Digital Electronics

This presentation will
• Define voltage, current, and resistance.
• Define and apply Ohm’s Law.
• Introduce series circuits.
o Current in a series circuit o Resistance in a series circuit o Voltage in a series circuit
• Define and apply Kirchhoff’s Voltage Law.
• Introduce parallel circuits.
o Current in a parallel circuit o Resistance in a parallel circuit o Voltage in a parallel circuit
• Define and apply Kirchhoff’s Current Law.
2

An understanding of the basics of electricity requires the understanding of three fundamental concepts.
•Voltage
•Current
•Resistance
A direct mathematical relationship exists between voltage, resistance, and current in all electronic circuits.
3

Current – Current is the flow of electrical charge through an electronic circuit. The direction of a current is opposite to the direction of electron flow. Current is measured in AMPERES (AMPS).
Andre Ampere
1775-1836
French Physicist
4

Voltage – Voltage is the electrical force that causes current to flow in a circuit. It is measured in VOLTS .
Alessandro Volta
1745-1827
Italian Physicist
5

Current – Current is the flow of electrical charge through an electronic circuit. The direction of a current is opposite to the direction of electron flow. Current is measured in AMPERES (AMPS).
Andre Ampere
1775-1836
French Physicist
6

The flow of water from one tank to another is a good analogy for an electrical circuit and the mathematical relationship between voltage, resistance, and current.
Force : The difference in the water levels ≡ Voltage
Flow : The flow of the water between the tanks ≡ Current
Opposition : The valve that limits the amount of water ≡ Resistance
Force
Flow
7
Opposition

Switch
Battery
Block Diagram
Light
Bulb
Switch
Light
Bulb
-
Battery
+
Schematic Diagram
8

Current
Resistance
-
Voltage
+
• Closed circuit (switch closed)
• Current flow
• Lamp is on
• Lamp is resistance, uses energy to produce light (and heat)
+
• Open circuit (switch open)
• No current flow
• Lamp is off
• Lamp is resistance, but is not using any energy
9

• Conventional Current assumes that current flows out of the positive side of the battery, through the circuit, and back to the negative side of the battery.
This was the convention established when electricity was first discovered, but it is incorrect!
• Electron Flow is what actually happens. The electrons flow out of the negative side of the battery, through the circuit, and back to the positive side of the battery.
Conventional
Current
Electron
Flow
10

• The direction that the current flows does not affect what the current is doing; thus, it doesn’t make any difference which convention is used as long as you are consistent.
• Both Conventional Current and Electron Flow are used. In general, the science disciplines use Electron Flow, whereas the engineering disciplines use Conventional Current.
• Since this is an engineering course, we will use Conventional
Current .
Electron
Flow
Conventional
Current
11

• Defines the relationship between voltage, current, and resistance in an electric circuit
• Ohm’s Law:
Current in a resistor varies in direct proportion to the voltage applied to it and is inversely proportional to the resistor’s value.
• Stated mathematically:
I

V
R
I R
Where: I is the current (amperes)
V is the potential difference (volts)
R is the resistance (ohms)

V
I R
V
I R
V
I R
I

V
R
( amperes , A )
R

V
( ohms ,

)
I
V

I R ( volts , V )

Example :
The flashlight shown uses a 6 volt battery and has a bulb with a resistance of 150

. When the flashlight is on, how much current will be drawn from the battery?
14

Example :
The flashlight shown uses a 6 volt battery and has a bulb with a resistance of 150

. When the flashlight is on, how much current will be drawn from the battery?
Solution :
Schematic Diagram
V
T
=
I
R
+
-
V
R
V
I R
I
R

V
R
R

6 V
150


0.04
A

40 mA
15

Components in a circuit can be connected in one of two ways.
Series Circuits
• Components are connected end-to-end.
• There is only a single path for current to flow.
Parallel Circuits
• Both ends of the components are connected together.
• There are multiple paths for current to flow.
Components
(i.e., resistors, batteries, capacitors, etc.)
16

