BASIC ELECTRICAL TRAINING COURSE

Transcription

1 BASIC ELECTRICAL TRAINING COURSE 5733WEST WHITTIER AVENUE HEMET, CA TELEPHONE (909) FAX (909)

2 Introduction This article is not an attempt at a basic course in electronics - rather, it is a course that will help you understand the electrical requirements that pertain to connectors. It s pretty basic stuff - we promise not to get much heavier than Ohm s Law and a few buzzwords. You ll also be introduced to wire and cable so you ll have a better understanding about connector applications. Buzzwords Voltage or EMF (E or V) = Volt (E or V) Current (l) = Ampere (A) Resistance (R) = Ohm ( Ω) Power (P) = Watt (W) Time (t) = Second (S) Frequency (F) = Hertz (Hz) Measuring Section 1 In the world of physics it seems we deal in either very big or very small numbers. For example, the leak rate for a Deutsch hermetic (glass sealed) connector is cubic centimeters per second. And the resistance of a Deutsch insert between adjacent contacts is 5,000,000,000 ohms. As you might be able to tell, these numbers can get out of hand rather quickly. That s why we use some special prefixes and/or express the values exponentially. Figure I covers some frequently use words. They are used as a prefix to the unit of measurement. Let s take micro, a term used to express a millionth. One hundred millionth would be called one hundred micro inches. A familiar term to you FM radio listeners is megahertz. If your favorite station is FM, it is broadcasting at a frequency of 105,100,000 hertz (or cycles per second). However, we generally use mega when speaking or writing these numbers so that we say megahertz, or write MHz. Amount of Multiplication (Figure 1) Prefixes Symbol Expressed in Words Expressed in FIgures 10 to the nth Tera T One trillion 1,000,000,000, Giga G One billion 1,000,000, Mega M One million 1,000, Kilo k One thousand 1, Milli m One-thousandth Micro u One millionth Nano n One-billionth Pico p One-trillionth

3 Another way of getting rid of all those zeros is to use exponents, both positive and negative. Let s review familiar Algebra 101. Exponents are a sort of shorthand used to write numbers. For example: 1 x 10 2 = 1 x 10 x 10 = x 10 3 = 1 x 10 x 10 x 10 = x 10 4 = 1 x 10 x 10 x 10 x 10 = 10,000 4 x 10 4 = 4 x 10 x 10 x 10 x 10 = 40,000 By now you ve gotten the idea. Where this comes in handy is in trying to express really big numbers like 1,000,000,000,000,000 - which written exponentially is 1 x * Here s how to convert large numbers to exponents; just add up the zeros and use that number as the exponent. Since 70,000,000,000,000 has 13 zeros, we write it as 7 x * To convert an exponent back, add the zeros back to the number. What we re doing, of course, is moving the decimal point to the right. For example: 6.04 x * = 6,040,000,000,000 Exponents also work for small numbers. Here we use a minus (-) sign before the exponent to tell us to count the other way. Back to our hermetic connector leak rate: instead of saying we have a leak rate of centimeters per second per inch of seal, we can say our leak rate is 1.04 x 10-7 * cc/sec. Like positive exponents, each change in the exponent by one represents a change in the number by ten. Therefore, we move the decimal to the left to convert an exponent to a decimal. For example: 1.04 x 10-7 * = To show a decimal fraction exponentially, move the decimal point to the right until you have a whole number, then use the number of places moved as your negative exponent: = 5.4 x 10-8 * You ve probably noticed that the negative exponent is one less than the number of zeros used in the decimal fraction. So we can write values several ways now: 2,800,000,000 ohms can be shown as 2.8 x 10-9 * ohms or 2.8 gigaohms or 2,800 megohms. Section 2 Understanding some basics about electricity will help us understand some of the electrical requirements of connectors. To understand electricity, you ll need to know about the makeup and behavior of atoms. Atoms are the building blocks of nature. All matter, from air to zinc, is made up of combinations of different kinds of atoms. Although the makeup of various atoms differ, they all contain three elements - neutrons, protons, and electrons. ATOM (FIGURE 2) Electrons ( - ) Protons ( + ) NUCLEUS Protons and neutrons are clustered in the center of the atom (called the nucleus), while electrons spin or orbit. Protons have a positive charge, neutrons are electrically neutral, and electrons have a negative charge. Atoms in a normal state have the same number of electrons and protons and therefore are themselves

