The specific transistor types used are not of critical importance in this design. You have the freedom to employ any pin-compatible equivalent type, and it is even feasible to use traditional leaded transistors instead of surface-mount devices (SMDs). For instance, BC547s can be substituted for the NPN transistors, and a BC557 can replace T2, which is the sole PNP transistor aside from the power transistor.
The table displays various measurements taken with an output voltage of 7.39 V, which was utilized to power a series-connected pair of white LEDs.
VIN = 9 V, VIN,min = 8.20 V.