Metric Based Prefixes Used for Electrical Calculations
In any system of measurements, a single set of units is usually not sufficient for all the computations involved in electrical repair and maintenance. Small distances, for example, can usually be measured in inches, but larger distances are more meaningfully expressed in feet, yards, or miles. Since electrical values often vary from numbers that are a millionth part of a basic unit of measurement to very large values, it is often necessary to use a wide range of numbers to represent the values of units, such as volts, amperes, or ohms. A series of prefixes that appear with the name of the unit have been devised for the various multiples or submultiples of the basic units. There are 12 of these prefixes, which are also known as conversion factors. Four of the most commonly used prefixes in electrical work are:
Mega (M) means one million (1,000,000).
Kilo (k) means one thousand (1,000).
Milli (m) means one-thousandth (1⁄1,000).
Micro (μ) means one-millionth (1⁄1,000,000).
Kilo is one of the most extensively used conversion factors. It explains the use of prefixes with basic units of measurement. Kilo means 1,000, and when used with volts, is expressed as kilovolt, meaning 1,000 volts. The symbol for kilo is the letter “k.” Thus, 1,000 volts is one kilovolt or 1 kV. Conversely, one volt would equal one-thousandth of a kV, or 1 ⁄1,000 kV. This could also be written 0.001 kV.
Similarly, the word “milli” means one-thousandth, and thus, 1 millivolt equals one-thousandth (1⁄1000) of a volt. Figure 12-5 contains a complete list of the multiples used to express electrical quantities, together with the prefixes and symbols used to represent each number.
Electricity is often described as being either static or dynamic. The difference between the two is based simply on whether the electrons are at rest (static) or in motion (dynamic). Static electricity is a buildup of an electrical charge on the surface of an object. It is considered “static” due to the fact that there is no current flowing as in alternate current (AC) or direct current (DC) electricity. Static electricity is usually caused when non-conductive materials, such as rubber, plastic, or glass, are rubbed together causing a transfer of electrons, which results in an imbalance of charges between the two materials. The fact that there is an imbalance of charges between the two materials means that the objects will exhibit an attractive or repulsive force.
Attractive and Repulsive Forces
One of the most fundamental laws of static electricity, as well as magnetics, deals with attraction and repulsion. Like charges repel each other and unlike charges attract each other. All electrons possess a negative charge and as such repel each other. Similarly, all protons possess a positive charge and as such repel each other. Electrons (negative) and protons (positive) are opposite in their charge and attract each other.
For example, if two pith balls are suspended, as shown in Figure 12-6, and each ball is touched with the charged glass rod, some of the charge from the rod is transferred to the balls. The balls now have similar charges and, consequently, repel each other as shown in part B of Figure 12-6. If a plastic rod is rubbed with fur, it becomes negatively charged and the fur is positively charged. By touching each ball with these differently charged sources, the balls obtain opposite charges and attract each other as shown in part C of Figure 12-6.
Although most objects become charged with static electricity by means of friction, a charged substance can also influence objects near it by contact. [Figure 12-7] If a positively-charged rod touches an uncharged metal bar, it draws electrons from the uncharged bar to the point of contact. Some electrons enter the rod, leaving the metal bar with a deficiency of electrons (positively charged) and making the rod less positive than it was or, perhaps, even neutralizing its charge completely.
A method of charging a metal bar by induction is demonstrated in Figure 12-8. A positively-charged rod is brought near, but does not touch, an uncharged metal bar. Electrons in the metal bar are attracted to the end of the bar nearest the positively-charged rod, leaving a deficiency of electrons at the opposite end of the bar. If this positively-charged end is touched by a neutral object, electrons will flow into the metal bar and neutralize the charge. The metal bar is left with an overall excess of electrons.
A field of force exists around a charged body. This field is an electrostatic field (sometimes called a dielectric field) and is represented by lines extending in all directions from the charged body and terminating where there is an equal and opposite charge. To explain the action of an electrostatic field, lines are used to represent the direction and intensity of the electric field of force. As illustrated in Figure 12-9, the intensity of the field is indicated by the number of lines per unit area, and the direction is shown by arrowheads on the lines pointing in the direction in which a small test charge would move (or tend to move) if acted upon by the field of force.
Either a positive or negative test charge can be used, but it has been arbitrarily agreed that a small positive charge is always used in determining the direction of the field. Thus, the direction of the field around a positive charge is always away from the charge because a positive test charge would be repelled. [Figure 12-9] On the other hand, the direction of the lines about a negative charge is toward the charge, since a positive test charge is attracted toward it.
Figure 12-10 illustrates the field around bodies having like charges. Positive charges are shown, but regardless of the type of charge, the lines of force would repel each other if the charges were alike. The lines terminate on material objects and always extend from a positive charge to a negative charge. These are imaginary lines used to show the direction a real force takes.
It is important to know how a charge is distributed on an object. Figure 12-11 shows a small metal disk on which a concentrated negative charge has been placed. By using an electrostatic detector, it can be shown that the charge is spread evenly over the entire surface of the disk. Since the metal disk provides uniform resistance everywhere on its surface, the mutual repulsion of electrons results in an even distribution over the entire surface.
Another example, shown in Figure 12-12, is the charge on a hollow sphere. Although the sphere is made of conducting material, the charge is evenly distributed over the outside surface. The inner surface is completely neutral. This phenomenon is used to safeguard operating personnel of the large Van de Graaff static generators used for atom smashing. The safest area for the operators is inside the large sphere, where millions of volts are being generated.
The distribution of the charge on an irregularly-shaped object differs from that on a regularly-shaped object. Figure 12-13 shows that the charge on such objects is not evenly distributed. The greatest charge is at the points, or areas of sharpest curvature, of the objects.
Electrostatic Discharge (ESD) Considerations
One of the most frequent causes of damage to a solidstate component or integrated circuits is the electrostatic discharge (ESD) from the human body when one of these devices is handled. Careless handling of line replaceable units (LRUs), circuit cards, and discrete components can cause unnecessarily time consuming and expensive repairs. This damage can occur if a technician touches the mating pins for a card or box. Other sources for ESD can be the top of a toolbox that is covered with a carpet. Damage can be avoided by discharging the static electricity from your body by touching the chassis of the removed box, by wearing a grounding wrist strap, and exercising good professional handling of the components in the aircraft. This can include placing protective caps over open connectors and not placing an ESD-sensitive component in an environment that causes damage. Parts that are ESD sensitive are typically shipped in bags specially designed to protect components from electrostatic damage.
Other precautions that should be taken with working with electronic components are:
- Always connect a ground between test equipment and circuit before attempting to inject or monitor a signal.
- Ensure test voltages do not exceed maximum allowable voltage for the circuit components and transistors.
- Ohmmeter ranges that require a current of more than one milliampere in the test circuit should not be used for testing transistors.
- The heat applied to a diode or transistor, when soldering is required, should be kept to a minimum by using low-wattage soldering irons and heat sinks.
- Do not pry components of a circuit board.
- Power must be removed from a circuit before replacing a component.
- When using test probes on equipment and the space between the test points is very close, keep the exposed portion of the leads as small as possible to prevent shorting.