Best Tutorial on Linear AC-DC Power Supplies

In this tutorial, we are going to learn about Linear AC-DC Power Supplies.

Linear AC-DC Power Supplies –

  • An AC-DC power supply converts AC electricity from a wall outlet into DC electricity that a sensitive electronic device can use.
  • This power supply is called an unregulated power supply.
  • The voltage it produces depends on the demand of the load and the capacitance provided.
  • Also, the output voltage is highly dependent on the input voltage across the transformer.

Examples of AC-DC Power Supplies-

  • Laptop power Adapter
  • Phone Charger
  • Desktop PC Power Supply

Using an Oscilloscope-

  • An oscilloscope is an instrument which allows us to view changes in voltage over time.

Using an Oscilloscope-

  • Time is on the x-axis.
  • Voltage is on the y-axis.
  • Each horizontal or vertical line is an increment of either unit.

Direct Current (DC)-

  • The direction of current is always the same because the voltage always remains greater than 0.

Alternating Current (AC)-

  • The direction of current is constantly changing because the voltage is constantly passing through 0V.

The First Step-

  • The voltage available at a wall outlet is much too high and is always changing polarity, making it useless for sensitive electronics.
  • The first step is to reduce the voltage using a transformer.


  • A transformer is a device that allows us to convert an AC voltage to a higher or lower level.
  • Transformers can only be used with AC.
  • They operate on the basis of an electromagnetic theory called mutual inductance.    

Effect of a Transformer-

  • This is the effect of a step-down transformer.
  • Note that only the amplitude (the voltage) has been reduced, not the frequency.

Examples of Transformers-

  • E-Core
  • Toroidal
  • High Frequency

The Second Step-

  • Though we have successfully reduced the voltage, we still have the problem of the voltage fluctuating from positive to negative.
  • To “force “the current to only flow in one direction we construct a bridge rectifier circuit ,using 4 diodes.

Effect of a Bridge Rectifier-

  • A bridge rectifier basically takes the absolute value of the AC waveform.
  • Now, instead of the current constantly reversing, it now has only one direction.

Examples of Bridge Rectifiers-

  • 4 discrete diodes
  • GBU –type bridge
  • High power bridge

The Third Step-

  • The fluctuation in voltage can have ill effects on the circuit at hand. Semiconductor devices have minimum  voltage needed to turn them on .(Typ. 0. 7V)
  • In the current waveform, we can observe that the voltage constantly dips to very low voltages (including 0V), which is problematic.
  • To fix this problem, we use filter capacitors.

The Filter Capacitor-

  • Electrolytic Capacitor

Effect of Filter Capacitor-

  •  As the capacitor discharges its voltage also decreases.
  • By using a higher capacitor can provide a greater voltage for longer time.

Examples of Filter Capacitors-

  • Electrolytic
  • Bulk (Electrolytic)
  • Solid Polymer

The Fourth Step-

  • Ideally, we want the output voltage of the power supply to remain constant regardless of the load, and fluctuations from the voltage source.
  • To accomplish this, we can use integrated circuit called a “voltage regulator” which does exactly what its name implies.

Effect of a Voltage Regulator-

  • Let’s say we need a steady 5.0V supply and the voltage regulator will take care of any fluctuations in voltage, provided the input voltage never dips below the desired output voltage (Vout + 2V or so).

Examples of Voltage Regulators-

  • TO – 220
  • TO – 3
  • TO – 92

Examples of Voltage Regulators-

  • Some part names of common voltage regulators:
  • – LM78xx          – LM723
  • – LM317            – L200
  • – LM338            – LT1085

Basic Power Supply Topology-

  • Mains Input
  • Trans former
  • Bridge Rectifier
  • Filter Capacitor
  • Voltage Regulator

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