EXPERIMENT 1 Safety, Instrumentation, and Measurement

Transcription

1 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 1 of 8 EXPERIMENT 1 Safety, Instrumentation, and Measurement Introduction The objectives of this experiment are to: Learn and apply principles of electrical safety Learn to connect electrical circuits Learn to use a digital multimeter (DMM) to measure voltage, current, and resistance Learn about and experimentally confirm several basic circuit laws including Ohm's law, Kirchhoff's Current Law, and Kirchhoff's Voltage Law. Electrical Safety Electrically powered equipment, such as oscilloscopes, power supplies, signal generators, soldering irons, etc. are essential elements of many ECE laboratories. In other laboratories one may find devices such as hot plates, stirrers, vacuum pumps, electrophoresis apparatus, lasers, heating mantles, ultrasonicators, and microwave ovens. These devices can pose a significant hazard to laboratory workers, particularly when mishandled or not maintained. Many laboratory electrical devices have high voltage or high power requirements, carrying even more risk. Large capacitors found in many laser flash lamps and other systems are capable of storing lethal amounts of electrical energy and pose a serious danger even if the power source has been disconnected. Electrical Hazards The severity and effects of an electrical shock depend on a number of factors, such as the pathway through the body, the amount of current, the length of time of the exposure, and whether the skin is wet or dry. Water is a great conductor of electricity, allowing current to flow more easily in wet conditions and through wet skin. The effect of the shock may range from a slight tingle to severe burns to cardiac arrest. The chart below shows the general relationship between the degree of injury and amount of current for a 60-cycle hand-to-foot path of one second's duration of shock. While reading this chart, keep in mind that most electrical circuits can provide, under normal conditions, up to 20,000 milliamperes of current flow. The major hazards associated with electricity are electrical shock and fire. Electrical shock occurs when the body becomes part of the electric circuit, either when an individual comes in contact with both wires of an electrical circuit, one wire of an energized circuit and the ground, or a metallic part that has become energized by contact with an electrical conductor. Current Reaction 1 Milliampere Perception level 5 Milliamperes Slight shock felt; not painful but disturbing 6-30 Milliamperes Painful shock; "let-go" range Milliamperes Extreme pain, respiratory arrest, severe muscular contraction ,300 Milliamperes Ventricular fibrillation 10,000+ Milliamperes Cardiac arrest, severe burns and probable death

2 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 2 of 8 In addition to the electrical shock hazards, sparks from electrical equipment can serve as an ignition source for flammable or explosive vapors. Even loss of electrical power can result in extremely hazardous situations. Flammable or toxic vapors may be released as a chemical warms when a refrigerator or freezer fails. Fume hoods may cease to operate, allowing vapors to be released into the laboratory. If magnetic or mechanical stirrers fail to operate, safe mixing of reagents may be compromised. Preventing Electrical Hazards There are various ways of protecting people from the hazards caused by electricity, including insulation, guarding, grounding, and electrical protective devices. Laboratory workers can significantly reduce electrical hazards by following some basic precautions: Inspect wiring of equipment before each use. Replace damaged or frayed electrical cords immediately. Use safe work practices every time electrical equipment is used. Know the location and how to operate shut-off switches and/or circuit breaker panels. Use these devices to shut off equipment in the event of a fire or electrocution. Limit the use of extension cords. Use only for temporary operations. In all other cases, request installation of a new electrical outlet. Use only multi-plug adapters equipped with circuit breakers or fuses. Place exposed electrical conductors (such as those sometimes used with electrophoresis devices) behind Plexiglas shields. Minimize the potential for water or chemical spills on or near electrical equipment. Insulation All electrical cords should have sufficient insulation to prevent direct contact with wires. In a laboratory, it is particularly important to check all cords before each use, since corrosive chemicals or solvent vapors may erode the insulation. Damaged cords should be repaired or taken out of service immediately, especially in wet environments such as cold rooms and near water baths. Guarding Live parts of electric equipment operating at 50 volts or more (i.e., electrophoresis devices) must be guarded against accidental contact. Plexiglas shields may be used to protect against exposed live parts. Grounding Only equipment with three-prong plugs should be used in the laboratory. The third prong provides a path to ground that helps prevent the buildup of voltages that may result in an electrical shock or spark. This does not guarantee that no one will receive a shock, be injured, or be killed. It will, however, substantially reduce the possibility of such accidents, especially when used in combination with other safety measures.

