Measuring Instruments--part 5: Oscilloscopes, Wattmeters, Recording Meters

The Oscilloscope

+++++Voltage is measured on the vertical or axis and time is measured on the horizontal or axis. axis (Time) Voltage is measured on the vertical or axis and time is measured on the horizontal or axis; axis (Voltage)

+++++ Voltage control of an analog oscilloscope.

+++++Voltage control knobs of a four channel digital oscilloscope.

+++++ Display of a digital oscilloscope.

Many of the electronic control systems in today's industry produce voltage pulses that are meaningless to a VOM. In many instances, it’s necessary to know not only the amount of voltage present at a particular point, but also the length or duration of the pulse and its frequency. Some pulses may be less than 1 volt and last for only a millisecond. A VOM would be useless for measuring such a pulse. It’s therefore necessary to use an oscilloscope to learn what is actually happening in the circuit.

The oscilloscope is a powerful tool in the hands of a trained technician.

The first thing to understand is that an oscilloscope is a voltmeter. It does not measure current, resistance, or watts. The oscilloscope measures an amount of voltage during a period of time and produces a two-dimensional image.

Voltage Range Selection: The oscilloscope is divided into two main sections. One section is the volt age section, and the other is the time base. The display of the oscilloscope is divided by vertical and horizontal lines. Voltage is measured on the vertical, or Y, axis of the display, and time is measured on the horizontal, or X, axis. When using a VOM, a range-selection switch is used to determine the full-scale value of the meter. Ranges of 600 volts, 300 volts, 60 volts, and 12 volts are common. The ability to change ranges permits more-accurate measurements to be made.

Oscilloscopes can be divided into two main types: analog and digital. Ana log oscilloscopes have been used for years and many are still in use; however, digital oscilloscopes are rapidly taking their place. Analog scopes generally employ some type of control knob to change their range of operation,. The setting indicates the volts per division instead of volts full scale. The settings indicate that Channel 1 is set for 0.2 volts per division and Channel 2 is set for 0.5 volts per division.

Digital oscilloscopes often indicate their setting on the display instead of marking them on the face of the oscilloscope. Voltage control knobs for a four-channel digital oscilloscope are shown. The display of a typical digital oscilloscope. In the lower left-hand corner of the display the notation CH1 200mV can be seen. This indicates that the voltage range has been set for 200 millivolts per division.

Oscilloscopes can display both positive and negative voltages. In the display, assume that a value of 0 volts has been set at the center line. The voltage shown at position A is positive with respect to 0 and the voltage at position B is negative with respect to 0. If the oscilloscope were set for a value of 2 volts per division, the value at point A would be 6 volts positive with respect to 0 and the value at point B would be 6 volts negative with respect to 0.

Another example of the oscilloscope's ability to display both positive and negative voltage values. The waveform is basically an AC square wave. Notice that the voltage peaks at the leading edge of both the negative and positive waves. This could never be detected with a common voltmeter.

Many oscilloscopes have the ability to display more than one voltage at a time. Each voltage is generally referred to as a trace or channel. The oscilloscope shown has the ability to display four voltages or four traces. Many digital-type oscilloscopes have the ability to display a different color for each trace.

+++++ The oscilloscope displays both positive and negative voltages.

The oscilloscope displays both positive and negative voltages.

+++++ AC waveform.

+++++ A four trace oscilloscope.

+++++53 Time base of an analog oscilloscope.

+++++54 Time base control for a typical digital oscilloscope.

The Time Base:

The next section of the oscilloscope to be discussed is the time base. The time base is calibrated in seconds per division and has ranges from seconds to microseconds. Analog-type oscilloscopes use a range selection switch similar to the one shown. The time base selection control for a typical digital-type oscilloscope is shown. Digital oscilloscopes generally indicate the time base setting on the display.

The lower middle section of the display shows M 100 microseconds, which indicates that the scope is set for 100 microseconds per division. With the time base set at this value, it would take the trace 1000 microseconds, or one milli second, to sweep across the face of the display.

