DC circuits and electronic principles

NAVEDTRA 14104 Ch. 12 — Resistance

Electrical resistance (R) is that property of an electric circuit that opposes the flow of current. The unit of resistance is known as the ohm (S2). WATT Power (P) is the rate of doing work. In a dc circuit, power is equal to the product of the current times the voltage, or P = I x E. The practical unit of power is the watt (W) or kilowatt (kW) (1,000 watts). Power in an ac circuit is computed in a slightly different way. If you are interested in how ac power is computed, see chapter 4 of Introduction to Alternating Current and Transformers, NAVEDTRA 172-02-00-85.

NAVEDTRA 14104 Ch. 2 — Ohm’s Law

In the early part of the 19th century, George Simon Ohm proved by experiment that a precise relationship exists between current, voltage, and resistance. This relationship is called Ohm’s law and is stated as follows: I = E/R, where: I = current in amperes, E = voltage in volts, and R = resistance in ohms. As stated in Ohm’s law, current is inversely proportional to resistance. This means, as the resistance in a circuit increases, the current decreases proportionately. In the equation I = E/R, if any two quantities are known, the third one can be determined.

NEETS Mod. 1 §3-1

Ohm's law and electrical power Ohm's law is the foundation of all circuit calculation: the current in a circuit is directly proportional to the voltage and inversely proportional to the resistance, expressed E = I × R, where E is electromotive force in volts, I is current in amperes, and R is resistance in ohms. Rearranged: I = E/R and R = E/I. If any two quantities are known the third is found. Resistance opposes current flow and depends on the conductor's material (resistivity), length (longer = more resistance), cross-sectional area (thicker = less resistance), and temperature (for metals, resistance rises with temperature). Electrical power — the rate of doing electrical work — is measured in watts: P = E × I. Combined with Ohm's law this gives the useful forms P = I²R and P = E²/R. T…

NEETS Mod. 1 §3-2

Series circuits and Kirchhoff's voltage law A series circuit provides a single path for current, so the same current flows through every component. Its rules follow from that: the total resistance is the sum of the individual resistances (Rt = R1 + R2 + R3 …); the current is the same at every point (It = I1 = I2 = I3); and the source voltage divides among the resistances in proportion to their size. Kirchhoff's voltage law states that the algebraic sum of all voltage drops around a closed loop equals the applied voltage (or, the sum of all voltages around any closed loop is zero). Thus Et = E1 + E2 + E3. Each resistor's drop is found by Ohm's law using the common current (E1 = I × R1), making series strings act as voltage dividers. A break anywhere (an open) stops all current and, because…

NEETS Module 16 §3.4.4

Voltage Tests Voltage tests must be made with the power applied; therefore, the prescribed safety precautions must be followed to prevent injury to personnel and damage to the equipment. You will find in your maintenance work that the voltage test is of utmost importance. It is used not only in isolating casualties to major components but also in the maintenance of subassemblies, units, and circuits. Before checking a circuit voltage, you should check the voltage of the power source to be sure that the normal voltage is being applied to the circuit. The voltmeter is used for voltage tests. In using the voltmeter, make certain that the meter used is designed for the type of current (ac or dc) to be tested and has a scale with a suitable range. Since defective parts in a circuit can cause h…

NEETS Module 16 §4.2.1.2

Measuring dc Voltages You set the multimeter to operate as a dc voltmeter by placing the function switch in either of two positions: +DC or −DC. The meter leads, as in the case of the ohmmeter function, must be connected to the proper meter jacks. When you measure dc voltages, be sure the red lead is the positive lead and the black lead is the negative, or common, lead. View A of figure 4-4 is a functional block diagram of dc voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc voltages. When the meter is connected in a circuit, it becomes a circuit component. Because all meters have some resistance, they alter the circuit by changing the current. The resistance presented by the voltmeter depends on the amount of voltage being measured and the pos…