Module 1 - Introduction to Matter, Energy, and Direct Current |
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Module 1 − Introduction to Matter, Energy, and Direct Current
Pages i, 1−1, 1−11, 1−21, 1−31, 1−41, 1−51, 1−61, 2−1, 2−11, 2−21, 3−1, 3−11, 3−21, 3−31, 3−41, 3−51, 3−61, 3−71, 3−81, 3−91, 3−101, 3−111, 3−121, Appendix I, II, III, IV, V, Index For example, a voltage divider can be designed to provide the voltage and current to three loads from a given source voltage. Given:
The circuit is drawn as shown in figure 3-66. Notice the placement the ground reference point. The values for resistors R1, R3, and R4 are computed exactly as was done in the last example. IR1 is the bleeder current and can be calculated as follows:
Figure 3-66. - Voltage divider providing both positive and negative voltages. Solution:
Calculate the value R1.
3-101 Solution:
Calculate the current through R2 using Kirchhoff's current law. At point A:
(or 195mA leaving point A) Since ER2 = E load 2, you can calculate the value R2. Solution:
Calculate the current through R3.
The voltage across R3 (ER3) equals the difference between the voltage requirements loads 3 and 2. Solution:
Calculate the value R3.
3-102 Solution:
Calculate the current through R4.
The voltage across ER4 equals the source voltage (Es) minus the voltage requirement load 3 and the voltage requirement load 1. Remember Kirchhoff's voltage law which states that the sum the voltage drops and EMFs around any closed loop is equal to zero. Solution:
Calculate the value R4. Solution:
With the calculations just explained, the values the resistors used in the voltage /divider are as follows:
From the information just calculated, any other circuit quantity, such as power, total current, or resistance the load, could be calculated.
3-103 PRACTICAL APPLICATION Voltage DIVIDERS In actual practice the computed value the bleeder resistor does not always come out to an even value. Since the rule--thumb for bleeder current is only an estimated value, the bleeder resistor can be a value close to the computed value. (If the computed value the resistance were 510 ohms, a 500-ohm resistor could be used.) Once the actual value the bleeder resistor is selected, the bleeder current must be recomputed. The voltage developed by the bleeder resistor must be equal to the voltage requirement the load in parallel with the bleeder resistor. The value the remaining resistors in the voltage divider is computed from the current through the remaining resistors and the voltage across them. These values must be used to provide the required voltage and current to the loads. If the computed values for the divider resistors are not even values; series, parallel, or series-parallel networks can be used to provide the required resistance. Example: a voltage divider is required to supply two loads from a 190.5 volts source. Load 1 requires +45 volts and 210 milliamps; load 2 requires +165 volts and 100 milliamps. Calculate the bleeder current using the rule--thumb. Given:
Solution:
Calculate the ohmic value the bleeder resistor. Given:
Solution:
Since it would be difficult to find a resistor 1451.6 ohms, a practical choice for R1 is 1500 ohms. Calculate the actual bleeder current using the selected value for R1.
3-104 Given:
Solution:
Using this value for IR1, calculate the resistance needed for the next divider resistor. The current (IR2) is equal to the bleeder current plus the current used by load 1. Given:
Solution:
The voltage across R2 (ER2) is equal to the difference between the voltage requirements loads 2 and 1, or 120 volts. Calculate the value R2. Given:
Solution:
3-105 The value the final divider resistor is calculated with IR3 (IR2 + I load 2) equal to 340 mA and ER3 (Es - E load 2) equal to 25.5V. Given:
Solution:
A 75-ohm resistor may not be easily obtainable, so a network resistors equal to 75 ohms can be used in place R3. Any combination resistor values adding up to 75 ohms could be placed in series to develop the required network. For example, if you had two 37.5-ohm resistors, you could connect them in series to get a network 75 ohms. One 50-ohm and one 25-ohm resistor or seven 10-ohm and one 5-ohm resistor could also be used. A parallel network could be constructed from two 150-ohm resistors or three 225-ohm resistors. Either these parallel networks would also be a network 75 ohms. The network used in this example will be a series-parallel network using three 50-ohm resistors. With the information given, you should be able to draw this voltage divider network. Once the values for the various divider resistors have been selected, you can compute the power used by each resistor using the methods previously explained. When the power used by each resistor is known, the wattage rating required each resistor determines the physical size and type needed for the circuit. This circuit is shown in figure 3-67.