Measuring Frequency

Because the oscilloscope has the ability to display the voltage with respect to time, it’s possible to calculate the frequency of the waveform. The frequency (f ) of an AC waveform can be found by dividing 1 by the time (t) it takes to complete one cycle (f = 1/t). For example, the time base is set at 100 microseconds per division. The AC sine wave being displayed completes one complete cycle in 167 microseconds. The frequency is 5988 hertz, or 5.988 kilohertz. (1/0.000167). The oscilloscope below displays these values. Many oscilloscopes have the ability to measure frequency automatically and display the value for you.

+++++ Oscilloscope attenuated probe.

Attenuated Probes:

Most oscilloscopes use a probe that acts as an attenuator. An attenuator is a de vice that divides or makes smaller the input signal. An attenuated probe is used to permit higher voltage readings than are normally possible. For example, most attenuated probes are 10 to 1. This means that if the voltage range switch is set for = volts per division, the display would actually indicate 50 volts per division. If the voltage range switch is set for 2 volts per division, each division on the display actually has a value of 20 volts per division.

Probe attenuators are made in different styles by different manufacturers.

On some probes, the attenuator is located in the probe head itself, whereas on others the attenuator is located at the scope input. Regardless of the type of attenuated probe used, it may have to be compensated or adjusted. In fact, probe compensation should be checked frequently. Different manufacturers use different methods for compensating their probes, so it’s generally necessary to follow the procedures given in the operator's manual for the probe being used.

Oscilloscope Controls:

The following is a list of common controls found on the oscilloscope. Refer to the oscilloscope shown.

1. POWER. The power switch is used to turn the oscilloscope ON or OFF.

2. BEAM FINDER. This control is used to locate the position of the trace if it’s off the display. The BEAM FINDER button will indicate the approximate location of the trace. The position controls are then used to move the trace back on the display.

3. PROBE ADJUST (sometimes called calibrate). This is a reference volt age point used when compensating the probe. Most probe adjust points produce a square wave signal of about 0.5 volts.

+++++ An oscilloscope.

4. INTENSITY and FOCUS. The INTENSITY control adjusts the brightness of the trace. A bright spot should never be left on the display because it will burn a spot on the face of the cathode ray tube (CRT). This burned spot results in permanent damage to the CRT. The FOCUS control sharpens the image of the trace.

5. VERTICAL POSITION. This is used to adjust the trace up or down on the display. If a dual-trace oscilloscope is being used, there will be two vertical POSITION controls. (A dual-trace oscilloscope contains two separate traces that can be used separately or together.)

6. CH 1-BOTH-CH 2. This control determines which channel of a dual trace oscilloscope is to be used, or whether they are both to be used at the same time.

7. ADD-ALT.-CHOP. This control is active only when both traces are being displayed at the same time. The ADD adds the two waves together. ALT. stands for alternate. This alternates the sweep between Channel 1 and Channel 2. The CHOP mode alternates several times during one sweep.

This generally makes the display appear more stable. The CHOP mode is generally used when displaying two traces at the same time.

8. AC-GND-DC. The AC is used to block any DC voltage when only the AC portion of the voltage is to be seen. For instance, assume an AC volt age of a few millivolts is riding on a DC voltage of several hundred volts.

If the voltage range is set high enough so that 100 VDC can be seen on the display, the AC voltage cannot be seen. The AC section of this switch inserts a capacitor in series with the probe. The capacitor blocks the DC voltage and permits the AC voltage to pass. Because the 100 VDC has been blocked, the voltage range can be adjusted for millivolts per division, which will permit the AC signal to be seen.

The GND section of the switch stands for ground. This section grounds the input so the sweep can be adjusted for 0 volt at any position on the display. The ground switch grounds at the scope and does not ground the probe. This permits the ground switch to be used when the probe is connected to a live circuit. The DC section permits the oscilloscope to display all of the voltage, both AC and DC, connected to the probe.

9. HORIZONTAL POSITION. This control adjusts the position of the trace from left to right.

10. AUTO-NORMAL. This determines whether the time base will be triggered automatically or operated in a free-running mode. If this control is operated in the NORM setting, the trigger signal is taken from the line to which the probe is connected. The scope is generally operated with the trigger set in the AUTO position.

