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Curiosity Insight

Curiosity Insight

Learning never exhausts the mind

How to Use Oscilloscope to Measure Voltage?

August 3, 2020 By Miranda Jackson Leave a Comment

With an oscilloscope different measurements are possible. To practice using the oscilloscope, I took some simple measurements myself and photographed the corresponding screens. All measurements are done with the PM 3240.

Preparatory settings

The following settings should be made before the measurements on the oscilloscope (PM 3240):

Settings on the device in general:

  • turn on a device
  • Adjust the brightness with the control ILLUM and focus on the control FOCUS

Settings on the vertical unit:

  • Press button A at the top of the keypad (display channel A)
  • Set the regulator AMPL / DIV to the highest value (here 2 V per DIV)
  • Engage the regulator GAIN in the correct position (calibration position)
  • Connect the signal to DC
  • Use the POSITION knob to move the beam to the center of the screen

As long as you work with only one channel (e.,g. A), it is sufficient to make these settings only on channel A. Otherwise; these default settings also apply to channel B.

Settings on the horizontal channel:

  • At the keypad above DEL’D TB-EXT.X-MAIN TB, press the key MAIN TB
  • Use the knob POSITION TB MARGIN to move the beam to the center of the screen

Settings on main time deflection generator:

  • Press the TRIG key on the AUTO-TRIG-SINGLE keypad (triggering in the normal way)
  • Set the TIME / DIV or DELAY TIME control to a medium value (e.g., ten milliseconds)
  • Press the DC button on the DC-LF-HF keypad (trigger signal is directly coupled)
  • Press key A on keypad AB-EXT-LINE (trigger signal of channel A)

The delayed time sweep generator is not needed here. I have never used it until now. If these settings are made, a straight line should appear on the screen:

The basic display: The electron beam on the screen

The screen grid can also be seen. A square is exactly 1 cm tall. For the PM 3240, the screen is 8 cm high and 10 cm wide. For exact reading, small auxiliary lines are drawn at a distance of 2 mm from the two middle grid lines (horizontal and vertical).

DC voltages

Before the measurement is again checked whether the electron beam is in the home position (screen center). If necessary adjust with the regulator POSITION on the vertical unit.

Then the measuring voltage is applied to channel A. Important: Be sure to observe the voltage limits! Without probe divider, the voltage must not exceed 16 V ( peak value )! When using a feeler divider, the limits are higher, but also here the maximum voltage for the probe divider must be observed!

Voltage measurement on a 9V battery

A DC voltage is displayed as a straight line, just like the basic display without a signal. As soon as the DC voltage is applied to the oscilloscope, the line of the electron beam on the screen moves slightly upwards, depending on the level of the measuring voltage and the setting of the switch AMPL / DIV. If the deflection is too low, set the switch AMPL / DIV one step smaller, e.g. 1V per cm or 0.5V per cm.

What is the voltage? For this purpose, the deflection of the line from the baseline is read on the screen. The voltage then results from the product of the read height (in cm) times the value to which the switch AMPL / DIV is set! In the picture above is the switch z. B. to 0.5 V per cm. The height of the beamline from the baseline is 1.8 cm. The bill is:

In this measurement, however, a key divider was used with the ratio 1:10. Therefore, the result must be multiplied by 10. The voltage is 9 V! The measurement was carried out on a 9 V block battery.

The abbreviation Div comes from “Divit,” which means part. Div stands for a grid unit.

AC voltages

Again, before the measurement again checks whether the electron beam is in the home position (screen center). If necessary, readjust. But first a warning:

Never go with an oscilloscope in the socket or voltages of 230 V !! Not even when using key dividers!

The first AC voltage measurements are best done with a sine wave generator. This also creates a clean sine. The voltage from the socket, however, is more or less distorted by harmonics and other disturbances.

Again, you start with the basic attitude described above. If the vertical deflection is too low after applying the measuring voltage, set the switch AMPL / DIV to a smaller value. Then set the horizontal deflection switch (TIME / DIV) to value until you see one or two periods of the sine wave on the screen. Here is a sample picture:

Sinusoidal AC voltage from a sine wave generator

First, the voltage level is determined again: In the picture above, the switch AMPL / DIV stood at 0.5 V per cm. The upper peak of the sinusoid is 3 cm above the baseline:

The peak value of the AC voltage is 1.5 V! The RMS value must still be calculated by multiplying the peak value by 0.707. That gives about 1.06 V!

I am missing still the determination of the frequency. The switch TIME / DIV stood at one millisecond per cm. One sine period has 3.8 cm on the screen. 

The sine wave on the picture thus has a frequency of 263 Hz and a peak voltage of 1.5 V!

To look at the mains voltage, one must use an isolating transformer, the voltage to a safe level, z. B. down 12V. It must necessarily be a real isolation transformer because the voltage signal must be potential-free!

The sinusoidal AC voltage from the socket is distorted by harmonics Measured at the output of a separating transformer with 12 V.

Another form of AC voltage is the square-wave voltage. In her, however, the flanks are not displayed on the screen.

With the square-wave voltage, the edges are not displayed

Ripple voltages

If DC voltage is generated from an AC voltage source, depending on the quality of the circuit, a more or less strong residual ripple remains, which is referred to as ripple voltage. Measure them by setting the oscilloscope to AC voltage (set keypad AC-0-DC on PM 3240 to AC). In this setting, a capacitor is connected in series with the input, which removes the DC voltage component from the measurement signal.

Not smooth DC voltage (ripple voltage) This is how an incorrectly smoothed DC voltage looks like; This is also called ripple tension.

Electricity

Currents can only be measured indirectly. For this purpose, a resistor of known value is installed in the circuit and measured at this voltage drop with the oscilloscope.

Phase shifts

Phase shifts are measured in two-channel operation, i.e., one measurement signal is applied to channel A, the other to channel B. For dual-channel operation, some settings on the oscilloscope must be different than in single-channel operation. The parameters for the vertical unit are available twice for a two-channel oscilloscope. So far we have only used channel A. First, channel B receives the following basic settings:

  • Set the regulator AMPL / DIV to the highest value (here 2 V per DIV)
  • Engage the regulator GAIN in the correct position (calibration position)
  • Connect the signal to DC
  • At the A-ALT-CHOP-ADD-B (vertical unit) keypad, press the ALT key. Thus, both channels are displayed simultaneously on the screen!

As an example, the phase shift on an RC element (resistor and capacitor) is measured here. For this purpose, one measuring signal is connected to the socket of channel A, the other to the socket of channel B.

As you can see, both sinusoids are offset (out of phase). How big is the phase shift? For this one first measures the length of a sinus period on the screen – it is 9 cm here. Then the distance between both sinusoids on the screen – that is about 0.9 cm. The bill is then:

The phase shift is 36 degrees here. On the next picture you can see a phase shift of 180 degrees:

Two sinewave signals with 180 degree phase shift The amplified signal is 180 degrees out of phase with the original signal! Measurement on the right output channel of the TDA 1519.

Measurements on amplifiers

Amplifiers should amplify signals as unadulterated as possible, that is, do not change the original waveform of the signals. With a sine wave generator and an oscilloscope, you can easily check this. You see z. B. at which input voltage an amplifier begins to overdrive. Furthermore, the operating points of the transistors can be precisely adjusted with an oscilloscope.

Original and amplified sine measurement on the left output of the TDA 1519 with approximately half the volume control turned up.

Filed Under: Science & Technology, Topics

About Miranda Jackson

I am a chemistry researcher and research various household products to make life easier and enjoyable for people around the world.

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