3-106
Figure 3-67. - Practical example a voltage divider. Q53. In figure 3-67, why is the value R1 calculated first, Q54. In figure 3-67, how is (a) the current through R2 and (b) the voltage drop across R2 computed,
Q55. In figure 3-67, what is the power dissipated in R1,
Q56. In figure 3-67, what is the purpose the series-parallel network R3, R4, and R5,
Q57. In figure 3-67, what should be the minimum wattage ratings R3 and R5,
Q58. If the load requirement consists both positive and negative voltages, what technique is used in the voltage divider to supply the loads from a single voltage source, EQUIVALENT Circuit TECHNIQUES The circuit solutions that you have studied up to this point have been obtained mainly through the use formulas derived from Ohm's law. As in many other fields science, electricity has its share special shortcut methods. Some the special circuit analysis techniques are: THEVENIN'S THEOREM, which uses a process circuit reduction to Thevenin's equivalent circuit; and NORTON'S THEOREM, which is reduction a circuit to Norton's equivalent. Another method is called Loop ANALYSIs. This uses Kirchhoff's voltage law to simultaneously solve problems in parallel branches a circuit. The use
3-107 these methods should be reserved until you have become thoroughly familiar with the methods covered thus far in this chapter. You may want to explore some the special techniques later in your career. ELECTRICAL Safety Safety precautions must always be observed by persons working around electric circuits and equipment to avoid injury from electric shock. Detailed safety precautions are contained in NAVMAT P-5100, Safety Precautions for Shore Activities and OPNAVINST 5l00-19, Navy Safety Precautions for Forces Afloat. The danger shock from a 450-volt ac electrical service system is well recognized by operating personnel. This is shown by the relatively low number reports serious shock received from this voltage, despite its widespread use. On the other hand, a number fatalities have been reported due to contact with low-voltage circuits. Despite a fairly widespread, but totally unfounded, popular belief to the contrary, low-voltage circuits (115 volts and below) are very dangerous and can cause death when the resistance the body is lowered. Fundamentally, current, rather than voltage, is the measure shock intensity. The passage even a very small current through a vital part the human body can cause DEATH. The voltage necessary to produce the fatal current is dependent upon the resistance the body, contact conditions, the path through the body, etc. For example, when a 60-hertz alternating current, is passed through a human body from hand to hand or from hand to foot, and the current is gradually increased, it will cause the following effects: At about 1 milliampere (0.001 ampere), the shock can be felt; at about 10 milliamperes (0.01 ampere), the shock is sufficient intensity to prevent voluntary control the muscles; and at about 100 milliamperes (0.1 ampere) the shock is fatal if it lasts for 1 second or more. The above figures are the results numerous investigations and are approximate because individuals differ in their resistance to electrical shock. It is most important to recognize that the resistance the human body cannot be relied upon to prevent a fatal shock from 115 volts or less - FATALITIES FROM VoltageS AS Low AS 30 VOLTS HAVE BEEN RECORDED. Tests have shown that body resistance under unfavorable conditions may be as low as 300 ohms, and possibly as low as 100 ohms from temple to temple if the skin is broken. Conditions aboard ship add to the chance receiving an electrical shock. Aboard ship the body is likely to be in contact with the metal structure the ship and the body resistance may be low because perspiration or damp clothing. Extra care and awareness electrical hazards aboard ship are needed. Short circuits can be caused by accidentally placing or dropping a metal tool, rule, flashlight case, or other conducting article across an energized line. The arc and fire which result, even on relatively low- voltage circuits, may cause extensive damage to equipment and serious injury to personnel. Since ship service power distribution systems are designed to be ungrounded, many persons believe it is safe to touch one conductor, since no electrical current would flow. This is not true, since the distribution system is not totally isolated from the hull the ship. If one conductor an ungrounded electrical system is touched while the body is in contact with the hull the ship or other metal equipment enclosure, a fatal electric current may pass through the body. ALL LIVE ELECTRIC Circuits SHALL BE TREATED AS POTENTIAL HAZARDS AT ALL TIMES. DANGER SignalS Personnel should constantly be on the alert for any signs which might indicate a malfunction electric equipment. Besides the more obvious visual signs, the reaction other senses, such as hearing, smell, and touch, should also make you aware possible electrical malfunctions. Examples signs which you must be alert for are: fire, smoke, sparks, arcing, or an unusual sound from an electric motor.
