In parts one and two of this series, I have only spoken about direct current (DC), were the source of voltage is trying to push the current in one direction. Another form of electricity is alternating current (AC) in which the voltage source alternates the current direction. Everything I have covered in parts one and two apply to both DC and AC. As I continue I will point out the differences.
Measuring voltage, current and resistance
Over the past one hundred years various instruments have been used to measure voltage, current and resistance. Two examples of instruments that can measure DC voltage are shown in photo 1.
Today, the most common electrical test instrument in use is the digital multimeter because of its ability to accurately measure voltage, current and resistance. We used to call them volt-ohm-meters (VOMs).
The accuracy of an instrument is a statement of the largest allowable error expressed as a percentage. Seventy years ago the most accurate instruments were ones designed to measure one thing. For example, the Weston DC voltmeter had six voltage ranges, 0 to 25 millivolts (0.025 volts), 0 to 50 millivolts, 0 to 500 millivolts, 0 to 5 volts, 0 to 50 volts, and 0 to 150 volts. The accuracy of the instrument is described on the meter in the following manner: “This instrument indicates International Volts and is correct within ½ of 1% of full scale value at any part of the scale at 75°F.” That means that the limit of inaccuracy for this instrument connected on the 0 to 150 volt scale is 0.005 x 150 = 0.75 volts plus or minus (±).
By “plus or minus,” I mean the reading could be off by as much as 0.75 volts high or low. In comparison, the Fluke Model 175, digital multimeter, set to measure DC volts has a range from 0.1 millivolts ( 0.0001 volts) to 1000 volts. The limit of inaccuracy for the instrument is ± 0.15% of the reading. That means the limit of inaccuracy when the meter is reading 120 volts is 0.0015 x 120 = 0.18 volts. The limit of inaccuracy when the meter is reading 1.0 milivolts is .0015 x 1 millivolts = 0.0015 milivolts (0.0000015 volts). The Fluke multimeter also measures AC volts from 0.1 millivolts to 1000 volts, DC and AC amps from 0.01 milliamps to 10.00 amps, and resistance from 0.1Wto 50.00 MW(50 millionW). I will talk about measuring currents greater than 10 amps in another segment.
Resistance variation with temperature
In part two of this series, I calculated the resistance of the filament of a 60-watt light bulb to be 240 ohms. That is the resistance when the bulb is energized at 120 volts. If you connect the terminals of a 60-watt bulb to digital multimeter set to measure resistance, the meter will read about 17W. The reason the multimeter measures 17Wis because the multimeter is measuring the resistance of the filament at room temperature. When the bulb is energized at 120 volts, the temperature of the filament is around 2200° F. When the bulb is first energized, for an instant the current drawn by the bulb is 120 V / 17 W = 7 A. In the first few milliseconds after closing the switch, the current heats up the filament from room temperature to 2200°F, the resistance changes from 17Wto 240W, and the current drops from 7 A to 0.5 A. If we wire an ammeter in the bulb circuit, the ammeter would not measure the surge in current because it happens too quickly. The bulb filament is a perfect example of the resistance of a material increasing with temperature.
The multimeter can measure the temperature of the filament at room temperature because the voltage being applied to the bulb by the multimeter is very low, typically 0.5 to 1.5 V. If the multimeter applies a voltage of 1.5 volts to measure resistance, the initial current in the bulb filament per Ohm’s law (equation 2 of part two) is the voltage divided by the resistance, 1.5 V / 17W= 0.088 A (88 milliamps). That is a very low current and yet if you continue to measure the resistance of the bulb filament with the multimeter for a few minutes, you will notice the resistance of the filament gradually increasing. That is because the 88 milliamp current is heating up the filament. The instrument measuring the resistance of the bulb filament is changing the resistance.
Instrument affect on the quantity being measured
When we use instruments to measure quantities like voltage, current, and resistance, it is very important that the instrument doesn’t affect the quantity being measured. Voltmeters must have a very high internal resistance and ammeters must have a very low resistance to keep the instrument from affecting the quantity being measured. The Weston DC voltmeter, vintage 1936, has an internal resistance of 3000Wwhen connected on the 0 to 150 volt scale. In contrast, the Fluke digital multimeter, vintage 2004, has an internal resistance of 10 MW(10 millionW) when set on the DC voltage scale. In general, modern multimeters have very little affect on the voltage, current, or resistance being measured in electric power circuits. When measuring these quantities in electronic circuits, the same is not always true.