3 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 3 of 8 Circuit Protection Devices Circuit protection devices are designed to automatically limit or shut off the flow of electricity in the event of a ground-fault, overload, or short circuit in the wiring system. Fuses, circuit breakers, and ground-fault circuit interrupters are three well-known examples of such devices. Fuses and circuit breakers prevent over-heating of wires and components that might otherwise create hazards for operators. They disconnect the circuit when it becomes overloaded. This overload protection is very useful for equipment that is left on for extended periods of time, such as stirrers, vacuum pumps, drying ovens, Variacs and other electrical equipment. The ground-fault circuit interrupter, or GFCI, is designed to shutoff electric power if a ground fault is detected. The GFCI is particularly useful near sinks and wet locations. Since GFCIs can cause equipment to shutdown unexpectedly, they may not be appropriate for certain apparatus. Portable GFCI adapters (available in most safety supply catalogs) may be used with a non-gfci outlet. Motors In laboratories where volatile flammable materials are used, motor-driven electrical equipment should be equipped with non-sparking induction motors or air motors. Avoid series-wound motors, such as those generally found in vacuum pumps, rotary evaporators and stirrers. Series-wound motors are also usually found in household appliances such as blenders, mixers, vacuum cleaners and power drills. These appliances should not be used unless flammable vapors are adequately controlled. Safe Work Practices The following practices may reduce risk of injury or fire when working with electrical equipment: Avoid contact with energized electrical circuits. Disconnect the power source before servicing or repairing electrical equipment. When it is necessary to handle equipment that is plugged in, be sure hands are dry and, when possible, wear nonconductive gloves and shoes with insulated soles. If it is not unsafe to do so, work with only one hand, keeping the other hand at your side or in your pocket, away from all conductive material. This precaution reduces the likelihood of accidents that result in current passing through the chest cavity. Minimize the use of electrical equipment in cold rooms or other areas where condensation is likely. If equipment must be used in such areas, mount the equipment on a wall or vertical panel. If water or a chemical is spilled onto equipment, shut off power at the main switch or circuit breaker and unplug the equipment. If an individual comes in contact with a live electrical conductor, do not touch the equipment, cord or person. Disconnect the power source from the circuit breaker or pull out the plug using a leather belt. High Voltage or Current Repairs of high voltage or high current equipment should be performed by trained technicians in the ECE Shop. The following additional precautions should be taken: Always assume a high voltage potential exists within a device while servicing it, even if it is deenergized and disconnected from its power source.

4 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 4 of 8 Avoid becoming grounded by staying at least 6 inches away from walls, water, and all metal materials, including pipes. Use voltmeters and test equipment with ratings and leads sufficient to measure the highest potential voltage expected to be found inside the equipment being serviced. After servicing, check equipment with a multimeter or appropriate device to ensure it is grounded before reconnecting to the power source. Theory: Circuits, Wiring, and Basic Electrical Laws Components Electrical circuits are made up of components (parts) connected by wires. There are many types of components, such as batteries, resistors, capacitors, transistors, transformers, lights, motors, etc. In this experiment, the components we will use are resistors and a power supply, and the wiring will be done with banana plug cables. Resistors Resistors are circuit components that produce a specified difficulty for current flow. They are used to direct current where it is desired, and to provide desired voltage at specific points in the circuit. They are also sometimes used to generate heat. For example, the glowing wires in a kitchen toaster are resistance wire made of an nickelchromium alloy called Nichrome. The circuit symbol for a resistor is shown below: A photograph of three real resistors is shown below: A resistor is a two-terminal, non-polarized component. Two-terminal means there are two connections to the outside world. (Some components have more terminals. For example, transistors have three or four.) Non-polarized means the two terminals are interchangeable, and current can flow in either direction with the same effect. Resistors are rated by the value of resistance in units of Ohms, by the tolerance in percent, and by the amount of power they can handle without overheating. For example, one resistor might be rated at 1 kω, 10%, ½ Watt, while another might be rated at 10 kω, 5%, 1/8 Watt.

