• BME 210
  • Lab 1: Introduction I - Equipment Overview

    1. Objectives

    By the end of the laboratory session, students will be able to:

    2. Background

    2.1. Safety

    Electricity follows the path of least resistance to ground. In this lab we will generally be working with low voltages (10s of volts), but even very small currents (microamps) can be dangerous if they flow through your body. Some general safety tips:

    2.2. Digital multimeter

    The digital multimeter (DMM) contains an ammeter, voltmeter, and ohmmeter, which are used to measure current, voltage, and resistance, respectively. They provide a single number for output, so they are generally used for DC current. AC measurements vary with time, but can be represented by a single number: the root-mean-square (rms). An AC voltage with a peak-to-peak amplitude of 10 V would be 3.54 Vrms. Vrms = Vpp/\({(2\sqrt{2})}\).

    Voltage

    Voltage is measured by placing the DMM in parallel with element you wish to measure. Think of the voltmeter as an open-circuit (i.e., its impact on the circuit is minimal). Since voltages across parallel elements are the same, the voltage across your DMM will be the same as across the element you are measuring. Figure 1 shows an example.

    Figure 1. Voltmeters are parallel to the circuit element being measured.

    Current

    Current is measured by placing the DMM in series with the element you wish to measure. Think of the ammeter as a short-circuit (i.e., it does not impede current flow). Current through series elements is the same, so the current through your ammeter will be the same as through the element you are measuring. Figure 2 shows an example.

    Figure 2. Ammeters are in series with the elements being measured.

    Resistance

    Resistance is measured using an ohmmeter by placing the resistor between the leads. To get the most accurate reading, be sure not to touch or hold the resistor with both hands.

    2.3. Power supply

    The DC power supply is used to provide a constant voltage and/or constant current to a circuit. In constant voltage mode (CV on the display) the supply provides a user-defined voltage and there is a limit to the amount of current that can be generated. In constant current mode (CC on the display) the user provides a user-defined current and there is a limit to the amount of voltage that can be generated. The user defines these limits. Use the + and – terminals to form a closed current path.

    2.4. Function generator

    The function generator is used to create AC voltages. Waveforms can have a variety of shapes (sinusoidal, square, etc.) and frequencies. All waveform characteristics (phase shift, amplitude, frequency, etc.) can be controlled with the function generator.

    2.5. Oscilloscope

    The oscilloscope is used to visualize time-varying voltages. Two signals can be compared. The horizontal and vertical scales can be adjusted to zoom in on signals of interest. One very important function on an oscilloscope is the ability to trigger when a new trace is drawn on the screen. This function helps reduce horizontal shifts between sequential traces and makes periodic signals easy to visualize. Triggering events are usually specified as a voltage threshold and direction (positive or negative transition) or an external trigger source is used.

    2.6. Resistors

    Resistors impede the flow of current. You can either measure resistance with a DMM or calculate it by using the color bands. Tolerance is how much the base value of the resistor may vary. Silver = 10%, Gold = 5%, Brown = 1%, Red = 2%, Green = 0.5%, Blue = 0.25%.


    Color 1st band 2nd band 3rd band \(\times 10^n\)
    Black 0 0 0
    Brown 1 1 1
    Red 2 2 2
    Orange 3 3 3
    Yellow 4 4 4
    Green 5 5 5
    Blue 6 6 6
    Violet 7 7 7
    Gray 8 8 8
    White 9 9 9


    2.7. Breadboard

    A breadboard allows you to easily make connections among circuit elements. Each leg of a circuit element is pushed into the holes (Figure 3a). Strips of metal are attached underneath the holes so that rows and columns constitute single nodes (Figure 3b). Each yellow strip represents one node. When wiring circuits, be sure to use the shortest length possible, as this will reduce the amount of noise in your circuit.

    (a)
    (b)
    Figure 3. (a) Breadboards are used to rapidly prototype circuits. (b) Metal strips on the bottom connect the leads of circuit elements inserted into the holes.

    3. Laboratory Equipment

    1. Oscilloscope (Agilent Technologies DSO3152A)
    2. Function generator (Agilent 33220A)
    3. Digital multimeter (Hewlett Packard 34401A)
    4. Agilent RCL impedance meter
    5. DC power supply (Agilent E3649A)
    6. Wires
    7. Breadboard (Global Specialties PB-503)
    8. Assortment of resistors
    9. Assortment of capacitors

    4. Procedure

    4.1. Resistance

    1. Watch the videos DMM Resistance 480 and Breadboard 480.
    2. To measure the resistance, plug the cables into the 1000V max pair of ports on the right hand side of the multimeter.
    3. Press the button labeled \(\mathrm{\Omega}\) 2W. You are setting the DMM to measure resistance using the 2-wire mode.
    4. Measure and record the resistance of a 100 \(\mathrm{\Omega}\), 200 \(\mathrm{\Omega}\), and 300 \(\mathrm{\Omega}\) resistor. Write down the color code for each.
    5. Assemble all three resistors on the breadboard in series and in parallel.
    6. Measure and record the equivalent resistance of both configurations.
    7. Disconnect the leads. Hold one ohmmeter lead in each hand (i.e., measure the resistance of your body) and record the value.

    4.2. Capacitance

    1. Pick up the blue Agilent RCL impedance meter. An impedance meter is a special ohmmeter that can measure capacitance and inductance in addition to resistance.
    2. Pick up 3 capacitors: 1 \(\mathrm{\mu}\)F, 0.47 \(\mathrm{\mu}\)F, 0.047 \(\mathrm{\mu}\)F
    3. To measure the value of a capacitor, Press the RLC button until C is displayed.
    4. Slip the capacitor leads into the slots on the meter and wait for the displayed value to stabilize.
    5. Record the value and units for the three capacitors. Be sure to record the identification number on each capacitor as well.

