Here are a few of the less common types of meters that you will occasionally encounter in electrical and electronics applications.
VU and Decibel Meters
In high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loudness metersare sometimes used. These are calibrated in decibels, a unit that you will often have to use, and interpret, in reference to electronic signal levels. A decibel is an increase or decrease in sound or signal level that you can just barely detect, if you are expecting the change.
Audio loudness is given in volume units(VU), and the meter that indicates it is called a VU meter.The typical VU meter has a zero marker with a red line to the right and a black line to the left, and is calibrated in decibels (dB) below the zero marker and volume units above it (Fig. 3-12). The meter might also be calibrated in watts rms, an expression for audio power. As music is played
through the system, or as a voice comes over it, the VU meter needle kicks up. The amplifier vol- ume should be kept down so that the meter doesn’t go past the zero mark and into the red range. If the meter does kick up into the red scale, it means that distortion is taking place within the ampli- fier circuit.
Sound level in general can be measured by means of a sound-level meter, calibrated in decibels (dB) and connected to the output of a precision amplifier with a microphone of known sensitivity (Fig. 3-13). Have you read that a vacuum cleaner will produce “80 dB” of sound, and a large truck going by will subject your ears to “90 dB”? These figures are determined by a sound-level meter, and are defined with respect to the threshold of hearing, which is the faintest sound that a person with good ears can hear.
Light Meters
The intensity of visible light is measured by means of a light meterorillumination meter.It is tempting to suppose that it’s easy to make this kind of meter by connecting a milliammeter to a solar (photovoltaic) cell. As things work out, this is a good way to construct an inexpensive light meter (Fig. 3-14). More sophisticated devices use dc amplifiers, similar to the type found in a FETVM, to enhance sensitivity and to allow for several different ranges of readings.
3-13 A meter for measuring sound levels. The output of the audio amplifier is rectified to produce dc that the meter can detect.
3-12 A VU (volume-unit) meter. The heavy portion of the scale (to the right of 0) is usually red, indicating the risk of audio distortion.
One problem with this design is that solar cells are not sensitive to light at exactly the same wavelengths as human eyes. This can be overcome by placing a colored filter in front of the solar cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as human eyes. Another problem is calibrating the meter. This must usually be done at the factory, in standard illumination units such as lumensorcandela.
With appropriate modification, meters such as the one in Fig. 3-14 can be used to measure in- frared(IR) or ultraviolet(UV) intensity. Various specialized photovoltaic cells have peak sensitivity at nonvisible wavelengths, including IR and UV.
Pen Recorders
A meter movement can be equipped with a marking device to keep a graphic record of the level of some quantity with respect to time. Such a device is called a pen recorder.The paper, with a cali- brated scale, is taped to a rotating drum. The drum, driven by a clock motor, turns at a slow rate, such as one revolution per hour or one revolution in 24 hours. A simplified drawing of a pen recorder is shown in Fig. 3-15.
Other Meter Types 49
3-14 A simple light meter. A microammeter can be substituted for the milliammeter if greater sensitivity is required.
A device of this kind, along with a wattmeter, can be employed to get a reading of the power consumed by your household at various times during the day. In this way you can find out when you use the most power, and at what particular times you might be using too much.
Oscilloscopes
Another graphic metering device is the oscilloscope.This measures and records quantities that vary rapidly, at rates of hundreds, thousands, or millions of times per second. It creates a “graph” by throwing a beam of electrons at a phosphor screen. A cathode-ray tube, similar to the kind in a tele- vision set, is employed. Some oscilloscopes have electronic conversion circuits that allow for the use of a solid-state liquid crystal display(LCD).
Oscilloscopes are useful for observing and analyzing the shapes of signal waveforms, and also for measuring peak signal levels (rather than just the effective levels). An oscilloscope can also be used to approximately measure the frequency of a waveform. The horizontal scale of an oscillo- scope shows time, and the vertical scale shows the instantaneous signal voltage. An oscilloscope can indirectly measure power or current, by using a known value of resistance across the input terminals.
Technicians and engineers develop a sense of what a signal waveform should look like, and then they can often tell, by observing the oscilloscope display, whether or not the circuit under test is be- having the way it should. This is a subjective measurement, because it is qualitative as well as quan- titative.
Bar-Graph Meters
A cheap, simple kind of meter can be made using a string of light-emitting diodes (LEDs) or an LCD along with a digital scale to indicate approximate levels of current, voltage, or power. This type of meter, like a digital meter, has no moving parts to break. To some extent, it offers the relative- reading feeling you get with an analog meter. Figure 3-16 is an example of a bar-graph meter that is used to show the power output, in kilowatts, for a radio transmitter. This meter can follow along quite well with rapid fluctuations in the reading. In this example, the meter indicates about 0.8 kW, or 800 W.
