Electric 101 Back to Basics Part V COLUMN BY BOB THOMAS Below are few questions to test your memory regarding what was covered last month. See if you can answer all of them correctly. 1 — When total resistance in any cir- cuit is increased, amperage will always (a) remain unchanged (b) increase (c) decrease (d) vary depending on type of resistance. 2 — In a series circuit, total resistance is always (a) equal to 1 ohm (b) equal to the sum of all resistance values in the cir- cuit (c) less than the path of least resist- ance (d) greater than circuit voltage. 3 – In parallel circuits, current flows (a) faster (b) through multiple paths (c) slower (d) through the path of least resistance. 4 — The formula used to determine total resistance in a parallel circuit is known as the (a) Resistance Formula (b) Parallel Formula (c) Reciprocal Formula (d) Ohm’s Law. 5 — In a parallel circuit, total resist- ance always as more paths are added. (a) increases (b) decreases (c) stays the same (d) varies with voltage 6 — In a parallel circuit, total resist- ance will always be the resistance of the path with least resist- ance. (a) less than (b) greater than (c) the same as (d) double You’ll find the answers at the end of this article. If you missed more than one, I’d recommend reading Part IV again. Understanding resistance and its affect on current flowing through both series and parallel circuits is important. The alternators, starters and generators we rebuild every day contain both types of circuits. In a previous article, we explained that conductors are materials through which electrons can flow easily. Although copper is the most common conductor used in alternators and starters, most other metals are conduc- tors too. Silver is the best conductor because it has the least resistance to electron flow. However, because of cost, it is only used in critical applications. Tron, aluminum, lead, zinc, tin, nickel, and even mercury are all used as con- ductors in automotive parts. Insulators, on the other hand, could be thought of as the opposite of conduc- tors. While we commonly think of them as materials through which electrons can- not flow, that is really not true. Insulators are materials through which electrons cannot flow through easily. Given enough voltage, any material can become a conductor. For example, even the best insulator will not stop a lightning bolt from going where it wants to. Insulators are rated by dielectric strength. Dielectric Strength is the maximum working voltage a material can with- stand without breaking down. It is represented in volts per millimeter in thickness. The best insulators will carry ratings well over several thousand volts. Having a rating this high may not seem too important when working with 12 volts DC. However, high voltages do occur for short periods, even in low voltage circuits. Most insulators are man-made mate- rials or naturally found compounds. Glass, mica, rubber, plastics, varnish and most epoxies are all good insula- tors. The best insulators have a high dielectric strength, remain solid at high temperatures, are flexible, and yet tough enough to withstand vibration and stress. In tightly wound solenoid and rotor coils, as well as stators, the integrity and dielectric strength of the insulating varnish is very important. High voltage testing is the only way to ensure the quality of this insulation. There is a third group of materials that can serve as either a conductor or an insulator, and we call them semiconduc- tors. Without semiconductors, our world would be a different place. They are used to manufacture solar cells, integrated cir- cuits, diodes, transistors and even lamps (LED — light emitting diode). While sili- con is the most common semiconductor utilized in our industry, selenium and germanium are also used. A semiconductor can act as either a conductor or/and insulator, depending on conditions, but they are really not very good at either one. There is a cost to pay for the ability to play a dual role. For example, when forward voltage is applied to a diode, using it as a con- ductor, internal resistance to the flow of electrons causes a drop in voltage (about .7 volts) and that energy is lost as heat. When reverse voltage is applied to the same diode, using it as an insulator, a small quantity of electrons will leak through (a few milliamps). When we test diodes, it is those readings that tell us the condition of the diode. In spite of those shortcomings, the unique properties of semiconductors seem to offer endless possible uses. In later parts of this series, we’ll explain how they are used to make diodes and transistors. The answers to the question at the beginning of this article are: 1-c, 2-b, 3- b, 4-c, 5-b, and 6-d. Next month Ill explain the most overlooked part in an alternator. Do you know what it is? E-mail me if you do: elebob @aug.com. Bob Thomas is owner of B & H Electric, Jacksonville, Fla.