Lab Electronics for Physics

In this lab we will play with items that we'll be using in many lab experiments this quarter. Here is a quick list of the tools we will be using:

  • Breadboard
  • Digital Multi-Meter (DMM)
  • Low-Voltage Power Supply
  • Function Generator
  • Oscilloscope

Breadboards
Often scientists and engineers will build small circuits with electronic components. Rather than designing a circuitboard and soldering in the various pieces, to save time and allow for easy modification they use a tool known as a breadboard. A diagram of a small breadboard is shown below:

Image of a breadboard, a circuit prototype tool that allows you to build circuits by placing wires and components into small holes


The breadboard in the diagram has a pair of lines of holes running between sets of red and blue stripes (yours will not have these colored stripes). These lines of holes are each at the same voltage, which means that any wire put into one of the holes is connected to any wires plugged into the same line. In the main body of the breadboard there are many columns of five holes. The same concept applies, each of those sets of five holes acts as a single voltage, so a wire plugged into one of those five holes is connected to any other wire plugged into one of the four remaining holes. This allows an experimenter to make connections between different parts of the circuit without soldering (and hence can build or modify the circuit painlessly).

Digital Multi-Meter (DMM)
Photo of a Digital Multi Meter, a tool that lets you measure voltage, current and resistance The second piece of equipment we will use today is the DMM. This quarter we will use DMMs to measure voltages, currents and resistances. We'll start with resistances. Plug cables with alligator clips into the DMM, set the DMM to measure resistances and grab a resistor with the alligator clips. Measure the value of a few resistors, and compare the measured values to their labels. How are resistors labelled? You have to know resistor code - there will be three colored stripes, followed by either a gold or silver stripe. The first two stripes correspond to two sig figs, and the third stripe gives how many trailing zeros. The colors represent numbers as follows:

Black

Brown

Red

Orange

Yellow

Green

Blue

Violet

Gray

White

0

1

2

3

4

5

6

7

8

9

The silver or gold bands represent tolerances. A silver-banded resistor can be expected to be within 10% of its labelled value, while a gold-banded resistor is within 5%. This means that a resistor labelled Red-Red-Green-Gold would in fact be a 2,200,000 ohm resistor (two-two-five zeros) with 5% tolerance. See below:

2
2
5 zeros
5%

Once you have measured three resistors and compared them to their stated values, experiment to find typical resistances for the human body. You can also break this down into finding the resistance of body parts. If your arm had a resistor code printed on it, what would it look like?

Low Voltage Power Supply
For most applications in these labs, we will not use high voltages. Instead we will use small direct-current power supplies. After switching your DMM to measure voltage, connect it to a power supply using bannana plugs. Starting a zero, raise the voltage on your power supply in three-volt increments, reading from the dial. Record the actual voltage output via your DMM, and comment on the accuracy of the power supply's dial.

Photo of a low-voltage power supply, which provides the power for our experiments

The next thing to do is to measure current. Connect your DMM and power supply to a resistor so that one wire goes from the power supply to the resistor. The other end of the resistor should be wired to the DMM, and to complete the circuit the DMM should have a return wire to the power supply. Before turning on power, make sure that your DMM is set to read current. Since you can read the voltage from the power supply, and know the value of the resistor, verify Ohm's Law, that V = i R.

Note the we usually use 1/4-watt resistors in our labs. Later on we will learn that power is equal to i2R, or, substituting in from Ohm's Law, V2/R. This means that if V2/R > 1/4, you will fry your resistor.

Function Generator and Oscilloscope
Use a BNC cable to directly connect the function generator to channel one of the oscilloscope. The function generator has a series of controls across its front panel. The left-hand-side controls the frequency of the signal. The large dial varies from zero than one to four. The buttons running across the front serve as a multiplier in steps of decades. To get a 30,000 hertz signal, turn the dial to three and push the 10K button. What if you want an 800 hertz signal, the dial only goes up to four? In that case set the dial to 0.8 and select the 1K button. The controls on the right hand side select the type of wave function (square, sine and triangle) along with the magnitude of the output voltage.

Photo of a function generator, which will allow us to put user-defined waveforms into our circuits.

How would we actually see these wave functions? With an oscilloscope! In the dark ages, the use of an oscilloscope was as much an art as it was a science. Current technology makes the life of a student in lab much easier. After turning on the oscilloscope you will need to wait a minute or two for it to warm up. Once it is ready, hit the autoset button. The proper waveform will then appear on the screen (needless to say, in the old days one could not solve oscilloscope problems simply by hitting a magic button).

Photo of an oscilloscope, which allows us to take close looks at the voltage signals in our circuits.

Now that you have the waveform displayed on your screen, play with the channel 1 volts/div knob. Note how it makes the displayed waveform bigger and smaller. It does not actually change the voltage of the signal (only a knob on the function generator could do that), but it changes the way the signal is displayed. For example, a 3-volt waveform displayed at one volt per division would be three divisions tall, but at two volts per division it would only be 1.5 divisions tall.

The horizontal axis corresponds to time, and playing with that knob will allow you to change the timescale that is displayed.

The triggering menu will be explained by the instructor.

Note that there are some helpful buttons across the top of your oscilloscope. With analog scopes, it was often hard to judge exactly where a point on the waveform was. With a digital scope you can press the cursor button and know the exact coordinates for features on your waveform. You will use this feature many times in the coming weeks. The display menu allows you to monitor things like peak-to-peak voltage and the frequency of each wave form. There is also a help menu, which is very extensive.

After playing with your oscilloscope, sketch a picture of a waveform from the function generator. Note to see if the period measured by the oscilloscope matches what you would expect from the settings on the function generator.

Note that this week's lab is graded on a pass/fail basis - the point of this lab is to learn as much as you can about the equipment you will be using for the rest of the quarter, that's more important than a one-week grade.


Pre-Lab: Simulated Oscilloscope
Learning how to use an oscilloscope is an important step in an engineering student's career. However, they are expensive, and most people don't have access to them outside of lab. You can visit an online simulation to play with a fake oscilloscope.

Before you go to lab, explore the online model. Write down the voltages and periods of the signals on the green, blue, tan and purple wires. Adjust the controls until you fill the screen with a sine wave, with the number of peaks close to the number of your birthmonth. Make the peaks take up the entire vertical size of the screen. Print this out. Can you make two different signals show up on the screen at the same time? Print this too. Note if you double-click on a control, it brings you to an informative screen that helps you understand that part of the oscilloscope.