velleman EDU06 Oscilloscope Tutor Board User Manual

velleman EDU06 Oscilloscope Tutor Board User Manual

Oscilloscope basics

While a multimeter shows an instant voltage level or an average voltage level, an oscilloscope is capable of displaying voltage levels over a period of time. Voltage is displayed vertically (X-axis) versus time (Y-axis).

Oscilloscopes can be used to for applications such as:

• Looking at the shape of a signal.
• Measuring the amplitude and frequency of a signal.
• Measuring the time between to events.
• Looking for anomalies such as clipping, noise, distortion, spikes, ripple, etc…

Analog versus digital:

There are two basic types of oscilloscopes: analog and digital scopes.

Each type has its typical applications, advantages and disadvantages. The advantage of digital scopes is that they are able to capture and store events for further study. They are also more user friendly, as they require less setup in order to show a picture of your signal. For our experiments, we will use a digital scope.

Waveforms

An oscilloscope will mainly display and measure waveforms. A wave is a pattern that repeats over time, e.g. the waves of the sea. One cycle or period of a wave is the part of the wave that repeats itself. When shown on the screen of an oscilloscope, it is called a waveform. There are many different waveforms. A couple of them will be used in our experiments:

1. Sine waves: A typical example: The AC power grid
2. Full-wave rectified AC: The output of a full wave diode rectifier
3. Half-wave rectified AC: The output of a half-wave diode rectifier
4. DC voltage: Yes, oscilloscopes can also measure DC
5. Sawtooth waveform: In this case: ripple
6. Square wave: The board features a simple two-transistor oscillator which produces a square wave

Measuring probe:
To be able to display waveforms, we need to connect the signal to the input of the oscilloscope. Similar to a multimeter, the oscilloscope uses a measurement lead, but here it is called a ‘probe’.

1. Probe tip: The (+) of your probe. It is needle-shaped to ensure a good electrical contact with your measuring point.
2. BNC connector: Connects to the input of your oscilloscope.
3. Ground lead with alligator clip: The (-) of your probe. Connect the alligator clip to the ground or (-) of the circuit you wish to examine.
4. Probe clip : Clips onto the probe tip and allows you to connect the probe to the measuring point in a permanent way, so you have your hands free.
5. x1/x10 switch: When set to ‘x1’, the signal is passed on to the scope without attenuation. When set to ‘x10’, the signal is divided by 10, so the signal that is seen by the scope is ten times smaller than it acuatally is. This allows you to measure higher voltages without damaging your scope.

Connecting the probe:

Setting up the board:

The board requires a 9VAC (Alternating Current) adaptor (e.g. Velleman PS905AC (230VAC)). A DC adaptor instead of an AC adaptor will not damage the board, but most experiments will not work correctly. Connect the adaptor at the left hand side of the board and plug-in the adaptor. Once powered, the LEDs (LD2 & LD3) should blink alternately. The board is now ready to perform experiments.

Experiment 1: Measuring AC voltage

Purpose:
The purpose of this experiment is to display and measure AC voltage. In this particular case, we will measure the AC voltage supplied to the board.

HOw?:

1. Turn on the HPS140 Handheld Pocket Scope (see HPS140 manual for How-To instructions).
2. Place the probe switch “x1/x10” to ‘x1’.
   The unit always starts up in ‘auto-setup’ mode. You can tell that it is in auto-setup mode, because the readouts for Volts/div and Time/div are displayed in ‘reverse video’ (white characters on dark background). Thanks to the auto- setup mode, the unit takes care of V/div and time/div for you, you do not have to do anything. In the lower right-hand corner, the RMS  value of the supplied AC voltage is displayed. More about auto-setup in the next experiment.

What we see:

• Select ‘AC coupling’ (see HPS140 manual for How-To instructions)
• Set time/div to 5ms/div
• Set volts/div: to 10V/div
  Note: the readout in the bottom right hand corner displays the RMS value of the measured AC voltage. Different readouts are available (see HPS140 manual for How-To instructions).
  Note: If you live in an area where the AC grid frequency is 60Hz instead of 50Hz (e.g. US), the image may slightly vary

Experiment 2: Adjustable AC voltage

Connection summary:
GND clip : 2
Probe tip : 3
Purpose:
The purpose of this experiment is to show the advantages of the auto-setup function to measure AC voltage.

How?:

1. Turn on the HPS140 Handheld Pocket Scope (see HPS140 manual for How-To instructions).
2. Place the probe switch “x1/x10” to ‘x1’.

Trimmer RV1 allows us to adjust the output voltage on test point 3 between 0V and the full input voltage. Turn RV1 fully anti-clockwise (0V output). The trace on our oscilloscope screen remains a flat line, as there is no input voltage.

 Next, set V/div to 50mV/div (see HPS140 manual for How-To instructions).

