Notes from Winter 2024.

  • TA in lab evaluation complicates their ability to help groups experiencing problems.
  • Too many students built circuits but did not measure anything. Perhaps a worksheet for them to fill out and hand in.
  • No more protoboards.
  • Clearly mark high impedance resistors.
  • Shorting issues when using one PS for 3 boards.
  • Maybe require all members of a group to complete a part before anyone can receive credit.
  • Add theory section for parallel and series resistances for those who have not seen it in lecture.

Electrical Measurements - PHYS142

Introduction


To this point the PHYS141 and PHYS142 labs have focused on how to do experimental physics. Understanding how to make measurements, determine how well something has been measured, drawing appropriate conclusions from data, etc. Each of these labs has been built around doing an experiment.

This two part lab will be different. The purpose of this lab is to teach you some fundamental skills which will be necessary for future experiments that you will perform. In particular we will be focusing on the following:

  • Reading basic circuit diagrams.
  • Designing and constructing simple circuits.
  • Making measurements of voltages and currents.
  • Understanding how the act of measuring electrical quantities can alter those quantities.
  • Using function generators and scopes.

These are foundational skills in the sense that so many experiments involve the use of detectors which produce an electrical signal which must be processed and displayed. You will not get far in understanding, setting up, debugging and using electronic apparatus if you are not comfortable using a DMM to make voltage and current measurements, or using a scope to look at electrical signals which vary in time.

This lab will mostly be a series of exercises where you will be tasked with figuring out how to use basic electronic components, configure and connect them properly, and then make measurements to confirm things are behaving as expected.

This lab is also different in the sense that developing these skills requires everyone in the group to work hands on with the apparatus. You will still collaborate as a group on how to do what needs to be done, but each member of the group will for example build and test their own circuits, or find and measure a signal on their own scope.

No Notebook Or Out Of Lab Component

This lab is a skill building exercise more than an experiment. As such you will receive credit for the lab by having the TA check off each part as you complete it. There is no group notebook and there are no conclusions to hand in after lab.

Tasks


Apparatus And Conventions


The apparatus you will use for the DC measurements are described below.

This is what the full setup looks like for one group of students. Each student has their own prototype board (protoboard), DMMs, cables and resistors. All three protoboards are powered by a single DC power supply.

A number of jumper wires with different colored insulation are provided as seen on the left hand side of the protoboard.

Here we see a close up view of an individual protoboard, which contains rows and columns of electrically connected sockets which various wires and components can be plugged into for building circuits.

To the right of the DMMs are two pair of red and black banana cables with alligator clips attached to one end for making connections from the DMMs to the protoboard.

Each student is also supplied with a set of 12 resistors. Two sets of five resistors in the kilo-ohm range, each set being a slightly different resistance value. One pair of higher resistance (mega-ohm range)resistors.

Resistor resistances can be measured using the DMM. Also, the colored bands on the resistors indicate their resistance. The color code is explained on this wikipedia page https://en.wikipedia.org/wiki/Electronic_color_code in the section on resistors.

Each group will also use an incandescent light bulb. This component will be given to you by your TA when you need it.

The DC Power Supply

Power to the proto boards is supplied by a dual output DC power supply. Each power supply has two independently controlled outputs, each of which is capable of 0V to 32V at currents up to 3A.

For each channel the display shows the voltage (in red) and current (in green) settings which are controlled by the dials labeled Volts and Current. A red led in between the Volts and Current dials shows whether the supply for that channel is in Constant Voltage (CV) or Constant Current (CC) mode. For this experiment we will use the power supply in Constant Voltage mode which is set by turning the CURRENT dial all the way up (full clockwise). What this means is that the power supply will adjust the current as needed in order to maintain a constant voltage at the setting you dial in.

At the bottom of the power supply are a series of output jacks which accept banana plugs. There is one group of output jacks in the center which are colored left to right as; Red, Black, Green, Red, Black. The red and black jacks to the left of the green are the outputs for CH1, the red and black pair to the right of the green jack are for CH2. There is also a Black and Red pair all the way on the right side of the supply, these are not used for this lab.

In the middle of the supply is a button labeled MODE which can be set to Free or TRACK. This button should be left in the out position (FREE).

Powering Your Proto Boards

To deliver power to your protoboards just connect the red and black output jacks of channel1 on the power supply to the black (ground) and red (V3) jacks on each of the protoboards. Note that there is a separate pair of wires from the power supply to each protoboard. When connected in this manner the circuit on each board will not impact the measurements made on the other boards.

By convention the color black on power outputs is defined as the zero volt reference point and red carries the voltage. For the sake of making it easier to be sure you have wired the power to the proto boards correctly always use black wires for the black jacks and red wires for the red jacks.