Characteristics of a series circuit
• The current flowing through every series component is equal.
• The total resistance (R
T
) is equal to the sum of all of the resistances
(i.e., R
1
+ R
2
+ R
3
).
• The sum of all of the voltage drops (V
R1 total applied voltage (V
T
+ V
R2
+ V
R2
) is equal to the
). This is called Kirchhoff’s Voltage Law.
I
T
+
V
R1
-
V
T
+
-
+
V
R2
R
T
-
V
R3
+
17

Example :
For the series circuit shown, use the laws of circuit theory to calculate the following:
• The total resistance (R
T
)
• The current flowing through each component (I
T
, I
R1
, I
R2
, & I
R3
)
• The voltage across each component (V
T
, V
R1
, V
R2
, & V
R3
)
• Use the results to verify Kirchhoff’s Voltage Law.
V
R1
I
T
+ -
V
T
+
-
R
T
I
R1
I
R3
-
V
R3
+
I
R2
-
+
V
R2
18

Solution :
Total Resistance:
R
T

R1

R2

R3
R
T

220
 
470
 
1.2
k

R
T

1890
 
1.89
k

Current Through Each Component:
I
T

V
T
R
T
(Ohm' s Law)
I
T

12 v
1.89
k


6.349
mAmp
Since this is a series circuit :
I
T

I
R1

I
R2

I
R3

6.349
mAmp
V
I R
19

Solution :
Voltage Across Each Component:
V
R1

I
R1

R1

(Ohm'
V
R1

6.349
mA

220 Ω s Law)

1.397
volts
V
R2

I
R2

R2 (Ohm'
V
R2

6.349
mA
 s Law)
470 Ω 
2.984
volts
V
R3

I
R3

R3 (Ohm' s Law)
V
R3

6.349
mA

1.2
K Ω 
7.619
volts
V
I R
20

Solution :
Verify Kirchhoff’s Voltage Law:
V
T

V
R1

V
R2

V
R3
12 v

1 .
397 v

2 .
984 v

7 .
619 v
12 v

12 v
21

Characteristics of a Parallel Circuit
• The voltage across every parallel component is equal.
• The total resistance (R
T
) is equal to the reciprocal of the sum of the reciprocal:
1
R
T

1
R
1

1
R
2

1
R
3
R
T

1
1
R
1

R
1
2

R
1
3
• The sum of all of the currents in each branch (I
R1 to the total current (I
T
+ I
R2
+ I
R3
) is equal
). This is called Kirchhoff’s Current Law.
I
T
+
V
T
-
+
V
R1
-
+
V
R2
-
+
V
R3
-
22
R
T

Example :
For the parallel circuit shown, use the laws of circuit theory to calculate the following:
• The total resistance (R
T
)
• The voltage across each component (V
T
, V
R1
, V
R2
, & V
R3
)
• The current flowing through each component (I
T
, I
R1
, I
R2
, & I
R3
)
• Use the results to verify Kirchhoff’s Current Law.
I
T
I
R1
I
R2
I
R3
V
T
+
-
+
V
R1
-
+
V
R2
-
+
V
R3
-
23 23
R
T

Solution :
Total Resistance:
R
T

1
1
R
1

1
R
2

1
R
3
R
T
R
T

1
1
470


1
2.2
k


1
3.3
k


346 .
59

Voltage Across Each Component:
Since this is a parallel circuit :
V
T

V
R1

V
R2

V
R3

15 volts
24

Solution :
Current Through Each Component:
I
R1

V
R1
R1
(Ohm' s Law)
I
R1

V
R1
R1

15 v
470


31.915
mAmps
I
R2

V
R2
R2

15 v
2.2
k


6.818
mAmps
V
I R
I
R3

V
R3
R3

15 v
3.3
k


4 .
545 mAmp
I
T

V
R
T
T

15 v
346.59


43.278
mAmp 25

Solution :
Verify Kirchhoff’s Current Law:
I
T

I
R1

I
R2

I
R3
43.278
mAmps

31.915
mA

6
43.278
mAmps

43.278
mAmps
.
818 mA

4 .
545 mA
26

Kirchhoff’s Voltage Law (KVL) :
The sum of all of the voltage drops in a series circuit equals the total applied voltage.
Gustav Kirchhoff
1824-1887
German Physicist
Kirchhoff’s Current Law (KCL) :
The total current in a parallel circuit equals the sum of the individual branch currents.
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