Slightly turn RV1 clockwise until a sine wave is displayed on the screen. You only need to turn it slightly before the signal appears. If the signal goes ‘off-screen’, turn RV1 anti-clockwise until the signal is correctly displayed. In the lower right hand corner, you can read the current RMS value of the AC voltage measured, e.g. 100mV (0.1V)

Turn RV1 a little further clockwise. The signal goes offscreen and the Vrms readout displays ?????mV, because the unit is no longer capable of calculating the correct Vrms.

How can we display the current signal correctly again?
Increase the V/div setting to 0.1V/div. As you will see, the signal fits the screen again.

If you turn RV1 clockwise again, the signal will go off screen again. Changing V/div to 0.2V/div once again restores the display.

Measuring AC grid frequency and period

Connection layout:

Connection summary:
GND clip : 2
Probe tip : 3
Purpose:
The purpose of this experiment is to demonstrate the use of the markers to perform on-screen measurement of frequency and period.
How?:

1. Turn on the HPS140 Handheld Pocket Scope (see HPS140 manual for How-To instructions).
2. Place the probe switch “x1/x10” to ‘x1’.

Press the lower right button, this will freeze the screen and turn-on the markers. The upper right button allows us to toggle between the different markers.

There are 4 markers, two horizontal and two vertical. The horizontal markers allow us to measure the amplitude of a displayed signal, i.e. it measures how many volts there are between both markers. The vertical markers allow us to measure time between the two markers. In order to measure the frequency of a periodic signal, we can use the vertical markers and isolate one period of the signal

Press the upper right button again to select vertical marker 2. Use the arrow keys to position this marker at the exact same location but further to the right of the screen. You have now selected one period or cycle of the displayed waveform. In the lower right corner, the unit displays the time between the two markers. In most cases, this will be 20ms (16.66ms). The value displayed is called the period of a waveform, i.e. the time before it repeats itself.

Now for the frequency (=the number of periods per second). Press and hold the upper right hand button until the menu appears

When the menu appears, release the button and press it again a number of times, until ‘time mark’ is displayed in reverse video. Next, press any arrow key once to change the mode from ‘time mark’ to ‘freq mark’. Release all buttons and wait until the unit exits the menu.

Look in the lower right corner. The readout now displays frequency. In most cases this will be 50Hz (60Hz).

Rectified AC, single phase

Connection summary:
GND clip : 4
Probe tip : 5

Purpose:
The purpose of this experiment is to show what single phase rectified AC looks like on a scope screen.

How?:

1. Turn on the HPS140 Handheld Pocket Scope (see HPS140 manual for How-To instructions).
2. Place the probe switch “x1/x10” to ‘x1’.
3. Make sure to flip SW1 to the correct position

A little theory:
With a single diode, we can convert an AC voltage to a DC voltage. As a diode only conducts the current in one way, only one half of the waveform can pass. The other half, with inverted polarity, is blocked. As you can see on the screen, the trace shows ‘interruptions’ where the voltage is 0. This is the part of the AC voltage that is blocked by the diode. If you move the probe clip from test point 5 and to test point 1, you remove the diode from the circuit and the display shows the complete waveform again.

Rectified AC, dual phase

Connection summary:
GND clip : 4
Probe tip : 5
Purpose:
The purpose of this experiment is to show what dual phase rectified AC looks like on a scope screen and to show the difference with single phase rectified AC.

How?:

1. Turn on the HPS140 Handheld Pocket Scope (see HPS140 manual for How-To instructions).
2. Place the probe switch “x1/x10” to ‘x1’.

As the switch is still set to single phase rectification , the display will show the same trace as with experiment 4.

Now, check what happens when you flip the switch from single phase to dual phase rectification. Flip the switch back and forward, to clearly see the difference between both settings.

A little theory:
As you can see, the ‘interruptions’ we have noticed with single phase rectification are gone. As opposed\ to single phase rectification, both halves of the sine wave are used. Instead of a single diode, we use 4 diodes to make a ‘diode bridge’

Single diode

Diode bridge

Exercise:
In experiment 3, we have learned how to measure the frequency of a repeating waveform.\ Can you measure the period and frequency of both the single phase and dual phase rectified signal? (Answer: 10ms/100Hz or 8.33ms/120Hz)

Smoothed versus unsmoothed DC (ripple)

Purpose:
The purpose of this experiment is to show what smoothed and unsmoothed DC looks like on a scope screen and how a scope can help you to determine the quality of your DC supply.
A little theory:
In the previous experiments, we have used one or more diodes to convert an AC voltage into a DC voltage. The result was OK, but far from perfect.\ Why? Because it was still far from the flat line one would expect when measuring a perfect DC voltage. It is clear that our rectified AC voltage needs ‘smoothing’. This can be done with an electrolytic capacitor (see diagram of this board)

How?:

1. Place the probe switch “x1/x10” to ‘x1’.
2. Make sure to flip SW1 to the correct position.
3. Turn on the HPS140 Handheld Pocket Scope, It will start-up in auto setup mode as always. Watch the screen closely. You will notice that the trace is almost flat, so the capacitor is doing a good job at smoothing our rectified AC voltage. Yet, it still wobbles a bit.

Why is this?

