Wednesday October 18 , 2017

Electric Circuits Basics

For Direct Current Circuits

Currents (I) and voltages (V) are constant with time.

 

For Alternating Current (AC) Circuits 

Current (i) and voltage (v) are time varying.

 

What Are AC Circuits?

• Any combination of Active and passive elements (resistors, capacitors and coils).

• The sources are AC voltage or current sources

 

 

 

Basic Concepts

  

 

  

The Electric Current (I)  

• Electric current results from the movement of electric charge.

• The SI unit of current is Ampere (A).

  

 

 

 

 • For most electronic circuits the ampere is a rather large unit, the mA unit is more common.

 

The Electric Voltage (V)

• Potential energy per unit charge.

• The measure of "push" available to motivate electrons.

• Work (W) in Joules= Volt (V) in Volts * Charge (Q) in Coulombs

• A positive charge will move from a higher to a lower potential.

• The SI unit of voltage is Volt (V).

 

• Current flows in a conductor if there is a potential difference between its ends.

 

Resistance and Ohm’s low

 

• The Resistance of an object is a function of its material and shape.

 

   

•Old Resistor values is indicated by a 3-color code.

• Resistor tolerances is in the order of 5%.

 

DC Circuit

• Any combination of Active and passive elements (usually resistors).

• Active elements are either voltage or current source.

 

 

 

 

• Power dissipated by loads and power supplied by sources.

 

• Active elements are either voltage or current, dependent or independent elements.

 

 

Active Elements

Active elements are either voltage or current, dependent or independent.

 

DC ideal independent sources

 

                

                 

            DC Voltage Source                                     DC Current Source

           

 

AC ideal independent sources

 

               

                

          AC Voltage Source                                     AC Current Source 

        

      

Ideal dependent voltage source

 

 

·   Voltage controlled voltage source (VCVS)

v= μ vx   Where, μ is dimensionless

 

• Current controlled voltage source (CCVS)

                                                       v= μ ix    Where, μ is in KΩ

 

Ideal dependent current source

·   Current controlled current source (CCCS)

i= μ ix   Where, μ is dimensionless

·   Voltage controlled current source (VCCS)

                                             i= μ vx   Where, μ is in (KΩ) ˉ¹

 Actual sources

 

 

Actual voltage source

       

          

  

·   Actual current source

      

 

 

Power & Energy Calculations

 

Power is the time rate of delivering or absorbing energy, measured in watts (W).

 

 

 

For a resistor, power is always dissipated.

 

For a linear resistor, V = I *R

 

  

For a source, power is either delivered or absorbed.

Voltage Source

 

                            

              P (delivered) =E*I                       P (delivered) =-E*I

                                                      P (absorbed) = E*I

 

Power balance:

 Σ Power delivered by sources =Σ Power dissipated by resistors.

 

Current source

  

           

                  

       P (delivered) =V*I0                           P (delivered) =-V*I0

                                                                    P (absorbed) =V*I0

 

Power balance:

 Σ Power delivered by sources =Σ Power dissipated by resistors.