4 electrically neutral. Electrical activity occurs with the changes of electrons in atoms. Different atoms, and therefore different materials, have different numbers of electrons in different configurations. Here are examples of two atoms, neon and sodium. (Figure 3) A quick look tells us that the outer ring, or energy level, is quite different between the two. The neon atom s outer energy ring is completely filled, while the sodium atom s outer ring has only one electron. This single electron is easily dislodged and transferred to another atom. When this happens, the sodium atom now has a positive charge. That s because there are now more positive protons in the atom than negative electrons. Now the atom will attach electrons to put itself back into equilibrium. This flow of electrons from atom to atom is the usual definition of electricity. 2 ATOM (FIGURE 3) Neon Atom Sodium Atom However, our neon atom with its outer ring filled with electrons is considered stable because its outer ring of electrons tends not to be dislodged. The bottom line here is that neon atoms - and other atoms like it - restrict the flow of electrons and are considered excellent insulators. Sodium atoms and other atoms such as copper, with their single atom in the outer ring, are unstable and therefore readily conduct electricity. Now we ll look at the flow of a large number of electrons through a mass of atoms. When a material with a surplus of electrons is connected to a copper wire there is a tendency for some of the excess electrons to flow into the copper wire. But since the electrons have nowhere to go, a state of equilibrium is quickly reached and electron flow stops. If we connect the other end of the wire to a material composed of atoms with a deficiency of electrons, things begin to happen. The first material ( with the excess electrons and negative charge) will allow its excess electrons to pass through the copper wire to the material with the deficiency of electrons ( with a positive charge). And we ve now formed the basic electrical circuit. Here s how it works: since the single electron in a copper atom s outermost energy level is easily dislodged, the electrons move from atom to atom as long as the circuit is completed. When an electron leaves a particular atom to travel to the next one, it leaves behind a vacant spot or hole in the last atom s outer energy level. This atom then takes on an overall positive charge since it now has one extra proton. The result is that a new electron is attracted to the atom to fill the hole and the process of electrical movement is continued until equilibrium is achieved. (Figure 4) PROCESS OF ELECRTICAL MOVEMENT (FIGURE 4)

5 Summary Atoms, the basic building blocks of matter, are composed of: neutrons... no charge electrons... negative charge protons... positive charge In a stable condition there are an equal number of electrons and protons. If an atom loses electrons, it will attempt to regain electrons. Atoms with an excess of electrons have a negative charge. Atoms with an excess of protons will have a positive charge. An atom with few electrons in its outer ring readily loses them and is considered a conductor An atom with a full complement of electrons in its outer ring is considered stable and is a good insulator. Figure 5 lists some common insulators and conductors. Since the ability or inability to control electricity isn t the only factor, we ve included other characteristics than connector manufacturers use in selecting materials. Conductors ( Figure 5) Material Relative Resistance* Relative Cost* Silver Copper Gold 1.4 5,600 Aluminum Zinc Brass Iron Nickel Tin Steel Solder Lead Nichrome * Average value, relative to copper equaling 1.00 Insulators (Figure 5) Material Dielectric Strength** Air 80 Ceramics 200 Glass 200 Mica 2,000 Mylar 7,000 Nylon 385 Oil 350 Paper 1,250 Phenolic 400 Polystyrene 1,500 Poly-imide (plastic) 600 Porcelain 750 Silicone 450 Teflon 480 Wax 1,000 **Average value, volts per mil

6 Current The orderly flow of electrons from one point to another is called an electrical circuit. In the last section we discussed how that occurred - electrons moving in a path from a material of excess electrons to a material with fewer electrons. Although it took a while to explain it, the process is actually very fast. The basic unit of measurement of current is the ampere, otherwise known as the amp. An amp is the flow of 6.26 x (*superscript) electrons per second. Electron flow can be compared to water flowing in a closed system. (Figure 6) The rate of water flow is measured in gallons per minute while electrical current is measured in amps. Like water, the bigger the pipe, the greater the flow. In electronics, the larger the electrical connector, or wire, the friction (resistance) and the resulting heat buildup is what limits the current-carrying capacity of a wire or other conductor. The other factor in current is time. That is, a given wire can handle very high currents for very short times. WATER FLOW (FIGURE 6) As you might realize by now, current can t be discussed without discussing two other key factors - resistance and electromotive force (voltage). Voltage While current defines the number of electrons passing a given point, voltage defines the amount of force required to push the electrons past that point. Using the water system analogy, the water flow (amps) is dependent upon the amount of pressure in the system as measured in pounds per square inch (PSI). It stands to reason that a lot more water comes out of a hose when the pressure is at 90 PSI than when at 30 PSI. The same is true of electricity. However, we measure electrical pressure in volts. Figure 7 continues our water analogy showing a water tank with three different levels. The more water in the tank, the higher the pressure and the higher the flow. This means that the higher the voltage, the higher the current. Voltage represents electrical pressure, also know as potential. So a high potential voltage between two materials doesn t mean anything unless they are connected in a circuit.