The chief drawback of the bar-graph meter is that it isn’t very accurate. For this reason it is not generally used in laboratory testing. In addition, the LED or LCD devices sometimes flicker when the level is between two values given by the bars. This creates an illusion of circuit instability. With bright LEDs, it can also be quite distracting.
3-16 A bar-graph meter. In this case, the
indication is about 80 percent of full-scale, representing 0.8 kW, or 800 W.
Quiz
Refer to the text in this chapter if necessary. A good score is 18 out of 20 correct. Answers are in the back of the book.
1. The attraction or repulsion between two electrically charged objects is called (a) electromagnetic deflection.
(b) electrostatic force. (c) magnetic force. (d) electroscopic force.
2. The change in the direction of a compass needle, when a current-carrying wire is brought near, is called
(a) electromagnetic deflection. (b) electrostatic force.
(c) magnetic force. (d) electroscopic force.
3. Suppose a certain current in a galvanometer causes the compass needle to deflect by 20 degrees, and then this current is doubled while the polarity stays the same. The angle of the needle deflection will
(a) decrease. (b) stay the same. (c) increase.
(d) reverse direction.
4. One important advantage of an electrostatic meter is the fact that (a) it measures very small currents.
(b) it can handle large currents.
(c) it can detect and indicate ac voltages as well as dc voltages. (d) it draws a large current from a power supply.
5. A thermocouple
(a) gets warm when dc flows through it. (b) is a thin, straight, special wire.
(c) generates dc when exposed to visible light. (d) generates ac when heated.
6. An important advantage of an electromagnet-type meter over a permanent-magnet meter is the fact that
(a) the electromagnet meter costs much less.
(b) the electromagnet meter need not be aligned with the earth’s magnetic field. (c) the permanent-magnet meter has a more sluggish coil.
(d) the electromagnet meter is more rugged.
7. Ammeter shunts are useful because (a) they increase meter sensitivity.
(b) they make a meter more physically rugged. (c) they allow for measurement of large currents. (d) they prevent overheating of the meter movement. 8. Voltmeters should generally have
(a) high internal resistance. (b) low internal resistance. (c) the greatest possible sensitivity. (d) the ability to withstand large currents.
9. In order to measure the power-supply voltage that is applied to an electrical circuit, a voltmeter should be placed
(a) in series with the circuit that works from the supply.
(b) between the negative pole of the supply and the circuit working from the supply. (c) between the positive pole of the supply and the circuit working from the supply. (d) in parallel with the circuit that works from the supply.
10. Which of the following will notnormally cause a largeerror in an ohmmeter reading? (a) A small voltage between points under test
(b) A slight change in switchable internal resistance (c) A small change in the resistance to be measured (d) A slight error in the range switch position
11. The ohmmeter in Fig. 3-17 shows a reading of approximately (a) 34,000 Ω.
(b) 3.4 kΩ. (c) 340 Ω. (d) 34 Ω.
12. The main advantage of a FETVM over a conventional voltmeter is the fact that the FETVM (a) can measure lower voltages.
(b) draws less current from the circuit under test. (c) can withstand higher voltages safely.
(d) is sensitive to ac voltage as well as to dc voltage. 13. Which of the following is nota function of a fuse?
(a) To ensure there is enough current available for an appliance to work right (b) To make it impossible to use appliances that are too large for a given circuit (c) To limit the amount of power that a device can draw from the electrical circuit (d) To make sure the current drawn by an appliance cannot exceed a certain limit
14. A utility meter’s motor speed depends directly on (a) the number of ampere-hours being used at the time. (b) the number of watt-hours being used at the time. (c) the number of watts being used at the time.
(d) the number of kilowatt-hours being used at the time. 15. A utility meter’s readout indicates
(a) voltage. (b) power. (c) current. (d) energy.
16. A typical frequency counter (a) has an analog readout.
(b) is accurate to six digits or more. (c) works by indirectly measuring current. (d) works by indirectly measuring voltage.
17. A VU meter is neverused to get a general indication of (a) sound intensity.
(b) decibels.
(c) power in an audio amplifier. (d) visible light intensity.
Quiz 53
3-17 Illustration for Quiz Question 11.
18. The meter movement in an illumination meter directly measures (a) current.
(b) voltage. (c) power. (d) energy.
19. An oscilloscope cannotbe used to indicate (a) frequency.
(b) wave shape. (c) energy.
(d) peak signal voltage.
20. What voltage would be expected to produce the reading on the bar-graph meter shown in Fig. 3-18?
(a) 6.0 V (b) 6.5 V (c) 7.0 V
(d) There is no way to tell because the meter, as shown, is malfunctioning.
3-18 Illustration for Quiz Question 20.
YOU’VE ALREADY SEEN SOME SIMPLE ELECTRICAL CIRCUIT DIAGRAMS. IN THIS CHAPTER,YOU’LL GET
more acquainted with this type of diagram. You’ll also learn more about how current, voltage, resis- tance, and power are related in dc and low-frequency ac circuits.