In this video, we will review the most commonly used components in electric circuits.
We will cover: Transistors, Capacitors, Inductors, Logic gates, Transformers, Operational amplifiers, and various other components.
Before we begin, let us first review the convention regarding current flow, as this causes a lot of confusion.
In an electric circuit, it is the negatively charged electrons that are free to move around, while the positively charged protons are fixed in place in the nuclei of the atoms.
However, this fact was not known when electricity was first discovered, and it was incorrectly assumed that it is the positively charged particles which are flowing in the opposite direction.
The convention for current flow was never updated, and the standard in industry and in education is still to show the direction of this so called "positive current", which is in the opposite direction of the actual electron current.
In this video, we will use the color red when referring to positive current.
And we will use the color blue when referring to electron current.
The differences in the elevation of different parts of a circuit will represent the differences in the voltage.
When a switch closes between two points of a circuit, it forces those two parts of the circuit to be at the same voltage, and hence at the same elevation.
Voltage is the energy per charged particle.
We will sometimes represent energy with boxes.
The first electric circuit component we will consider is an ideal battery.
An ideal battery will maintain a specified voltage difference across its terminals, so long as it has energy remaining.
We can increase the total energy stored by placing multiple batteries in parallel.
This increases the amount of time we have before the batteries run out of energy.
When we place multiple batteries in parallel, the total voltage output is the same as if we only had one battery by itself.
If we want to increase the total output voltage, we can do this by placing multiple batteries in series.
Higher elevations represent higher voltages.
Higher voltages mean more energy per charged particle.
Therefore, the charged particles are shown gaining energy boxes as they rise in elevation.
The greater the voltage drop across a light bulb, the greater the current flowing through it.
The charged particles lose energy boxes as they fall in elevation, and this energy is now associated with the light and heat coming from the bulb.
If we apply an AC voltage across the light bulb, it will behave as shown.
If we want to add the energy boxes to this picture, it will look as shown.
The next circuit component we will consider is an ideal transformer.
Let’s keep the AC input voltage the same, but add a transformer to the circuit.
A transformer has the ability to increase the amplitude of an AC voltage.
A transformer also has the ability to decrease the amplitude of an AC voltage.
It all depends on the ratio of the number of turns in each of the coils.
Let’s remove the transformer.
Resistors are the next component we will consider.
An equal amount of energy is lost as heat in both of these resistors.
The amount of energy lost each second is the power dissipation.
The power dissipation through a resistor is the current multiplied by the voltage drop across the resistor.
The voltage drop across a resistor is the current multiplied by the resistance.
The voltage drop across these resistors should not be confused with the voltage of the voltage source, as these are two different quantities.
It is only when a resistor is in parallel with a voltage source that the resistor’s voltage drop is equal to the voltage source.
If we double the voltage drop across the resistor by placing two batteries in series, we will double the current flowing through the resistor.
Now suppose that we keep the number of batteries constant, but instead increase the number of resistors in parallel.
Multiple resistors in parallel have the same effect as a single resistor with a smaller resistance.
On the other hand, multiple resistors in series have the same effect as a single resistor with a larger resistance.
Let us now consider diodes.
A diode allows current to flow in the forward direction, but blocks current from flowing in the reverse direction.
When a diode blocks current in the reverse direction, the two sides of the diode can be at significantly different voltages.
For an ideal diode, when current flows in the forward direction, the two sides of the diode are at approximately the same voltage.
There is a special type of diode called a Zener diode.
When current flows in the forward direction, a Zener diode acts like a normal diode.
What makes a Zener diode special is that it has a reverse breakdown voltage.
If a voltage smaller than the breakdown voltage is applied in the reverse direction, the Zener diode will block all current just like a normal diode.
But, if we try to increase the voltage beyond the breakdown voltage, the Zener diode will begin to conduct, while keeping the voltage drop across it at the breakdown voltage.
Diodes are semiconductor devices.
Another popular semiconductor device is a NPN bipolar transistor.
In a NPN bipolar transistor, a small positive current flowing into the middle terminal controls a much larger current flowing through the two other terminals.
A PNP bipolar transistor is similar to this, except that it is the positive current flowing “out” of the middle terminal that controls the current flowing through the two other terminals.
Let’s now consider a N channel Field Effect Transistor.