7 WATER WATER SYSTEM ANALOGY: PSI (FIGURE 7) WATER WATER Sources of Voltage or Electromotive Force Batteries - produce direct current through a chemical reaction. Generators - produce alternating current by converting mechanical energy. Photoelectric cell - produces direct current by converting light energy. Thermocouple - produces direct current through the reaction of two dissimilar metals. Fuel Cell - produces direct current by converting gases. Solar Cell - produces direct current by directly converting sunlight. Alternating and Direct Current Direct current is a constant output from an EMF source. On a grid that shows voltage and time (Figure 8), the output from a battery would be a straight line. Of course if we were to chart the life of a battery, the line would dip down as the battery s EMF was used up. Voltage Current DIRECT CURRENT (FIGURE 8) Time Alternating current is much different. The current produced by a generator reverses direction several times each second. A graph plotting the output of a generator is shown in Figure 10. Notice that a negative

8 voltage is shown as the current reverses. The shape of the curve is known as a sine wave. Some generators are built to produce DC. The number of cycles per second that a generator operates at is known as hertz (Hz). (Figure 9) House current (and the current in factories and test labs) operates at 60 Hz. That means that all electrical appliances turn off and on 60 times each second. 1 Cycle (Hertz) ALTERNATING CURRENT (FIGURE 9) 1 Second AC vs. DC Generally speaking, appliances that are designed to operate with DC do not work with AC. Since most electrical systems are designed to operate on DC, most customers will need to know voltage ratings in DC. Most electrical testing, however, is done in AC because AC voltages are easier to obtain. Comparing AC and DC voltages takes a bit of explanation and math. That is because the voltage is computed by a procedure called Root Mean Square. This effective voltage in Alternating Current (AC) can be compared to the Direct Current (DC) voltage. (Figure 10) 1.0 AC peak vs. AC (RMS) AC PEAK VS. AC (RMS) (FIGURE 10) To convert AC peak to AC (RMS) or DC, multiply by.707. To convert AC (RMS) or DC to AC peak, multiply by Application Can a connector rated at 2,000 volts DC be used for an application that requires 1,500 volts AC (RMS)? No. That connector will see 2,020 volts (1,500 x 1.414) at peak, which is above its rated DC voltage. Can a connector rated at 1,500 volts AC (RMS) operate at 2,000 DC? Yes. Take the AC (RMS) rating (1,500) and convert to peak ( x 1.414). Compare the peak (2,120 volts) to the DC voltage required (2,000). Since the connector is rated higher than the requirement, it can be used. AC to DC : AC x.707 = DC DC to AC : DC x = AC Resistance Now for the other part of our electrical factors - resistance. Resistance is caused by friction, in this case by all those billions of electrons moving along a conductor. As you can imagine, the amount of friction or resistance depends on how many electrons are going by (amps) and how fast they are going (volts). In the water analogy we show resistance by the diameter of the pipe. In Figure 11 we show the effect on water flow and PSI by decreasing the size of the pipe. In electronics, resistance is measured in Ohms, usually shown by the Greek capitol letter Omega (Ω).

9 RESISTANCE (FIGURE 11) WATER Remember that all other things being equal, a larger size conductor will have a lower resistance. So when we increase resistance, we decrease amperage or must increase voltage to maintain the same amperage. Luckily, there s a simple formula that shows this relationship. It s called Ohm s Law. Ohm s Law Ohm s law states that the voltage in a circuit is equal to the current times the resistance. This formula can also be used to solve for current or resistance. Figure 12 shows this. To use the magic triangle, just pull out the piece you want to solve for and the remaining pieces are in the correct relationship. Problem Pipe Constriction (Resistance) Find the resistance of a contact if the voltage drop across the contact is 1.3 millivolts at 1 amp. CONVERSION CHART (FIGURE 12) I E R Solution Using our triangle, E/I=R, where E is 1.3 millivolts or.0013v and I is 1 amp, then.0013/1 = The answer is.0013 ohms or 1.3 milliohms. Below are two examples of questions you may encounter. 1. Deutsch connector specifications requires the connector to have an insulation resistance between contacts of 5,000 megohms at 500 volts DC. Your specifications call for a maximum leakage current of 10 microamps between connectors at 500 volts. Can Deutsch pass that spec? Compare the two specifications. 2. You have a requirement for minimum shell conductivity of 50 ohms at 5 volts. You have Deutsch test data that indicates a similar connector was tested with the same results except the test current was at 50 milliamps. Will Deutsch pass? Compare the two specs. The examples you ve just used are typical of the ways you ll need to use Ohm s Law to make an accurate comparison of performance data from two sources so that we can make sure we re comparing apples and apples. You might not be required to do the math, but you should understand the concepts behind these kinds of problems.

