Electrical Theory Lesson 2: Basic Instruments and Measurements This document contains the transcript for the entire lesson. Page 1: Welcome to Lesson 2 of Electrical Theory. This lesson covers the following objectives: • Explain the correct procedure for using an ammeter, a voltmeter, and an ohmmeter. • Interpret a linear scale. • Compute shunt resistor values. • Compute multiplier resistor values. • Interpret a nonlinear scale. • Discuss the concept of meter sensitivity. • Understand basic electrical diagrams. • State and explain Ohm’s Law Page 2: Meters come in two formats; analog and digital. Analog meters use a continuous scale readout that must be interpreted by means of a needle, while the digital multimeter uses a liquid crystal display (LCD) readout that needs no interpretation. Page 3: A common type of meter movement is the D'Arsonval movement. The movement consists of a permanent‐type magnet and a rotating coil in the magnetic field. An indicating needle is attached to the rotating coil. When a current passes through the moving coil, a magnetic field is produced. The field reacts with the stationary field and causes rotation (deflection) of the needle. This deflection force is proportional to the strength of the current flowing through the coil. When the current ceases to flow, the moving coil is returned to its "at rest" position by hair springs. The coil that rotates in the magnetic field is mounted on precision‐type jewel bearings, much like a fine watch. The jewel‐type bearings and mount is known as a D'Arsonval movement. Page 4: When connecting a meter to an electrical circuit, proper polarity must be maintained. The meter is equipped with polarity marking, usually a red plus sign (+) and a black negative sign (‐). Some meters use the abbreviation COM, which stands for common, for the negative polarity marking. The meter coil rotates inside the permanent magnet field. If proper polarity is not used, the coil will deflect in the direction opposite to that which it was designed. Page 5: The operation principle of the iron vane meter movement is shown here. Two pieces of the iron are placed in the hollow core of a solenoid. When the current passes through the solenoid, both pieces of metal become magnetized with the same polarity. Because like poles repel each other, the two pieces of iron are repelled from each other. One piece of metal is fixed in its position. The other piece of metal pivots. The pivoting piece can turn away from the fixed metal. An indicating needle is attached to the moving vane. The needle is equipped with hair springs so that the vane must move against the spring tension for accurate readings. An applied voltage causes current to flow in the solenoid and creates the magnetic field. The moving vane is repelled against the spring according to the strength of the magnetic field. The needle may indicate 1 Electrical Theory Lesson 2: Basic Instruments and Measurements This document contains the transcript for the entire lesson. either voltage or current. When the iron vane movement is used for a voltmeter, the solenoid is commonly wound with many turns of fine wire. Proper multiplier resistance may be used to increase the range of the meter. A selector switch is used to select proper ranges. When used as an ammeter, the solenoid has a few turns of heavy wire. This is because the coil must be connected in series with the circuit and carry the circuit current. The iron vane meter movement always deflects in the same direction regardless of polarity. Either AC or DC may be measured with this instrument. This type of meter is best suited for high power circuit measurements. Page 6: The meter scale used to interpret ampere and voltage values is the linear type. A linear meter scale has evenly spaced marks used to indicate the amount of current flowing or voltage present, in the meter movement. This figure shows a typical linear scale for an ammeter. To determine the value of each mark between the major divisions, divide the value of the first major division by the number of spaces in that division. The dial to the right of each scale in this figure is the range selector. The range selector must be correlated to the scale to determine full scale deflection. As the range switch is rotated through the different ranges: – 5 amps, 0.5 amps, and 0.05 amps, interpreting the scale also changes with each major divisions – 0, 1, 2, 3, 4, and 5 and minor divisions. For example, with the scale on 5 amps, then each minor division becomes 0.1 amps, and changing to the 0.5 amp scale, each minor division becomes 0.01 amps. Page 7: None Page 8: An ammeter measures electrical current in a circuit. An ammeter will usually measure in amperes, milliamperes, or microamperes, depending on the scale or design of the instrument. The coil in the meter movement of an ammeter is wound with many turns of fine wire. If a large current is allowed to flow through this coil, the ammeter will quickly burn out. In order to measure larger currents, a shunt, or alternate path, is provided for current. Most of the current flows through the shunt, leaving only enough current to safely work the movement coil. The shunt is a precision resistor connected in parallel with the meter coil. Page 9: This figure shows the proper way to connect an ammeter to an electrical circuit. When an ammeter is connected into the circuit, it becomes part of the circuit in order to allow the current to flow through the meter coil. You are connecting the meter in series with the circuit of device you are trying to measure. Page 10: None Page 11: The same basic meter movement that is used in an ammeter is also used to measure voltage. This is providing that the impressed voltage across the coil never exceeds 0.1 volt, as computed, for full scale deflection. To arrange the meter to measure higher voltages, multiplier resistors are placed in 2 Electrical Theory Lesson 2: Basic Instruments and Measurements This document contains the transcript for the entire lesson. series with the meter movement coil, using a switch. Voltmeters are always connected in parallel with the device being measured. Page 12: A voltmeter is always connected in parallel or across the circuit. When measuring with the meter set on its highest range. Adjust downward to the proper range to avoid damaging the meter. In addition, be sure that the leads are connected with the correct polarity. The black lead is negative and the red lead is positive. Page 13: The sensitivity of a meter can be used to gauge meter quality. A quality meter has a sensitivity of at least 20,000 ohms/volt. Precision laboratory meters measure as high as 200,000 ohms/volt. Accuracy, of the meter is commonly expressed as a percentage, such as 1 percent. This means that the true value will be within one percent of the scale reading. Another system of rating meters is the accuracy expressed as a percentage of full scale reading. A meter may have a rating of ±0.05 percent or less. In general, the smaller the percentage, the higher the quality of the meter. Page 14: When a voltmeter is connected across a circuit to measure a potential difference, it is in parallel with the load in the circuit. This situation can introduce errors in voltage measurement. In this figure two 10,000 ohm resistors from a voltage divider circuit across a ten volt source. The voltage drops across both R1 and R2 are 5 volts each. If a meter with a sensitivity of 1000 ohms/volt on the ten volt range is used to measure the voltage across R1, the meter resistance will be in parallel with R1. The addition of this meter cuts the effective resistance of R1 in half. The combined resistance of the meter and R1 is equal to what’s shown here: With the meter connected, the total circuit resistance becomes what’s shown here: Using Ohm’s law, the current can be calculated at approximately 0.00067 amps. Using Ohm’s law again, ER1 = 3.35 V and ER2 = 6.7 V. The meter has caused an error of more than one volt due to its shunting effect. Page 15: None Page 16: A meter used to measure the value of an unknown resistance is called an ohmmeter. The same meter movement that was used in the volt and ammeter can be used for the ohmmeter. A voltage source and a variable resistor are added to the ohmmeter's circuit. A series type ohmmeter is shown here. A three volt battery is used as the source for the ohmmeter. The battery is built into the meter case. The meter movement permits only 0.1 volt for the current of 0.001 amps for full scale deflection. Therefore, a multiplier resistor is placed in series with the meter coil to reduce the voltage applied to the meter coil. The 2900 ohm multiplier resistor, plus the meter coil resistance, is equal to 3000 ohms. Part of this resistance is made up of a variable resistor to allow the total resistance to vary. Because temperature changes or weak batteries can affect the total resistance of the circuit, the 3 Electrical Theory Lesson 2: Basic Instruments and Measurements This document contains the transcript for the entire lesson. ohmmeter must be calibrated before each use. To use the ohmmeter, first short the test leads together. This applies 0 ohms across the meter.