Basically, the capacitor acts as a temporary storage device. It provides power to the rest of the circuit during the ‘interruption’ of the waveform (remember, single phase rectification?). With dual phase rectification this interruption is not present, so the capacitor has less work to do. The remaining ‘wobble’ of the waveform is called ‘ripple’. One of the key features of a good DC supply is low ripple.

Can we measure this amount of ripple?
Yes we can, a scope is the ideal tool for ripple measuremen.

Default, your scope starts up with ‘DC-coupling’ selected.
Change that to ‘AC-coupling’ (see manual for how-to instructions).

Now, the scope will only show the AC part of the signal, the DC part will be blocked. Make sure it is still in ‘auto-setup’ mode. If you look at the screen below and your scope screen, you will see a kind of ‘sawtooth’ waveform, this is the ‘ripple’ voltage that rides on top of your DC voltage. During the rising edge the power supply charges the capacitor, during the falling edge the capacitor supplies current to the circuit. If more current is drawn from the supply the ripple will be higher, as the capacitor will be drained more, so it will not be able to keep the output as steady as it would with a small load

How can we reduce the ripple?
Try flipping SW1 for single phase to dual phase rectification and watch the screen. Check the lower right hand corner. It displays the rms ripple voltage. Flip the switch back and forward. It is clear that ripple is reduced when dual phase rectification is used.

DC measurement

Connection summary:
GND clip : 4
Probe tip : 6

Purpose:
The purpose of this experiment is to show that a scope is also suited to measure DC voltages. In general, scopes are used to measure AC voltages. For DC voltages, a multimeter is fine. However, if you don’t have a multimeter at hand, you can still perform DC measurements with a scope.

make sure the scope is set up correctLy for dc measurements

How?:

1. Place the probe switch “x1/x10” to ‘x1’.
2. Turn on the HPS140 Handheld Pocket Scope, it will turn on in ‘auto set’ mode. ‘Auto setup’ will also work for DC measurements. There are
3. Important settings that need to be performed for correct DC measurement:

• DC input coupling
• DC readout
• DC reference

DC input coupling:
Input coupling needs to be set to DC (=). When set to AC, the scope will block any DC signal, so we won’t be able to perform DC measurements. At start-up, the unit is automatically set to DC coupling.

Next, we will set the readout in the bottom right hand corner to DC. Press and hold the upper right hand button until the menu appears. Release the button and press it again a number of times until the ‘readout’ setting is highlighted (123 appears in the lower left hand corner). Next, press any of the arrow keys repeatedly until Vdc is displayed. Wait for the unit to return to scope screen.

It now displays the measured DC voltage in the lower right hand corne

What happens if you swap the probe tip and ground clip?

Waveform with adjustable frequency

The purpose of this experiment is to demonstrate the use of the ‘trigger’-function.
How?:

1. Place the probe switch “x1/x10” to ‘x1’.
2. Flip SW1 into the ‘full wave’-position.
3. Turn on the HPS140 Handheld Pocket Scope, it will turn on in ‘auto setup’ mode. Select DC coupling.

Set unit to 10ms/div and 2V/div. Adjust RV2 and RV3 in such a way that the waveform looks like the the screenshot below. The unit displays a square wave. The rising edge of the square wave is not perfectly ‘square’, due to the limitations of this simple two-transistor circuit. Anyway, the resulting waveform is fine for our experiment. As you can see, the displayed waveform is perfectly stable, it does not jump from left to right. The circuit responsible for this is the triggering circuit.

How does it work?
Take a close look at the left hand side of the screen, where the waveform
starts. You will see a vertical line with a small ‘gap’. In this gap, a ‘slope’- symbol is displayed. This ‘gap’ determines the trigger point, the place where the scope will ‘trigger’ or will start drawing the waveform on the screen.

What is the purpose of the ‘slope’-symbol?

Let’s change the slope and see what happens. Press and hold the upper right button to enter the menu. Release the button and press it a number of times until ‘Slope’ is highlighted. Wait for the unit to exit he menu. Now look at the bottom left hand side of the screen, the slope symbol is displayed. Press any arrow key to toggle between rising and falling slope. Take a look at the waveform and see what happens

GLOSSARY

1. Volts/div: Determines how many volts the signal at the input must swing for the trace to move one division.
2. Time/div: Determines the time the trace needs to scan from the the left hand side to the right hand side of a division.
3. Division: Imaginary or visible grid on the oscilloscope screen. It helps estimating signal amplitude and period.
4. Period (T): Duration of one cycle of the AC waveform (= 1/f)
5. Frequency (f): The number cycles of the AC waveform per second
6. Trace: ‘line’ that is drawn on the screen, which represents the signal at the input
7. Amplitude: How far does the signal ‘swing’in a direction. Expressed in mV or V. For repetitive signals: Vpeak.
8. Peak-to-peak: Difference between most positive and most negative swing of the signal. 2xVpeak for sinusoidal signals.

Diagram

Documents / Resources

| velleman EDU06 Oscilloscope Tutor Board [pdf] User Manual
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