Fixed DC regulated power supply (III)

The fixed DC regulated power supply circuit introduced in this example uses a digital display composed of seven light emitting diodes (LEDs) to display the output voltage (with two levels of 3V and 6V). It has the characteristics of simple circuit, practicality and easy production.

How the Circuit Works

The fixed DC regulated power supply circuit consists of a voltage stabilizing circuit and a display circuit, as shown in Figure 5-4.

The voltage-stabilized power supply circuit consists of a power transformer T, rectifier diodes VDl-VD4, filter capacitors Cl, C2, a three-terminal voltage-stabilizing integrated circuit IC, resistors Rl-R3 and a voltage selection switch S (Sa, Sb).

The display circuit is composed of resistor R4, light emitting diodes VL1-VL7 and Sa.

The 220V AC voltage is divided into two paths after being stepped down by T, rectified by VD1-VD4 and filtered by Cl: one path is used as the working power supply of the display circuit after being limited and stepped down by R4; the other path is output after being stabilized by IC and filtered by C2.

When S is placed at the "6V" position, R3 is connected to the voltage stabilization control circuit, and the output voltage of the IC is +6V; at this time, VL2 (stroke b) in the light-emitting diodes VL1-VL7 does not emit light because the cathode is disconnected by Sb, and the display shows the number "6".

When in the "3V" position, R3 is short-circuited by Sa, and the output voltage of the IC is +3V. At this time, the light-emitting diodes VL5 (stroke e) and VL7 (stroke g) do not emit light, and the display shows the number "3".

Component Selection

R1-R4 are all 1/2W metal film resistors.

Cl and C2 are both aluminum electrolytic capacitors, the withstand voltage of Cl is 25V, and the withstand voltage of C2 is 16V.

VD1-VD4 uses 1N4001 or lN4007 silicon rectifier diodes.

VL1-VL7 all use 2mmx5mm flat square light emitting diodes.

IC uses LM317 or SW317 three-terminal adjustable integrated voltage regulator.

T uses a power transformer with a 3-5W and a secondary voltage of 15-18V.