10 Another important aspect of understanding Ohm s Law is in understanding the relationship between voltage, current, and resistance - they are all interrelated, that increase in the resistance will drop the voltage, that a decreased resistance will drop the voltage, or that a decreased resistance will increase current. If you have any problems with questions such as these, please contact Deutsch Engineering. They will be happy to assist you in obtaining the correct answer. Section 3 Connector Testing Connectors, whether they re military or commercial, are generally required to pass a series of tests. These tests are designed to prove that the connector tested will meet its performance requirements. Tests are performed by the manufacturer, by the customer, and/or by a qualifying agency such as the military. Here are a few testing buzzwords: Developmental testing - Usually done by the manufacturer during the design and development stage for a new type of connector. Qualification testing - Used to prove that a connector design will meet the stated performance. Acceptance testing - Usually done on a continuing basis on every part shipped or received by a customer to verify that production parts are meeting stated performance. These tests are usually quick and easy to perform and, of course, nondestructive. Lot testing - To verify that a particular production run will meet stated performance. Since the lot test is usually extensive and contains destructive tests, it s run from a randomly selected group from a specified production lot. Usually the entire lot is not shipped or used until successful completion of the test. Failure analysis - Tests run after a failure to determine the underlying cause of the failure. Buzzwords Section 4 Wire and Cable AWG - American Wire Gauge. A standard numbering system for wire diameter. Coaxial Cable - Transmission line with a center conductor and an outer shield. It has a characteristic appearance. Flexible circuits - Conductors printed on a flat flexible material. Hookup wire - Single conductor cable used a general purpose wiring. Impedance - Total opposition a circuit offers to the flow of electrical energy. O.D. - Outside Diameter. Planar cable (flat cable) - Round or flat conductors laid side by side and adaptable to mass termination technicalities. Triaxial cable - A coaxial cable with an outer shield. Twinax cable - Two conductor cables where the conductors are laid side by side. VSWR - Voltage standing wave ratio; a way of measuring power loss (and therefore efficiency) in high frequency transmission lines. Any discussion of connectors should include wire. Both must have the same electrical requirements. Usually manufacturers specify wire first, since the choice of wire size must be consistent with the system s electrical requirements. Other important factors include cost and strength. The three basic types of wire are hookup wire, high frequency transmission lines, and shielded cable. Hookup Wire This is the standard single conductor wire with a plastic insulator. Conductors are copper and may be either solid or stranded. Although more expensive, stranded wire is stronger, more flexible and resistant to flexing failure than single conductors. Stranded conductors are usually plated in silver or nickel. HOOKUP WIRE (FIGURE 13) Solid Stranded

11 Wire sizes are specified in American Wire Gauge (AWG). (Figure 14) This system uses numbers to designate the diameter of the conductor. These sizes may range from 0000 (pronounced four ought), the size of your thumb, to 40 gauge, about the size of a hair. Figure 15 lists size, resistance and weight for most wire gauges. You ll notice that the bigger the number the smaller the wire. Also you can see the dramatic effect on weight by using a smaller gauge wire. For example, there s a 50% weight difference between wire gauges 20 and 23. This translates into several hundred pounds in an aircraft electrical system. However, the constraint to size is conductivity. As the size goes down, so does the conductivity. Looking at the chart again, our switch from 20 to 23 gauge wire also doubled the resistance of the wire. Since the big wire can carry more current, power circuits will use large gauge wire. Grounding circuits, with their need for low resistance, will also use large wire sizes. As we mentioned, a plastic or rubber jacket (typical materials include silicone rubber and plastics such as polyethylene, TFE, PVC, or FEP) insulates the conductor. Insulator types are chosen based on voltage, frequency, temperature requirements (a combination of outside air temperature and the heat buildup from the electrical current), cost and resistance to physical abuse and fluids. An important consideration for persons who specify connectors is the outside diameter of the wire insulation. The rear seal used in environmental connectors will only work with wires of specific diameters. As an example, the sealing range for a size 20 contact grommet is from.040 to.083 inches. That means that the outside diameter (O.D.) of the wire must be bigger than.040 inches and smaller than.083 inches to seal properly. So a wire with an insulation O.D. of.062 would work okay, but not wire O.D. s of.036 or.090. (See Figure 14 next page)