3-108 Frayed and damaged cords or plugs; receptacles, plugs, and cords which feel warm to the touch; slight shocks felt when handling electrical equipment; unusually hot running electric motors and other electrical equipment; an odor burning or overheated insulation; electrical equipment which either fails to operate or operates irregularly; and electrical equipment which produces excessive vibrations are also indications malfunctions. When any the above signs are noted, they are to be reported immediately to a qualified technician. DO NOT DELAY. Do not operate faulty equipment. Above all, do not attempt to make any repairs yourself if you are not qualified to do so. Stand clear any suspected hazard and instruct others to do likewise. . Warning Signs - They have been placed for your protection. To disregard them is to invite personal injury as well as possible damage to equipment. Switches and receptacles with a temporary warning tag, indicating work is being performed, are not to be touched. . Working Near Electrical Equipment - When work must be performed in the immediate vicinity electrical equipment, check with the technician responsible for the maintenance the equipment so you can avoid any potential hazards which you may not be immediately aware. . Authorized Personnel 0nly - Because the danger fire, damage to equipment, and injury to personnel, all repair and maintenance work on electrical equipment shall be done only by authorized persons. Keep your hands f all equipment which you have not been specifically authorized to handle. Particularly stay clear electrical equipment opened for inspection, testing, or servicing. . Circuit Breakers and Fuses - Covers for all fuse boxes, junction boxes, switch boxes, and wiring accessories should be kept closed. Any cover which is not closed or is missing should be reported to the technician responsible for its maintenance. Failure to do so may result in injury to personnel or damage to equipment in the event accidental contact is made with exposed live circuits. ELECTRICAL FIRES Carbon dioxide (C02) is used in fighting electrical fires. It is nonconductive and, therefore, the safest to use in terms electrical safety. It also fers the least likelihood damaging equipment. However, if the discharge horn a C02 extinguisher is allowed to accidentally touch an energized circuit, the horn may transmit a shock to the person handling the extinguisher. The very qualities which cause C02 to be a valuable extinguishing agent also make it dangerous to life. When it replaces oxygen in the air to the extent that combustion cannot be sustained, respiration also cannot be sustained. Exposure a person to an atmosphere high concentration C02 will cause suffocation.
First AID for ELECTRIC SHOCK A person who has stopped breathing is not necessarily dead, but is in immediate danger. Life is dependent upon oxygen, which is breathed into the lungs and then carried by the blood to every body cell. Since body cells cannot store oxygen, and since the blood can hold only a limited amount (and that only for a short time), death will surely result from continued lack breathing. However, the heart may continue to beat for some time after breathing has stopped, and the blood may still be circulated to the body cells. Since the blood will, for a short time, contain a small supply
3-109 of oxygen, the body cells will not die immediately. For a very few minutes, there is some chance that the person's life may be saved. The process by which a person who has stopped breathing can be saved is called ARTIFICIAL VENTILATION (RESPIRATION). The purpose artificial ventilation is to force air out the lungs and into the lungs, in rhythmic alternation, until natural breathing is reestablished. Artificial ventilation should be given only when natural breathing has stopped; it should NOT be given to any person who is breathing naturally. You should not assume that an individual who is unconscious due to electrical shock has stopped breathing. To tell if someone suffering from an electrical shock is breathing, place your hands on the person's sides, at the level the lowest ribs. If the victim is breathing, you will usually be able to feel the movement. Remember: DO NOT GIVE ARTIFICIAL VENTILATION to a PERSON WHO Is BREATHING NATURALLY. Records show that seven out ten victims electric shock were revived when artificial respiration was started in less than 3 minutes. After 3 minutes, the chances revival decrease rapidly. Once it has been determined that breathing has stopped, the person nearest the victim should start the artificial ventilation without delay and send others for assistance and medical aid. The only logical, permissible delay is that required to free the victim from contact with the electricity in the quickest, safest way. This step, while it must be taken quickly, must be done with great care; otherwise, there may be two victims instead one. In the case portable electric tools, lights, appliances, equipment, or portable outlet extensions, this should be done by turning f the supply switch or by removing the plug from its receptacle. If the switch or receptacle cannot be quickly located, the suspected electrical device may be pulled free the victim. Other persons arriving on the scene must be clearly warned not to touch the suspected equipment until it is deenergized. Aid should be enlisted to unplug the device as soon as possible. The injured person should be pulled free contact with stationary equipment (such as a bus bar) if the equipment cannot be quickly deenergized, or if considerations military operation or unit survival prevent immediate shutdown the circuits. This can be done quickly and safely by carefully applying the following procedures: 1. Protect yourself with dry insulating material. 2. use a dry board, belt, clothing, or other available nonconductive material to free the victim from electrical contact. DO NOT TOUCH The VICTIM UNTIL The SOURCE ELECTRICITY HAS BEEN REMOVED. Once the victim has been removed from the electrical source, it should be determined, if the person is breathing. If the person is not breathing, a method artificial ventilation is used. Sometimes victims electrical shock suffer cardiac arrest (heart stoppage) as well as loss breathing. Artificial ventilation alone is not enough in cases where the heart has stopped. a technique known as Cardiopulmonary Resuscitation (CPR) has been developed to provide aid to a person who has stopped breathing and suffered a cardiac arrest. Because you most likely will be working in the field electricity, the risk electrical shock is higher than most other Navy occupations. You should, at your earliest opportunity, learn the technique CPR. CPR is relatively easy to learn and is taught in courses available from the American Red Cross, some Navy Medical Departments, and in the Standard First Aid Training Course (NAVEDTRA 12081).
3-110
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