These days electronic circuitry is being used in many different types of equipment and this means that an understanding of basic electronics is required to be able to service these items. This section will present some basic electronic laws and formulas for the novice to electronics. These formulas, with examples can be used to make some of the basic calculations required in electronic circuits.
Voltage – Voltage is a measure of the force required to move an electrical charge. Named after Alessandro Volta who invented the forerunner to the modern battery – the voltaic cell. The symbol for voltage is V (sometimes E is also used and stands for Electro Motive Force (EMF).
Current – Current is the movement of electrons through a conductor. The symbol for Amperes is (I). Discovered by Luigi Galvani a physiologist – look up the story of how he discovered this – very interesting.
Resistance – Resistance is opposition to current flow. Symbol R. All substances have some resistance to current flow. Metals, such as gold or silver have very low resistance, plastics and porcelain have very high resistance. Temperature can also affect res istance.
Ohm’s Law – Ohm’s law is the most useful formula in electricity and electronics. Discovered by a German physicist Georg Ohm. Ohm’s law is expressed by the formula V=IR where I is the current in amperes, V is the voltage and R is the resistance. This formula is applicable for both DC and AC circuits.
V = IR
I = V/R
R = V/I
Series & Parallel Circuits – An illustration of series and parallel circuits are shown below.
Kirchoff’s Laws – Kirchoff’s laws are used to analyze arbitrary circuits that may contain several (or more) series and parallel circuits.
The first of Kirchoff’s laws states that at any junction in a circuit through which a steady current is flowing, the sum of the currents flowing to the point is equal to the sum of the currents flowing away from that point.
Applying the junction rule:
A: I2 = I1 + I3
B: I6 = I2 + I4
C: I6 = I3 + I5
D: I5 = I1 + I4
Kirchoff’s second rule, or the loop rule, states: around any closed loop in a circuit, the sum of the potential difference across all elements is zero.
Now take a look at the equation for each path:
Starting point B: E3 = R2 x I6
Starting point D: E2 = R1 x I5
Starting point A: E1 = R3 x I4
When analysing circuit using Kirchoff’s laws, it is helpful to use following guidelines.
1./ Draw the circuit and assign labels to the known and unknown quantities, including currents in each branch. You must assign directions to currents; don’t worry if you guess incorrectly the directions of a particular unknown current, as the answer resulting from the analysis in this case will simple come out negative, but with right magnitude.
2./ Apply the junction rule to as many junctions in the circuit as possible to obtain the maximum number of independent relations.
3./ Apply the loop rule to as many loops in the circuit as necessary in order to solve for the unknowns. Note that if there are n unknowns in a circuit you will need n independent equations. Generally there will be more loops present in a circuit than you need to solve for all the unknowns; the relations resulting from these extra loops can be used as a consistency check on your final answers.
4./ Solve the resulting set of simultaneous equations for the unknown quantities.