    4.3. DC circuits

    1. Watch the videos DC Power Supply 480, DMM Voltage 480, and DMM Current 480.
    2. Assemble the circuit shown below. Be sure to have your TA check the circuit before you turn the power on.

    3. Turn the DC supply by pressing Power.

    4. Set the display to Limit Mode by pressing Display Limit.

    5. Press Voltage/Current and turn the knob to 0.100 A of current. If the voltage value is changing, press Voltage/Current again and adjust current to 0.100 A.

    6. Press Voltage/Current to adjust the voltage limit to 10 V.

    7. Press Display Limit to return to Meter Mode.

    8. Press Output On/Off to turn on output.

    9. Turn the knob to adjust the output voltage to 10 V

    10. Measure and record the voltage drop across R2 using the DMM. Place the probes in parallel with the resistor and press DC V.

    11. Measure and record the voltage drop across R1 using the DMM.

    12. Press Output On/Off on the DC supply.

    13. Place the DMM probes in series with R1 and R2. Unplug the DMM cable that is plugged into the port labeled Input. Plug it into the port on the bottom labeled I, for current. This cable swap is required for you to measure the current in the circuit.

    14. Turn the output back on.

    15. Measure and record the DC current by pressing Shift and then DC V.

    4.4. AC circuits

    1. Watch the videos Function Generator 480 and O-Scope 480 on the course site.
    2. Later in the semester when you study AC circuits you will need to use a function generator to provide AC input to your circuit.
    3. Disconnect the DC power supply and hook up the leads from the function generator to your circuit. The figure below shows the AC circuit.

    4. Turn on the function generator.

    5. First, set the output impedance of the function generator to a High Z Load. Press Utility and then the blue button under Output Setup. Press the blue button under Load High Z until High Z is highlighted. Press Done.

    6. Next, set the frequency. If Freq is highlighted and selected then turn the knob on the right until the frequency is 100 Hz. The frequency can also be set by using the keypad.

    7. Finally, set the amplitude of the signal. Press the blue button under Ampl to set the amplitude. Set the amplitude of your waveform to 10 V peak-to-peak (Vpp). You can move the cursor to the left or right by pressing the arrow keys. Or you can key in the exact value in the number pad followed by Vpp. This will help you set your new value much quicker.

    8. Have your TA check the function generator to make sure you have entered the correct values.

    9. Turn the output on by pressing Output.

    10. Using the DMM. Measure the AC voltage across R1 and R2 following the procedures in section 4.3. Note that instead of DC V and DC I you will be using the AC V and AC I buttons on the multimeter. Record the AC voltages across both resistors. Record the AC current through both resistors.

    11. Disconnect the DMM.

    12. Using the oscilloscope. Attach the grey part of the Channel 1 probe on the node connecting R1 to the function generator. Attach the black part of the probe to the node between R2 to the function generator. You have just attached the Channel 1 oscilloscope probe across R1 and R2 and your output will be the voltage across both of these resistors. Press Auto-Scale.

    13. Press Measure, Voltage, and then Vpp. Measure and record the combined voltage across R1 and R2. Record the units per division on the bottom left of the screen. Also record how tall the waveform is in terms of divisions.

    14. If the Measure menu disappeared, press Measure again to bring it back. Press Vrms and record the rms voltage value.

    15. Attach the grey part of the probe to the node between R1 and R2. Measure and record the voltage (Vpp and Vrms) across R2 only.

    16. Press Clear on the Measure menu, followed by the Measure button to remove the menu.

    17. Press Cursors button once followed by Source until it reads CH1 and Type until it reads Voltage. This tells the oscilloscope to use data from the Channel 1 probe and for the cursors to display the voltage value of the waveform.

    18. Use the CurA and CurB buttons on the menu to select between cursor A and B. Use the dial with the clockwise arrow above it to position the cursors.

    19. Manually adjust cursor A and cursor B until they barely touch the top and bottom of the waveform. Record the value for \(\mathrm{\Delta}\)Y.

    20. Turn the function generator output off by pressing the Output button.

    4.5. Triggering

    1. Watch the video O-Scope triggering 480.
    2. Wire the output of the function generator by connecting the clips to the oscilloscope probe. The red clip should hook directly to the probe and the black clip should hook to the probe ground.
    3. Once the probes have been hooked up, turn the function generator back on by pressing the Output button. The button will light up when output is on.
    4. Press Auto-Scale.
    5. Turn the trigger Level knob on the right clockwise until it is above the waveform. Sketch the waveform. Turn the knob counterclockwise until the trigger threshold is at approximately 0 mV. Sketch the waveform.

    5. Results

    1. Are you less likely to get electrocuted by a circuit if you are touching it with both hands or just one hand? Why or why not?
    2. How much error was there between the color code on the resistors and their measured value? Create a table with three columns and include the data showing the measured resistor values, the ideal resistor values, and the percent error. Is the percent error less than the rated tolerance? Percent error is calculated with the formula

    $$\frac{|\mathrm{measured}-\mathrm{ideal}|}{\mathrm{ideal}} \times 100$$

    1. Report the results from steps 4 and 6 in Section 4.1.
    2. What resistance did you measure when you held both DMM leads in your hands? Is this result reasonable? Consult a reliable source on the internet or a book in the library. Cite your source.
    3. Report the results from Section 4.2., step 5.
    4. Report the results from Section 4.3. steps 10, 11, and 15.
    5. Calculate the current and voltage through the DC circuit. Do your measured values match the predicted values?
    6. Report results from Section 4.4., steps 10, 13, 14, 15, 19.
    7. In Section 4.4., does the measured Vpp match the voltage if you add up the number of divisions?
    8. What does triggering do?




    Last updated:
    January 7, 2018