Here, it is not the current, but the "voltage" on the middle terminal which controls the current through the other two terminals.
This is similar for a P channel Field Effect Transistor, except that the effect of the voltage is reversed.
Field effect transistors are unable to block in the reverse direction because they intrinsically have a diode built into them as a part of their physics.
Transistors can be used to create logic gates.
In logic gates, the input voltages and output voltages are one of two values, which we refer to as either "High" or "Low."
For an "Or" logic gate: The output is high if one "or" the other input is high.
These outputs are all reversed for a "Nor" logic gate.
For a "And" logic gate, The output is high only if both one "and" the other input is high.
These outputs are all reversed for a "Nand" logic gate.
For a "Xor" logic gate, The output is high if one or the other input is high, but not both.
For a "Not" logic gate, also known as an "inverter": The output is the opposite of the input.
Logic gates are created from transistors.
Transistors can also be used to create operational amplifiers, also known as Op Amps.
In an ideal op amp, no current flows into the input terminals, and negative feedback causes the voltages at the two input terminals to be equal to each other.
Op Amps can be used to create many wonderful circuits.
This circuit is called an inverting amplifier, because when the input is positive the output is negative.
This is another common Op Amp circuit and it is called a non-inverting amplifier, because when the input is positive, the output is also positive.
In this circuit, all the resistors have the same resistance.
The output voltage is the negative of the sum of all the input voltages.
In this circuit, the output voltage is proportional to the negative of the integral of the input voltage.
In this circuit, the output voltage is proportional to the negative of the derivative of the input voltage.
These last two examples used capacitors, and capacitors are the next component we will consider.
A capacitor consists of two metal plates separated by a dielectric material.
A capacitor is able to store electric charge and hence store the energy associated with this electric charge.
A capacitor with a larger capacitance is able to store more charge and more energy for the same amount of voltage.
If we connect a battery to a capacitor through a resistor, the capacitor will charge to the voltage of the battery.
If we then disconnect the battery, and connect the capacitor to a light bulb, the capacitor will discharge its stored energy through the light bulb.
Suppose we increase the capacitance of the capacitor.
The capacitor will still charge to the voltage of the battery.
But, charging the capacitor to the voltage of the battery will now take a longer amount of time.
With a larger capacitance, the same voltage now represents a larger amount of charge, and hence a larger amount of energy.
Let’s apply an AC input voltage.
Even if we keep the amplitude of the input voltage constant, we can increase the current through the capacitor by increasing the frequency of the AC input voltage.
The voltage of a capacitor can’t change instantaneously.
By contrast, for an inductor, it is the current that can’t change instantaneously.
Even if we keep the amplitude of the input voltage constant, we can increase the current through the inductor by decreasing the frequency of the AC input voltage.
An inductor tries to keep the current flowing through it constant, and tries to oppose any change to the current.
Suppose we connect a battery to an inductor through a resistor.
The inductor will initially try to oppose the increase in current flow.
Once the current reaches a steady state value, the inductor will just act like an ordinary wire.
If we then connect the inductor to a resistor, the inductor will oppose the "decrease" in the current flow.
Suppose we connect an inductor to a capacitor as shown.
The current and voltage will oscillate at a resonant frequency that is determined by the values of the inductance and capacitance.
If this capacitor and inductor are connected in parallel with an AC voltage source with this exact same resonant frequency,
then from the perspective of the AC voltage source,
the capacitor and inductor parallel combination will behave like an open circuit.
On the other hand,
if this capacitor and inductor are connected in series with an AC voltage source with this exact same resonant frequency,
then from the perspective of the AC voltage source,
the capacitor and inductor series combination will behave like a short circuit.
All wires trying to conduct electricity have some inductance and some capacitance to them.
These capacitors and inductors shown are not components that we deliberately add to the circuit, but are an intrinsic part of all wires in all circuits.
This causes the voltages and currents to act as waves which travel down the wires.
These waves will reflect at the ends of the wires, unless the wires are properly terminated.
Electric circuit theory is based on Maxwell’s Laws of Electromagnetism.
To fully understand electric circuits, it is necessary to first understand Maxwell’s Laws of Electromagnetism.
These laws are explained in detail in the video titled "Electromagnetism – Maxwell’s Laws."
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