11. LEVEL. The LEVEL control determines the amplitude the signal must be before the scope triggers.

12. SLOPE. The SLOPE permits selection as to whether the trace is triggered by a negative or positive waveform.

13. INT.-LINE-EXT. The INT. stands for internal. The scope is generally operated in this mode. In this setting, the trigger signal is provided by the scope. In the LINE mode, the trigger signal is provided from a sample of the line. The EXT, or external, mode permits the trigger pulse to be applied from an external source.

These are not all the controls shown on the oscilloscope, but they are the major controls. Most oscilloscopes contain these controls.

+++++ AC square wave. An AC square wave

+++++ A DC waveform. A DC waveform

+++++ A chopped DC waveform. A chopped DC waveform

Interpreting Waveforms:

The ability to interpret the waveforms on the display of the oscilloscope takes time and practice. When using the oscilloscope, one must keep in mind that the display shows the voltage with respect to time.

It’s assumed that the voltage range has been set for 0.5 volts per division, and the time base is set for 2 milliseconds per division. It’s also assumed that 0 volt has been set on the center line of the display. The waveform shown is a square wave. The display shows that the voltage rises in the positive direction to a value of 1.4 volts and remains there for 2 milliseconds. The voltage then drops to 1.4 volts negative and remains there for 2 milliseconds before going back to positive. Because the voltage changes between positive and negative, it’s an AC voltage. The length of one cycle is 4 milliseconds. The frequency is therefore 250 hertz (1/0.004s = 250 Hz).

The oscilloscope has been set for 50 millivolts per division and 20 microseconds per division. The display shows a voltage that is negative to the probe's ground lead and has a peak value of 150 millivolts. The waveform lasts for 20 microseconds and produces a frequency of 50 kilohertz (1/0.000020s = 50,000 Hz). The voltage is DC because it never crosses the zero reference and goes in the positive direction. This type of voltage is called pulsating DC.

Assume the oscilloscope has been set for a value of 50 volts per division and 4 milliseconds per division. The waveform shown rises from 0 volts to about 45 volts in a period of about 1.5 milliseconds. The volt age gradually increases to about 50 volts in the space of 1 millisecond and then rises to a value of about 100 volts in the next 2 milliseconds. The voltage then decreases to 0 in the next 4 milliseconds. It then increases to a value of about 10 volts in 0.5 milliseconds and remains at that level for about 8 milli seconds.

This is one complete cycle for the waveform. The length of the one cycle is about 16.6 milliseconds, which is a frequency of 60.2 hertz. (1/0.0166). The voltage is DC because it remains positive and never drops below the 0 line.

Learning to interpret the waveforms seen on the display of an oscilloscope will take time and practice, but it’s well worth the effort. The oscilloscope is the only means by which many of the waveforms and voltages found in electronic circuits can be understood. Consequently, the oscilloscope is the single most valuable piece of equipment a technician can use.

The Wattmeter

The wattmeter is used to measure true power in a circuit. There are two basic types of wattmeters, dynamic and electronic. Dynamic wattmeters differ from d'Arsonval-type meters in that they don’t contain a permanent magnet. They contain an electromagnet and a moving coil. The electromagnets are connected in series with the load in the same manner that an ammeter is connected. The moving coil has resistance connected in series with it and is connected directly across the power source in the same manner as a voltmeter.

+++++ The wattmeter contains two coils-one for voltage and the other for current. Voltage coil; Current coil

+++++ The current section of the wattmeter is connected in series with the load, and the voltage section is connected in parallel with the load.

+++++ Portable wattmeters often make connection to the voltage and current terminals inside the meter. VA A V Power in terminals; Power out terminals; Wattmeter

Because the electromagnet is connected in series with the load, the current flow through the load determines the magnetic field strength of the stationary magnet. The magnetic field strength of the moving coil is determined by the amount of line voltage. The turning force of the coil is proportional to the strength of these two magnetic fields. The deflection of the meter against the spring is proportional to the amount of current flow and voltage.

