Enginer

1. Direct Current (DC) Electric Circuits
A Direct Current (DC) Electric Circuit consists of a source of DC electricity such as a battery
with a conducting wire going from one of the source terminals to a set of electric devices and then
back to the other terminal, in a complete circuit.
A DC circuit is necessary for DC electricity to exist. DC circuits may be in series, parallel or a
combination. Understanding DC circuits is important for learning about the more complex AC
circuits, like those used in the home.

2. DC waveform
DC circuits have a unidirectional flow of current and it is not changing the direction periodically.
DC waveform
DC sources:
Direct current source can be denoted by these symbols.
DC voltage source symbol

3. DC Voltage The dc voltages induce the direct current. The waves only in one direction and the magnitude of the
voltage always remain constant. The generation of the DC voltage is quite simple and easy. The voltage induces
by rotating the coil in the field of the magnet. The coil consists the split ring and commentator which converts
the alternating voltage into the direct voltage. The DC voltage measure by volt.
Significant Characteristics of DC Voltage
The DC Voltage induces the direct current between the two points.
The frequency and power factor are equal zero.
The polarity remain constant.
The direction remain same.
Obtained from cell or battery.
Passive parameter (resistance only).
The voltage converted by using rectifier.
The transformer not require.
Don’t have phase and neutral.

4. Kilovolt and Millivolt are derived term of volt.
1 kilovolt is 1000 volts
1 volt is 1000 millivolts
As nouns the difference between kilovolt and volt is that kilovolt is one thousand ( 103 ) volts symbol: kV while volt
is in the international system of units, the derived unit of electrical potential and electromotive force (voltage); the
potential difference across a conductor when a current of one ampere uses one watt of power symbol: V.

5. Electrical current
An electric current is the rate of flow of electric charge past a point or region. An
electric current is said to exist when there is a net flow of electric charge through a
region. The conventional symbol for current is I.

6. Direct Current Resistance – The DCR is defined as the resistance of an inductor as a result of the resistance of the
wire used in the winding.
Resistance is a circuit element measured in Ohms, Greek symbol ( Ω, Omega ) with prefixes used to denote Kilo-
ohms ( kΩ = 103Ω ) and Mega-ohms ( MΩ = 106Ω ). Note that resistance cannot be negative in value only positive.
Resistor Symbols

7. Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportion to the
potential difference across the two points. Introducing the constant of proportionality, the resistance, one
arrives at the usual mathematical equation that describes this relationship.
I=
𝑽
𝑹
where I is the current through the conductor in units of amperes, V is the potential difference measured
across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More
specifically, Ohm's law states that the R in this relation is constant, independent of the current.

8. Ohm's Law is a very simple and useful tool for analyzing electric
circuits. It is used so often in the study of electricity and
electronics that it needs to be committed to memory by the
serious student. For those who are not yet comfortable with
algebra, there's a trick to remembering how to solve for any one
quantity, given the other two. First, arrange the letters E, I, and R
in a triangle like this:
If you know E and I, and wish to determine R, just eliminate R
from the picture and see what's left:

9. If you know E and R, and wish to determine I, eliminate I and see what's
left:
Lastly, if you know I and R, and wish to determine E, eliminate E and see
what's left:

10. Examples
Example 1
Find the current I through a resistor of resistance R = 2 Ω if the voltage across the resistor is 6 V.
Solution to Example 1
Substitute R by 2 and V by 6 in Ohm's law V = R I.
6 = 2 I
Solve for I
I = 6 / 2 = 3 A
Example 2
What is the amount of current (I) in this circuit?
Solution to Example 2
Substitute R by 3 and V by 12 in Ohm's law V = R I.
6 = 2 I
Solve for I
I = 12 / 3 = 4A

11. Example 3
What is the amount of resistance (R) offered by the lamp?
Solution to Example 3
Substitute V by 36 and I by 4 in Ohm's law E= R I.
R = E/ I
Solve for R
R = 36 / 4 = 9Ω
Example 4
What is the amount of voltage provided by the battery?
Solution to Example 4
Substitute I by 2 and R by 7 in Ohm's law E= R I.
E= 2× 7 =14 V

12. REVIEW
•Voltage measured in volts, symbolized by the letters "E" or "V".
•Current measured in amps, symbolized by the letter "I".
•Resistance measured in ohms, symbolized by the letter "R".
•Ohm's Law: E = IR ; I = E/R ; R = E/I

13. Homework
1. Find the current I through a resistor of resistance R = 5Ω if the voltage across
the resistor is 4V?
2. What is the amount of resistance (R) offered by the lamp if the current (I)=3
and E=9?
3. What is the amount of voltage provided by the battery if the resistance(R)=2
and the current through the resistance (I) = 12?