12 American Wire Gauge AWG Size (Dia. in mils) Resistance (Ohms over 1000 ft.) Weight (Lbs. per 1000 ft.)

13 High Frequency Transmission lines This category of cable includes coax, triax, twisted pair, twinax, and twin lead types. At high frequencies (radio frequencies and above), the signal starts to bulge and radiate away from the conductor. The higher the frequency, the worse the problem, so to keep the signal from radiating out into space, a second conductor is placed next to the conductor that is transmitting the signal. This second conductor reflects the signal back. In order to do this efficiently(without losing a lot of power) the cable and connectors must be of the same impedance. Impedance is a term used to describe the total opposition a circuit offers to the flow of alternating current. It s made up of both resistance and capacitance and measured in Ohms. The impedance of a cable is determined by the ratio of the diameter of the current-carrying conductor. Matching the impedance between cable and connectors must be maintained to keep the system from losing power. This efficiency is expressed as a ratio of the power put into a system to the power reflected back. It s known as the voltage standing wave ratio (VSWR). You ll see something like, VSWR 1: 1.05 at 5 MHz. Since frequency has everything to do with VSWR it must be specified. Just one thing to remember about VSWR - the lower the ratio the better it is. Although there a several types of transmission lines, the one you may use the most is coaxial cable. Deutsch makes coaxial contacts that fit into standard multipin connectors. Coaxial cable is specified by an RG number. It s like a specification that describes dimensions, electrical characteristics and materials. A typical number would run: RG 59/U. Coaxial Triaxial TRANSMISSION LINES (FIGURE 15) Twisted Pair Twinax Twin Lead Triaxial cable is similar to coaxial but adds an additional shield. Twisted pair is simply two pieces of hookup type wire twisted together. It s terminated to a connector with standard contacts and procedures. Twinax is shielded over a twisted pair. It s also known as twisted, shielded pair. Twin lead is pair of conductors placed in parallel. You know it as TV antenna lead. Shielded Cable Shielded cable is coaxial cable without any specified impedance. It is used to shield individual wires from outside interference, or used to keep signals from radiating out and disturbing others. Since there s no concern for power loss, there s no requirement for specified impedance or VSWR. Deutsch makes several types and sizes of shielded wire contacts. And if they fit, coaxial contacts can be used to terminate shielded wire. Other Types of Cables Planar (or flat) cable (Figure 16) consists of either rounded or flat conductors laid side by side in a flat plane. Flat conductor cable is normally mass terminated tail conductors (terminated one at a time) by solder. Round conductor cable is built in two versions - one for solderless mass termination techniques and one for standard crimp termination. Both types of round conductor cable be terminated to standard Deutsch connectors.

14 PLANAR CABLE (FIGURE 16) A flexible circuit (Figure 17) is a technique to print conductors on a thin, flexible material. It s usually found in enclosures to save space and assembly time. Flexible circuits are terminated to connectors by plugging in special contacts directly into a hole printed on the circuit for that purpose, in the same way that DIP sockets are plugged into printed circuit boards. FLEXIBLE CIRCUITS (FIGURE 17) These three books are readily available. Electrical/Electronics Interconnection Systems by Roland Lawrence Published by The Deutsch Company, Banning, California For Further Study Roland Lawrence was a Senior Vice President at Deutsch. The book is a guide to connector design and covers the common termination system, termination techniques, selection of connectors, plus a complete listing of wire and cable. Please Speak My Language by Eric Wyland An unpublished manuscript from The Deutsch Company (Available by writing to Deutsch Marketing Communications) Mr. Wyland was Chief Engineer at Deutsch. The book is nontechnical. Introduction to Electronics by Forrest M. Mims, III Published by Tandy Corporation (Available by writing to Deutsch Marketing Communications) A good introduction to electronics that goes from theory through passive and active components.