Because the wattmeter contains an electromagnet instead of a permanent magnet, the polarity of the magnetic field is determined by the direction of cur rent flow. The same is true of the polarity of the moving coil connected across the source of voltage. If the wattmeter is connected into an AC circuit, the polarity of the two coils will reverse at the same time, producing a continuous torque. For this reason, the wattmeter can be used to measure power in either a DC or an AC circuit. However, if the connection of the stationary coil or the moving coil is reversed, the meter will attempt to read backward.

Dynamic-type wattmeters are being replaced by wattmeters that contain electronic circuitry to determine true power. They are less expensive and generally more accurate than the dynamic type. Like dynamic wattmeters, electronic type meters contain amperage terminals that connect in series with the load and voltage terminals that connect in parallel with the load. Portable-type wattmeters often have terminals labeled "power in" and "power out." Connection to the current and voltage section of the meter is made inside the meter. Analog-type electronic wattmeters use a standard d'Arsonval type movement to indicate watts. The electronic circuit determines the true power of the circuit and then supplies the appropriate power to the meter movement. Wattmeters with digital displays are also available.

+++++ (A) Single-line recording volt-ammeter. (B) A kilowatt-kilo-VAR recording meter. (C) A single-phase or three-phase voltage and current recording meter.

+++++ The Wheatstone bridge circuit is used to make accurate measurements of resistance and operates on the principle that the sum of the voltage drops in a series circuit must equal the applied voltage.

Recording Meters

On occasion, it becomes necessary to make a recording of an electrical value over a long period of time. Recording meters produce a graph of metered values during a certain length of time. They are used to detect spike voltages, or currents of short duration, or sudden drops in voltage, current, or power. Recording meters can show the amount of voltage or current, its duration, and the time of occurrence. Some meters have the ability to store information in memory over a period of several days. This information can be recalled later by the service technician. Several types of recording meters are shown. The meter shown is a single-line-recording volt-ammeter. It will record voltage or current or both for a single phase. A kilowatt-kiloVARs recording meter is shown below. This meter will record true power (kilowatts) or reactive power (kiloVARs) on a time share basis. It can be used on single- or three-phase circuits and can also be used to determine circuit power factors. A chartless recorder is shown below. This instrument can record voltages and currents on single- or three-phase lines.

The readings can be stored in memory for as long as 41 days.

Bridge Circuits

One of the most common devices used to measure values of resistance, inductance, and capacitance accurately is a bridge circuit. A bridge is constructed by connecting four components to form a parallel-series circuit. All four components are of the same type, such as four resistors, four inductors, or four capacitors. The bridge used to measure resistance is called a Wheatstone bridge. The basic circuit for a Wheatstone bridge is shown below.

The bridge operates on the principle that the sum of the voltage drops in a series circuit must equal the applied voltage. A galvanometer is used to measure the voltage between points B and D. The galvanometer can be connected to different values of resistance or directly between points B and D. Values of resistance are used to determine the sensitivity of the meter circuit. When the meter is connected directly across the two points, its sensitivity is maximum.

Assume the battery has a voltage of 12 volts and that R1 and R2 are precision resistors and have the same value of resistance. Because R1 and R2 are connected in series and have the same value, each will have a voltage drop equal to one-half of the applied voltage, or 6 volts. This means that point B is 6 volts more negative than point A and 6 volts more positive than point C.

RV (variable) and RX (unknown) are connected in series with each other.

RX represents the unknown value of resistance to be measured. RV can be adjusted for different resistive values. If the value of RV is greater than the value of RX, the voltage at point D will be more negative than the voltage at point B. This will cause the pointer of the zero-center galvanometer to move in one direction. If the value of RV is less than RX, the voltage at point D will be more positive than the voltage at point B, causing the pointer to move in the opposite direction. When the value of RV becomes equal to that of RX, the voltage at point D will become equal to the voltage at point B. When this occurs, the galvanometer will indicate zero. A Wheatstone bridge is shown below.

+++++ Wheatstone bridge.

Next: part 4   ---  Prev: part 6: Summary/Quiz

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