# 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.

Transformers-

• 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