To learn electronics, we want to understand:
1) What is voltage and current?
2) What is an electronic component?
3) How are components connected together?
Voltage and Current
Voltage is a potential differnce (a difference in electrical energy) so that when a charge is present it will want to move to the place of lower energy. Current is the flow of charge. If you were to count the number of charges passing a specific point in a wire every second, you would be measuring the current. Current is measureed in colombs (a unit of charge) per second which is also abreviated as Ampere (A).
Most people use a water analogy. More water flowing though a pipe as analagous to more current flowing though a wire. We can have more water (current) flowing by at a given moment if:
1) there is more pressure (voltage) pushing the water (charge) though the pipe (wire)
2) the pipe (wire) is wider (more conductive) so that more water (charge) can fit in the pipe (wire).
The exact relationshop between the voltage and current will varry based on what components we are using.
Components
A "component" can be any electronic device. There are many basic components including: resistors, capacitors, inductors, diodes, transistors, and many others.
First, we will focus on the component itself. For now, we will only use two terminal components. This means there are only two connections between the component and the rest of the circuit. Below is an example of a component:
The box represents the component, and the lines connected to the box represent wires. Components can either absorb power from the circuit or give power to the circuit. For example, the electric company supplies power into the electric grid and your kitchen stove absorbs that power and converts it into heat. Each component will have a voltage (potential difference) between each of its ends (indivated as V) and a current (indicated as I) that flows through it.
Sign Convention
There is a sign convention which is used with electric circuits. We say that current flows into the negative side (and out of the positive side) of a component that is supplying power. Likewise, current flows into the positive side (and out of the negative side) or a component that is absorbing power:
Loops
When we put the component together, this forms what is called a circuit. The circuit below shows a battery (supplying power) and a resistor (absorbing power) in a closed loop. Current flows from the battery to the resistor and back again to the battery in this loop. Power is being supplied from the battery to the resistor. The loops must be closed in order for the current to flow.
The charge that come out of the positive side of the battery is at a high electric potential. It then flows through the resistor, losing it's potential and returns to the battery and the cycle repeats.
Kirchhoff's Law's: The Voltage Law
Kirchhoff's voltage law (KVL) is a simple re-statement of the law of conservation of energy which states that energy can only be converted from one form to another, it cannot be created or destroyed. In terms of a circuit, this means that the sum of voltages around any loop in the circuit must be zero. In other words, all voltage from a power source must be used up - there can't be anything left over. The best way to write the mathematical formula is to pick a starting point (usually the negative terminal of a battery) and go around the loop.
1) If while you are going around the loop and you go from the negative side to the positive side, then the voltage increased (write a positive number for the voltage).
2) If while you are going around the loop and you go from the positive side to the negative side, then the voltage decreased (write a negative number for the voltage).
Finally, when you get back to where you started write equals zero ( = 0)
In the circuit above, V1 is a voltage increase, so it is positive and V2 is a voltage decrease, so it is negative. So we can write:
If there are multiple loops in the circuit, a KVL equation can be written for each one. For example, here is a circuit with three loops. Assume the component with voltage V1 is a battery (supplying power) and other components are resistors (absorbing power).
I chose to start at the negative side of V1 - which is always a good place to start.
These equations can be later used to solve for voltages in the circuit.
Nodes and Ground
Notice that there is no component between the negative side of component V1 and the negative side of component V3, they are simply connected by a wire. Wherever elemnets connect, this is called a node. Note that if a node is connected all together at one point (diagram A) or has a wire between two connection points (diagram B), it is still the same node because there are no components between the connection points.
Nodes are often labelled with voltages. Since voltage is always measured between two points, it is useful to have a reference point. Ground is simply a name used for the reference point in the circuit. The ⏚ symbol should be placed at the point where you selected to be ground. There are different types of ground, some mathematical and some the physical earth, but this is for a future blog.
Below we can see there are 4 different nodes. x, y, z, and ground (g)
To calculate, for example, the voltage Vy (meaning the voltage between node y and ground) we can start at ground and work our way towards y:
Rearranging, we get:
We can also see from the picture that Vy is equal to V3 since Vy is the voltage between node y and ground just as V3 is the voltage over component 3, which on one side is Vy and the other side ground.
Kirchhoff's Law's: The Current Law
Similar to KVL, Kirchhoff's current law (KCL) states that charge cannot be created or destroyed. In practical terms, this means that whatever charge enters a node most also leave that node. For example, I1 and I2 are entering the node below and I3 and I4 are leaving the node.
Note: A dot is usually placed wherever wires connect.
To write an equation, you can either write:
currents entering node = currents leaving node
or
currents entering node - currents leaving node = 0
or
- currents entering node + currents leaving node = 0
They are all just mathematical rearrangements of the same statement.
Using the first form:
Going back to our circuit, we can write a KCL statement at the point y and at ground:
This gives the KCL statements:
Note: We don't need to write a KCL statement at node x or node z because there are only two connections (one continuous wire).
Labeling Circuits with the Sign Convention
Finally, if a circuit is not labelled with voltage polarities and directions for current, what should I do?
1) Start with the components that supply power since you will know which side is positive.
Draw the current leaving the positive side of the component that supplies power and going into the next component or node from all components that are supplying power.
2) Make an assumption for the current in other components that are absorbing power.
For components absorbing power, make a guess for the direction of the current might be. Which direction you guess does not matter as long the voltage is drawn correctly according to the sign convention. If you made a wrong assumption about the direction of current, then your calculated answer will be negative - meaning your arrow for direction should be the other way.
3) Draw the voltage polarities for elements absorbing power.
Draw the voltage polarities according to the sign convention based on the direction of current which you drew in step 2.
Example
Draw the direction of current and label the voltage polarities on each component below. Assume V1 is supplying power and all other components are absorbing power.
The first step is to draw the current leaving the positive of V1 (shown in green). After this is is a good assumption to assume that current will split in two parts each going to component 3 (blue) and 4 (orange). Then they will rejoin and flow though component 6 back to component 1.
After this, we label the components according to the direction of current: Remember, current flows into the positive side of components absorbing power.
