Designing Switch-Selected Fixed-Voltage Power Supply

The power supply provides five fixed DC voltages of 1.5V, 5V, 6V, 9V, and 12V at 1.5A by choosing the correct resistor value. It is useful for electronic circuits that need stable regulated voltages.

Circuit Diagram

Schematic of the switch-selected fixed-voltage power supply circuit is shown below.

Circuit Explanation

Firstly, the 220V mains power is connected through a 0.5A fuse to the primary winding of a 0-15V 2A step-down transformer. The transformer's secondary winding outputs an AC voltage, which is then rectified by a bridge consisting of four 1N5408 diodes. The rectified voltage is smoothed by a 4700uF capacitor.

Additionally, a 3mm LED with a 2.2k resistor is connected in parallel with the capacitor to serve as a power indicator.

The unregulated DC voltage from the previous stage is then regulated using an LM317 voltage regulator. A 33uF capacitor (C2) helps to stabilize the voltage at the adjustment pin, which helps reject noise. The 1N4007 diode (D5) blocks the reverse voltage, preventing it from reaching and potentially damaging the internal Darlington transistor of the LM317 IC. Another 1N4007 diode (D6) is used to discharge (C2) in case the output is shorted to ground. The output voltage of the LM317 is determined by a series of fixed resistors, and the output voltage is adjusted by selecting the appropriate resistor using a 5-position rotary switch (SW1).

The formula for LM317 regulator circuit is, Vout = Vref x (1 + (Rx/R1)), where Vref is the 1.25V reference voltage of the LM317 IC, and R1 is a default 220 Ohm resistor. By substituting the known values into the formula, we can determine the resistance value (Rx) needed for the desired output voltage.

For instance, to achieve an output voltage of 1.5V, 5V, 6V, 9V, and 12V, the required resistance values are 44 Ohms, 660 Ohms, 836 Ohms, 1.36k Ohms, and 18.9k Ohms, respectively. However, these exact fixed-resistance values are not commonly available in the market, so they must be approximated using a series-parallel combination of standard resistors.