Electronics Information Study Guide for McGraw-Hill's ASVAB (page 3)

By — McGraw-Hill Professional
Updated on Jun 26, 2011


Electric current exists only when electrons can flow through a circuit. There are two basic types of circuits, and a third type that blends the two. Let's start with the basics: the series and parallel circuits.

In a series circuit, all moving electrons pass through every part of the circuit, including all the loads and switches. Current (the quantity of electrons) is the same at all points of the circuit, but voltage drops as the current goes through each device. The total voltage of the loads must equal the voltage of the circuit: In a series circuit supplied by a 12-volt car battery, a single light bulb must have a 12-volt drop. If the circuit has two lights, their combined voltage drop is 12 volts.

When a series circuit is used in a string of Christmas tree bulbs, the whole string goes dark if any bulb burns out. Thus, although they are simple, series circuits are less common than the next basic type, the parallel circuit.

Series, Parallel, And Series-parallel Circuits

In a parallel circuit, the loads are placed between the two supply wires, so that they all get the same voltage. A second advantage of the parallel circuit is this: Current can flow through any of the loads, even if one is switched off. (In a series circuit, a switch controls the current in the entire circuit.)

Series, Parallel, And Series-parallel Circuits

Remember this rule: Current is the same at all points in a series circuit. Voltage is the same at all points in a parallel circuit.

To find the total resistance in a series circuit, add the resistance of each load. For example, in a series circuit with one 12-Ω and one 8-Ω resistor, total resistance = 12 + 8 = 20 Ω.

It's more complicated to calculate total resistance in a parallel circuit since you must add the inverse of the resistances. What is the total resistance in a parallel circuit with one 12- Ω and one 8- Ω resistor?

1/Rtotal = 1/12 + 1/8 = 2/24 + 3/24 = 5/24 = 1/Rtotal

Solve for Rtotal:

Multiply both sides by Rtotal: 5 × Rtotal/24 = 1

Multiply both sides by 24: 5 × Rtotal = 24

Divide both sides by 5: Rtotal = 24/5 ohms

Note that Rtotal, 24/5, is simply the inverse of 5/24, the fraction equaling 1/Rtotal.

The third type of circuit is the series-parallelcircuit, which combines features of series and parallel circuits. The series-parallel circuit is a hybrid with many advantages of each. You'll find seriesparallel circuits throughout your house. The branch circuits that bring electric power to the lights and outlets are series-parallel: All the current goes through a fuse or circuit breaker, but then it is distributed in parallel. Why is this circuit needed? Because voltage must be the same for all outlets and lights, and because the circuit must work whether any particular light is switched on or not.

Series, Parallel, And Series-parallel Circuits


So far, we've talked about insulators and conductors. But there is an important category of materials between these two categories.

A semiconductor can act as a conductor or as an insulator. Silicon, the main semiconductor, is the basis for computer memory and logic boards. Chemicals called dopantsare applied to the silicon to determine whether it will act as an insulator or as a conductor. They do this by making electrons available or not available to flow. (When electrons can flow, a material becomes a conductor.)

The basis for computer applications is a group of components, particularly transistors and diodes.

Transistorsare devices that can switch a current, regulate its flow, or amplify a current, all based on the presence of a smaller current. Millions of tiny transistors are built on small pieces of semiconductor, which are the basis of computer logic and memory.


Diodesare devices that allow a current to flow in one direction only. In addition to electronics, diodes are also used in devices called rectifiers, which convert AC into DC.



The ASVAB will also ask you about more practical matters, including simple electric circuits. You'll have an advantage here if you've ever worked on your home wiring (which is usually much easier than most people think).

In a simple electric circuit like the one in your home, electricity is distributed at a fuse box or circuit breaker box. The box has two functions:

  • Breaking up the load in the building into a number of circuits
  • Preventing excess current from flowing into the circuits

Many simple circuits have two separate conductors: hot and grounded. The hot conductor is usually black, but it can be red or another color. The grounded (sometimes called neutral) conductor is white. Together, the black and white wires are called the supplywires, because they form the circuit that electric current needs to travel.

To work safely on any electric circuit, you need to shut off the electricity, and check that it is off. The fuses and circuit breakers can shut off the circuit. Before starting work, use an electrical tester to test whether the hot wires are energized.

Bigger (heavier) wires can carry more current. Home wiring systems are usually rated for 15 or 20 amperes. 15-ampere circuits require 14-gauge wires. Larger, 12-gauge wires are needed for 20-ampere circuits.

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