5 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 5 of 8 Resistance is the ratio of voltage produced between the terminals to the current flowing through the resistor. This is expressed formally by Ohm s Law. Tolerance is the manufacturer s statement of the variation of resistance in a production batch around the nominal rated value. For example, a resistor marked as 1 kω, 5% might have an actual value anywhere in the range shown below: so the value could be between 950 Ω and 1,050 Ω. ( ) ( ) 1kΩ R 1kΩ ACTUAL The actual value of resistance for a given resistor can only be determined by direct measurement using a suitably calibrated ohm-meter. The measurement must be made while the resistor is NOT connected in a circuit. Power Supplies Energy is supplied to a circuit using one or more power supplies. Common types are batteries (electrochemical cells), AC to DC converters (AC power supplies), and solar cells (photovoltaics). In this experiment, we will use an AC power supply. Although we will treat it as a single component, this is actually itself a complete circuit that converts the 60 Hz power from the wall plug into a constant DC voltage. Two widely accepted circuit symbols for a DC voltage supply are shown below: DC The + symbol indicates the positive terminal, and the symbol indicates the negative terminal. Power supplies are rated by the output voltage they can produce, and the maximum current in Amperes (A) they can deliver. Sometimes the current rating is replaced by the power rating, which is the product of voltage and current in Watts (W). For example, one power supply might be rated at 12 1 A (or 12 V, 12 W), and another might be rated at 5 25 A (or 5 V, 125 W). Ground Although not a component in the same sense as resistors and power supplies, ground, (or common, sometimes also called common ground) is a crucial part of every circuit. This is the node or connection point which provides the path for current to return to the power supply. Most measurement errors, circuit malfunctions, and accidental electrocutions can be traced to ground problems, such as mis-wired connections to ground, equipment not properly grounded, or poor choice of ground point. It is important to make sure that all equipment used to make measurements on a circuit is referenced to the circuit ground point. Wires Wires are an important part of every circuit. They connect components and determine the paths that current will take through a circuit. The most important ratings of a wire are the wire gauge, which determines the maximum current

6 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 6 of 8 that the wire can safely carry without overheating, and the insulation voltage, which determines the maximum voltage that can safely be applied between the wire and ground before dangerous arcing may occur. For example, common house wiring may be rated at 14 gauge, 600 V, while we often use 22 gauge, 300 V wires in low-voltage, low-current lab experiments. The types of ends a wire has is determined by the intended application. Wire ends (terminations) range from bare wire to very complex mechanical locking mechanisms. Wires with special-purpose ends are often referred to as cables. Below is a photograph of some cables we use in our laboratory experiments. The shorter cable has banana-plug terminals on both ends. The other cable is actually composed of two wires. The two wires each have banana plugs on one end, and are joined into a coaxial cable terminating in a BNC plug on the other end. Circuit Terminology From your study of basic circuits, you should be familiar with some fundamental circuit terms including the following: Node, branch, and mesh Series and parallel Your lab instructor will review these with you. However, if these are not familiar to you, you should look them up in the index of your circuits book and read the definitions and examples. Ohm s Law Perhaps the most famous circuit law is Ohm s Law. Ohm s law gives the relationship between current and voltage in a resistor with value R: + V I V = I R

7 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 7 of 8 The picture is an essential part of Ohm s Law, because the polarity of the voltage and the direction of the current are linked. If the current flows in the opposite direction relative to the voltage polarity, then there must be a minus sign in the equation. This is shown below: + V I V = I R Kirchhoff s Current Law (KCL) KCL states that the sum of currents entering any node must be zero. Another way to state this is to say that the total current entering a node must equal the total current leaving a node. This law is based on the conservation of electrical charge. If it were not true, there would be an accumulation or depletion of charge at a node, which is physically impossible. Kirchhoff s Voltage Law (KVL) KVL states that the sum of the voltage drops around any closed path must be zero. Another way to state this is that the sum of voltage rises must equal the sum of voltage drops. This law is based on conservation of energy. Voltage is the potential energy of electrons, and therefore obeys the same energy conservation rules you learned in physics.

8 ELEC 2010 Laboratory Manual Experiment 1 - Prelab Page 8 of 8 Your Name Prelab Questions and Quiz (20 points) (Answer these questions in lab and turn them into your instructor before beginning the in-lab procedure. For subsequent weeks, the Prelab (10 points) will be done before lab, and the Quiz (10 points) will be given in lab.) ELECTRICAL SAFETY 1. What should you do if an individual comes in contact with a live electrical conductor? (See "Safe Work Practices" on page 3.) 2. What should you do if water or a chemical is spilled onto electrical equipment? (See "Safe Work Practices" on page 3.) 3. What is the smallest amount of electrical current that can be felt by the average person? (See "Electrical Hazards" on page 1.) 4. What is the current range which results in extremely painful shock, and the victim probably cannot let go due to severe muscular contraction? (See "Electrical Hazards" on page 1.) DC CIRCUIT THEORY For each question 5-6, write the appropriate equation and then solve for the answer. 5. What is the current I 3 in the circuit segment below? Let I 1 = 2 ma and I 2 = 3 ma. [Answer: 1 ma]. I 3 I 1 I 2 6. What is the voltage V 3 in the circuit segment below? Each box represents a circuit element. Let V 1 = 2 V and V 2 = 3 V. [Answer: +5 V] V V 1 + V 3