Parallel Circuit    
    A parallel circuit is one that has two or more paths for the electricity to flow. In other words, the loads are parallel to each other. If the loads in this circuit were light bulbs and one blew out there is still current flowing to the others as they are still in a direct path from the negative to positive terminals of the battery

Combination Circuit
    A combination circuit is one that has a "combination" of series and parallel paths for the electricity to flow. Its properties are a synthesis of the two. In this example, the parallel section of the circuit is like a sub-circuit and actually is part of an over-all series circuit.

    In 1887 direct current (DC) was king. At that time there were 121 Edison power stations scattered across the United States delivering DC electricity to its customers. But DC had a great limitation, namely, that power plants could only send DC electricity about a mile before the electricity began to lose power. So when George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power, people naturally took notice. A "battle of the currents" ensued, but in the end, Westinghouse's AC prevailed.

    This is a simple explanation that shows the difference between AC and DC. A generator consists of some magnets and a wire that is wrapped to form several coils, known as an armature. A hydro turbo engine or some other outside source of motion moves the armature through the magnetic field created by the magnets.  In the example a loop of wire is spinning within a magnetic field. Because it is always moving through the field, a current is sustained, but because the loop is spinning, it's moving across the field first in one direction and then in the other, which means that the flow of electrons keeps changing.

    One advantage that AC has over DC is that it can easily be "stepped up" or "stepped down" with a transformer. In other words, a transformer can take a low-voltage current and make it a high-voltage current, and vice versa.

    This comes in handy in transmitting electricity over long distances. Since AC travels more efficiently at high voltages, transformers are used to step up the voltage before the electricity is sent out, and then other transformers are used to step down the voltage for use in homes and businesses.

A transformer has two windings; the primary and the secondary. The primary is the coil which receives the energy (Ep voltage). The secondary is the coil which discharges the energy (Es voltage) at the transformer or the changed voltage. If the secondary voltage is lower, it is called a step down transformer. If the secondary voltage is higher, it is called a step up transformer.

   The secondary voltage is dependent upon the voltage of the primary , the number of turns of the primary windings, and the number of turns of the secondary windings.

    Switches like this are used to rewind cassettes in cassette tape players, but industrial motors require more complex switching and  motor starting contactors because of the higher voltage and current requirements. Three phase power is required for most industrial uses.

    Contacts are referred to as normally open or normally closed. When a coil of a relay is energized, the plunger moves through the coil, closing the normally open contacts and opening the normally closed contacts.  When the contact is initially closed, there is a inrush of current which is about six to eight times the sealed current. As the plunger moves into the coil, closing the gap, the current levels drop to a low level. The contacts tend to get burned after thousands of repeated uses.

    Mechanical switches are hand operated, linear motion, or rotary motion components that control current flow in a circuit. They are often used as signal devises, to switch circuits on or off, or even to control the direction of motors by reversing the current flow. In machine control these switches are used to supply the plc with information relative to the position of a machine part like cylinder extend or retract.

    Proximity switches consist of a sensor used to detect the presence of an object without physical contact such as by magnetic fields, radio frequency, capacitive fields, and light rays. There are many more types of electrical switches, but these are the two most used in machine control.