Components:
Step-down transformer 30V
Bridge rectifier (DF02M) or 4 1N4003 diodes
2200µF, 50V capacitor
2 1N4002
2 Resistors: 1K & 180ohms
Potentiometer: 5K
LM317
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lm317 pin configuration
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In this circuit, I used LM317 (an adjustable voltage regulator IC). The input voltage to the IC is approximately 41V. You can check my 12V regulated DC power supply and Dua DC power supply where I explained in a simple way voltage entering voltage regulator IC with Vrms-Vpeak relationship and diode p-n junction voltage drop. In this circuit, p-n junction voltage concept play a vital role: it will be discussed here.
LM317 provides an internal reference voltage of 1.25 V between the output and adjustments terminals. This is used to set a constant current flow across an external resistor divider giving an output voltage VO of:
 Click here for LM317 calculator
note that 'VO = VREF (1 + R2/R1) + IADJR2' is what you will see in the data sheet. Since I am using it to explain circuit on this page I have to change 'R2' to 'VR' to correspond with the circuit.
The device was designed to minimize the term IADJ (100 µA max) and to maintain it very constant with line and load changes.
VO is 1.25V (the minimum output voltage) when potentiometer VR turned to zero and is 36.47V when VR turned fully to give 5K. In order to get an output voltage of 0V when VR is turned to 'zero' diode D1 and D2 are connected in series to the output as it in the circuit. P-N junction voltage drop of 0.7V of each diode give total of 1.4V which completely remove its minimum output of 1.25V. Also 1.4V is cut from the maximum to get 35.07V.
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