Lab 1 part 1: Using a multimeter to verify Ohm’s Law

Objective: Verify Ohm’s Law and resistor power ratings. Build a voltage divider circuit block.

Skills Learned: How to use a multimeter.

Ohm’s Law: : the voltage across a resistor is the current through it multiplied by its resistance (V = IR). Note the language used – voltage across and current through (never never never refer to the voltage through something or the current across it.)

(image)
Usually the horizontally dashed lines in the middle will be used to symbolize ground, but they all mean 0V.

A multimeter is a tool that can make many important measurements – resistance, voltage across a component, current through a component (and sometimes capacitance, inductance, forward voltage, and more). To measure voltage, turn the knob to Vbar and touch the probes of the multimeter to either end of the component. To measure current, turn the knob to Abar, “break open” the circuit, and insert the probes so that the current has to go through the multimeter. Note how the sign of the voltage depends on where you have placed the red vs. black leads of the multimeter, and the maximum value you can read is based on the setting of the knob.

  1. Measure and plot the voltage across a 1k resistor as you vary the current through it. Put a resistor “R” in series with the 1k resistor, from 5V to ground. Using two multimeters (with a partner), simultaneously measure the voltage and current. Use different values of “R” so that you get different amounts of current through the resistors and fill out the table and plot the data in the graph below. (image, table, and graph)
  2. Power is defined as the voltage across multiplied by the current through a component (P = IV). In resistors, power is dissipated as heat. Typical breadboard sized resistors can dissipate 0.25W before something bad happens to the resistor. What would happen to the resistor if you dissipated more than 0.25W ?
  3. Push a resistor to the rated specification. How much resistance do you need to dissipate about 0.25W using your +5V to ground supply?
  4. Place the resistance (calculated above) between +5V and ground, and simultaneously measure the voltage across the resistor and the current through it to verify the actual power dissipated.
    Measure the voltage:
    Measure the current:
    What is the power?
    Physically, what happens to the resistor?
  5. Add another of the same value resistor between +5V and ground. What power does the nScope software report?
    At what power level does the nScope turn itself off for protection?
    Compare the value of the “+5V” rail when you are drawing no current to when you draw just below the amount of current before the nScope turns off. How well does the nScope maintain “+5V”?
  6. Ideally, you could draw as much current from the power supply as you could ever need. But realistically, the USB port can only deliver so much. Do a little research online: how much power can a standard USB 2.0 port supply?
  7. Resistors…

  8. Use your multimeter to measure the voltage across:
    • VR1
    • VR2
    • VD1
    • VD2
  9. To measure voltage with the nScope, cut a long wire and strip both ends. Plug one end into the voltage you want to read, and the other into a CH pin on the nScope.
  10. Use the nScope to measure the voltage at points:
    • VA
    • VB
  11. Use the multimeter to measure the current through:
    • ID1
    • ID2
  12. Replace R1 and R2 with 100(ohm) resistors. Use your multimeter to measure the voltages and currents:
    • VR1
    • VR2
    • VD1
    • VD2
    • ID1
    • ID2
  13. Replace D1 with a green LED. Use your multimeter to measure the voltages and currents again:
    • VR1
    • VD1
    • ID1
  14. (image) Replace the +5V supply with the function generator output A1. Set the output to a sine wave, unipolar, with an amplitude of 4.75V and a frequency of 0.1Hz. Simultaneously measure the voltage at A1, the voltage at point C, and the current through D3. Describe the difference between the two voltages, and how they relate to the current through the LED and the LED brightness. What is weird about the LED?