3.3V DC Power Supply Circuit by Using L78L33

Digital Circuits and CMOS Logic Circuits are widely uses 3.3V DC as a Operating Power supply. These digital circuits considers less than 0.4V as a logic 0 (LOW) and Greater than 2.4V as logic 1 (HIGH). Hence we need to apply constant Regulated DC supply to these circuits. In this Article 3.3V DC power Supply circuit by using L78L33 is designed and tested to observe the Regulation characteristics and operation of Positive voltage Regulator L78L33.

Here we have taken TO-92 package L78L33 Positive Voltage Regulator from st.com, then employed this IC in to an standalone Stepdown Rectifier circuit and 3.3V DC Vout obtained with maximum 100 mA output Current.

Circuit Diagram

Components Required (BOM)

S.NoDesignatorValuePart NumberQuantity
1TR1230V to 0-6V AC0-6V Stepdown Transformer1
Disc capacitor
Disc capacitor
3D1Bridge Rectifier Module1

Construction & Working

Operation and working methods are very simple for this 3.3V DC Regulator Power supply Circuit by using L78L33. First thing is this is an independent source regulated power supply circuit hence stepdown Transformer (0-6 VAC) used to reduce the 230V AC mains supply into 6V AC then Bridge Rectifier module used to convert AC into DC supply. After Rectification DC supply may contains some AC ripples, these are removed by using filter capacitors C1, C2 then filtered 6V DC supply is applied to the input terminal of L78L33 and output is taken from terminal Vo. Here Regulated 3.3V DC obtained without any fluctuations. This IC has common GND terminal to Vi and Vo.


We are familiar with 78XX Positive voltage Regulators and 79XX Negative voltage Regulators. By the way these L78L series positive voltage Regulators are used to obtain low current 3.3V, 5V, 6V, 8V, 9V, 10V, 12V, 15V, 18V and 24V Output and L79L Series Negative voltage Regulators are used to obtain low current -5V, -8V, -12V and -15V Output.

This L78L Series of three terminal positive voltage Regulators has internal thermal shutdown and current limiting ability, and these devices are delivers 100 mA output current. You can add heat sink if needed to avoid excessive heat on PCB board. These devices are designed to give fixed Regulated output voltage and to eliminate noise and distortion problems associated with single point Regulation.

Pin Configuration of L78LXX

Available Package of L78LXX

Some of the notable features of L78L Series positive voltage regulators are,

  • Short circuit protection
  • Thermal overload protection
  • Output current up to 100mA (most suitable for digital circuits and CMOS Logic circuits)
  • No operational external components are required
  • Available Packages: TO-92, SO-8 and SOT-89
  • Refer datasheet for more information.

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