An AC voltmeter reads RMS voltage. They measure the AC voltage by converting the AC to DC then reporting a voltage that is 70.7% of the peak voltage. AC voltmeters are only accurate for measuring sine waves. Any other wave shape will not give a true RMS reading. The exception is a "true RMS" voltmeter which will be clearly labeled "true RMS".

An oscilloscope is an instrument that draws a time domain graph of voltage. Another way to look at it is that an oscilloscope is an analog voltmeter that measures voltage as it changes over time. A luminous spot moves across the screen horizontally (along the X-axis) over a predetermined period of time. Therefore, the horizontal axis (the X-axis) of the oscilloscope represents time.

Voltage is represented along the vertical axis (the Y-axis). The higher the voltage the higher the spot moves along the vertical scale. The lower the voltage the lower the spot goes. Esentially, as the spot moves horizontally over a given period of time and vertically for a given voltage, the oscilloscope tells you what voltage appeared at what time. As the voltages changes over time, the oscilloscope gives a pictorial representation of the changes.

The oscilloscope's most common use is to simply give a picture of voltage over time. This picture can tell you a lot about what a circuit is doing. For example, as shown in AC Waveforms, an oscilloscope can show how a circuit distorts a waveform. This tells you how the circuit responds to different frequencies. An oscilloscope can also be used for voltage and time measurements. These can be combined to calculate the frequency of a wave also. To make voltage and time measurements, the voltage and time scales are adjusted so that each line of the scale represents a particular interval of voltage or time. For example, the oscilloscope can be set so that each horizontal line represents a difference of 5 volts. Then, if the spot moves up 3 lines, this represents a change of 15 volts to the positive. Likewise, the oscilloscope can be set so that each vertical line represents an elapsed time of 100 milliseconds. In this case, it will take 1 second (10 X .1 seconds) to move from the left side of the screen to the right side. If the oscilloscope is connected to an oscillator that produces a sine wave, the distinctive sine wave shape will be traced on the screen.

An oscilloscope can be used as a simple DC voltmeter. This may sound like a humble use for such a sophisticated instrument. However, if you already have an oscilloscope running, why set up a voltmeter too if the oscilloscope will do?

To use an oscilloscope as a DC voltmeter, first set the vertical scale for a range that can include any voltage you may read. For example, if you expect a maximum of 20 volts, and there are 10 horizontal lines on the scale, set the vertical range for 2 volts per line (2 volts per division in technician speak). Then, the entire 20 volts will fit the display. If you expect to encounter both positive and negative voltages, Set the scale to accommodate any voltage you expect and center the trace using the vertical positioning knob. For example, if you expect your voltage range to be between -20 and +20 volts, set the scale to 5 volts per division and center the trace. Then, -20 volts will move the trace down 4 lines and +20 volts will move the trace up 4 lines.

The following procedure shows how to set up an oscilloscope to measure between -10 and +10 volts (on a scope that has 10 divisions vertically).

To make AC measurements with an Oscilloscope you have to set the time base to a setting that will give a usable trace as the voltage changes over time. For example, if you are measuring the 60Hz sine wave coming from a wall socket, you need to set the time base so that approximately one cycle is traced in one sweep of the spot. Since it takes approximately 16 milliseconds for one cycle at 60 Hz, a setting of 2 mS per division will display 16 mS in just over 8 of the 10 divisions.

Just as above, you need to set the voltage (vertical) scale to include any voltage you expect. Since the voltage from a wall socket swings between -165 to +165 volts, which is 330 V peak-to-peak, you need to set the voltage scale to accommodate this. If you set the voltage scale to 50 volts per division, the 330 volts will swing just over 6 divisions.

Once you determine the period of a wave (how long it takes to complete one cycle) you can easily calculate the frequency. The frequency is simply the reciprocal of the period. In the above text example, we determined that it took just over 8 divisions for one cycle when the time base was set at 2 mS per division. If you count the divisions, and interpolate the actual time to where the wave is completed, you will see that it takes about 16.5 milliseconds for the wave to complete one cycle. If you divide 1 by 16.5 mS (1/.0165) you get a frequency of 60.6 Hz. Since it is difficult to be extremely accurate with an oscilloscope, this is a reasonable result.

A dual trace oscilloscope has two vertical (Y axis) inputs. Although the spots move horizontally in unison, they move vertically independently. A dual trace oscilloscope can be used to measure to different voltages at the same time. This is useful to compare the timing of one signal to another.

A dual trace oscilloscope has two Y-axis inputs that measure voltage independently.

Any test instrument takes some current from the circuit that is being tested; an oscilloscope tends to take more than others. This can be enough current to cause a voltage drop and change the way the circuit is operating. Attenuating (X10) probes are available to reduce this circuit loading. A X10 probe has a voltage divider built into the probe. The voltage divider increases the impedance of the probe so it doesn’t load the circuit under test as much as a X1 probe (which has nothing between the probe and the oscilloscope input). You must multiply all your voltage readings by 10 when using a X10 probe. Most oscilloscopes have different indicators on the voltage settings for X1 and X10 probes.