The article assumes that the reader has the following knowledge:
- Ohm’s Law
- Series Circuits
- Parallel Circuits
Review of Ohm’s Law
Ohm’s law states that the electrical pressure is equal to the electrical current multiplied by the resistance.
Electrical pressure in measured in unit called volts
Electrical current is measured in units called amperes
Resistance is measured in units called ohms
Review of Series Circuits
Two electrical components are said to be in series if there is only one electrical path through the two components. In other words, if one component is removed, the electrical path is broken.
Review of Parallel Circuits
Two electrical components are said to be in parallel if there is two electrical paths: One path through each component. If one component is removed, current still flows through the other component
How Electricity Flows
To understand how electricity flows, we need to briefly examine the atom. Everything is made up of atoms. Each atom contains electrons orbiting around a nucleus. Each electron has a negative charge. Protons and neutrons are inside the nucleus. Neutrons have no electrical charge. Each proton has a positive charge. The number of protons inside the nucleus equals the number of electrons circling the nucleus. The charges of protons and electrons are the same magnitude, but opposite polarity. Therefore, an atom that has not lost any electrons has no charge. The electrons that break free from an atom are known as free electrons. These free electrons are the medium of electricity. They carry their electrical energy from one point to another. A material with an excess of free electrons is a conductor. The material does not resist the flow of electricity. A material with a lack of free electrons resists the flow of electricity.
Originally, it was assumed that electricity flowed from positive to negative. When an electron breaks away from an atom, the atom is left with more protons than electrons. Since the proton has a positive charge, the atom now has a positive charge and called an ion. Any electron that comes near this atom will be attracted to the atom. Hence, the atom soon gains an electron neutralizing the atom. The effect is that current flow consists of electrons moving from atom to atom. But while the electrons are moving from atom to atom, it looks like a positive charge is moving in the opposite direction. Of course, the ions are not actually moving, but that is the appearance.
To get a clearer picture of the atom, think of our solar system. The sun is the nucleus of our solar system. Each planet has it’s own orbit. Instead of one planet in each orbit, imagine many planets in each orbit. Now you have a more accurate picture of the atom. A band is a group of orbits that are close together. In the atom, there are many bands of electrons. Only the outermost band is affected by an external pressure.
What Is a Diode?
A diode is an electronic component that only conducts in one direction. It is used extensively in electronic circuits. The symbol for a diode is shown in Figure One. Figure One shows a diode and indicates the diode’s anode and cathode.
A diode consists of two types of materials. A P-type and an N- type. In the P type material, some of the atoms have less electrons than protons. Hence, these atoms have a positive charge. The part of the diode that has P type material is called the anode.
In the N-type material, some of the atoms have more electrons than protons. These atoms have a negative charge. The part of the diode that has N type material is called the cathode.
If a positive voltage exists at the anode and a less positive voltage exists at the cathode, the diode will conduct. When a diode is conducting, it is said to be forward biased. Typically, a forward biased diode’s anode is 0.6 volts higher than it’s cathode.
If a negative voltage exists at the anode and a less negative voltage appears at the cathode, the diode will have a very high resistance and will not conduct. Under these conditions, the diode is said to be reversed biased. For practical purposes we consider a reverse biased diode to be an open circuit meaning that there is no electrical path through the diode and no current through the diode.
A Diode and a Resistor in Series
Look at the circuit in figure two. The diode D1 is in series with a resistor. The positive terminal of the DC voltage source is connected to the anode of the diode. There is a complete path from the positive terminal of the voltage source through the diode and the resistor to the negative terminal of the voltage source.
In the circuit of figure 2, the output of the voltage source is 5 volts DC. The value of the resistor is 200 ohms. What is the total current through the circuit?
We know that a forward biased diode has a 0.6 volt drop across it. Therefore the voltage drop across the resistor equals the source voltage minus the voltage drop across the diode.
5 volts – 0.6 volts = 4.4 volts.
There is 4.4 volts across the 200 ohm resistor. The current is
It = 4.4 volts/200 ohms = 0.022 amperes
In the circuit in figure 3, the output of the voltage source is 10 volts. Resistor R1 is 50 ohms and resistor R2 is 20 ohms. What is the total current?
The positive terminal of the voltage source is connected to the anode of diode D1 and the anode of diode D2. Therefore both diode D1 and diode D2 are forward biased. There is a 0.6 volt drop across each diode. Hence the voltage across R1 is equal to the source voltage minus the voltage across D1.
Vr1 = 10 volts – 0.6 volts = 9.4 volts
The current through R1 equals the voltage across R1 divided by the resistance of R1
Ir1 = 9.4 volts/50 ohms = 0.188 amperes
Likewise the voltage across R2 is equal to the source voltage minus the voltage across D2.
Vr2 = 10 volts – 0.6 volts = 9.4 volts
The current through R2 equals the voltage across R2 divided by the resistance of R2.
Ir2 = 9.4 volts/20 ohms = 0.47 amperes
The total current is the sum of the currents in each branch.
It = Ir1 + Ir2 = 0.188 + 0.47 = 0.658 amperes
I have a Bachelor of Science in Electrical Engineering along with experience in digital logic design,testing and troubleshooting.
Digital and Analog Circuits and Systems