To begin the experiment set the voltage for YOUR channel to 5V. Later in the lab when you are taking current vs voltage data it is ok to turn the voltage all the way up to 32V, doing so will not hurt any of the components.

Note that the Current control needs to be turned fully clockwise.

Circuit Symbols

For the purposes of creating and reading circuit diagrams we will use a commonly accepted set of symbols to represent circuit elements. When you design your circuits as part of the exercises, use these symbols to draw them.

DC Power
Resistor
LED
Lamp
DMM used as a Voltmeter
DMM used as an Ammeter

There is no need to draw the circuits on the computer, that tends to take more time than it is worth. Just use paper and pencil and get used to making use of these symbols to sketch out organized, neat and readable circuit ideas. Below is an example of a perfectly acceptable circuit diagram I made of a resistor in series with a lamp and a DMM setup to measure the voltage drop across the resistor.

DMM Fuses

One potential stumbling block is the fact that the DMM's have a built in fuse protecting the current input to the ammeter functionality. There are several ways that this fuse can be blown, trying to measure too large of a current for example. Unfortunately it is not obvious when the fuse is blown because the DMM still turns on and still works in the voltage and resistance measurement modes. If you suspect that the fuse in your meter is blown you can easily use a second DMM to check the fuse on the suspect DMM.

Help! I'm not seeing any current!

There are a few reasons you might not be seeing a current in your circuit. 

To start, make sure that your meter is on the current setting (i.e. the dial is turned to $\text{mA}$) and that the leads are in the correct socket. If this is the case and you're still seeing nothing, the fuse in your meter may be blown.  Here's how to check:

Connect one meter in resistance mode to the other in current mode, as shown below. If the meter being tested reads $0.1$ to $0.2\text{ mA}$ and the other reads a couple of ohms, then the fuse is fine.

If, as shown below, the meter being tested reads $0\text{ mA}$ and the other reads overload (O.L) then the fuse is probably blown. Fortunately, replacing them is easy.

Exercises


For each of the circuits specified below:

  • Work as a group to design the appropriate circuit. Each person in the group should draw their own copy of the circuit.
  • Calculate the voltage across and current through each element for a +5v output from the power supply.
  • Each member then builds the circuit on their own proto-board.
  • Each member uses the DMM to confirm that the voltages across, and the currents through each circuit element are as expected.

The TA will check off each students progress for all parts of each task and this will for the basis for the participation portion of your lab grade.

Series Circuit

Design, build and test a circuit with at least two resistors of different values in series.

Use only resistors in the kΩ range, not the higher resistance ones in the MΩ range.

Parallel Circuit

Design, build and test a circuit with at least two resistors of different values in parallel.

Use only resistors in the kΩ range, not the higher resistance ones in the MΩ range.

Combination Series Parallel Circuit

Design, build and test a circuit with at least three resistors connected in a combination of both series and parallel configurations.

Use only resistors in the kΩ range, not the higher resistance ones in the MΩ range.

High Resistance Voltage Measurements

Build a series circuit using two resistors whose values are greater than 10MΩs. Measure the voltage drops across each resistor individually as well as across both. You should see something puzzling about the results of the measurements, think about what is different about measuring voltage drops across large resistances.

Ohmic and non-Ohmic Devices

Ohm's Law predicts a linear relationship between Voltage and Current.

$ V = IR $

Components which obey Ohm's Law are referred to as being Ohmic. However not all electrical components used in circuits have linear relationships between Voltage and Current. Such components are referred to as being non-Ohmic. For your final exercise your TA will provide you with a light bulb. One person in the group should select the resistor and another person the light bulb. The third person can enter data to the co-lab notebook.

As a group measure the current through each component as a function of the voltage drop across it for voltages from 0V to +5V.

Use the co-lab Notebook to plot the data for each device. Compare the I vs V curves of the two components. It should be obvious that the light bulb does not follow Ohms Law.

Before You Leave The Lab


It is absolutely imperative that you return your station to a neatly ordered state before leaving the lab. For the sake of your fellow students who come to lab after you, please take 10 minutes at the end of the period to clean up your work area and put everything back where it should be, your station should look like the photo at the top of this wiki page.

Specifically make sure that you have done the following:

  • Disconnect all components and jumper wired from your proto-board, with the exception of the two wires leading from the red and black banana jacks to the top two rows of the board.
  • Test the DMM's to make sure their fuses are good. Replace any blown fuses you find.
  • Turn the DMM's off.
  • Put all of your resistors back in their envelope.
  • Neatly organize your banana cables.
  • Return the lamp and LED to the TA.
  • Set the power supply to 5V and turn it off.
  • Ensure all three protoboards are properly connected to the power supply.

Not doing the above may result in an unnecessarily difficult lab experience for your fellow students.