Hand's On Introduction to Ohm’s Law with Example and Counter Example

Education
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Introduction

Not every device follows Ohm’s Law. Not all materials follow the oversimplified linear relationship. We will show that a resistor follows the ohmic relationship while an incandescent light bulb has a non-ohmic (curved) current to voltage (I-V) relationship.

Figure 1: The incandescent lamp is connected directly to the output of the DC power supply.
Figure 1: The incandescent lamp is connected directly to the output of the DC power supply.847×1128 113 KB

Figure 1: The incandescent lamp is connected directly to the output of the DC power supply.

Parts Required to Duplicate the Experiment

The Ohm’s Law demonstration shown in Figure 1 was conducted using the following components. DigiKey links are included for core components.

Procedure

  1. Work with your lab instructor to construct the incandescent lamp circuit as shown in Figure 1.

  2. Verify that the circuit is operating by observing the lamp’s glow.

  3. Set the power supply to 0.5 VDC.

  4. Record the corresponding current in the table.

  5. Repeat steps 3 and 4 for all voltages in the table as shown in Figure 2.

Figure 2: Current and voltage data for the incandescent lamp and resistor.
Figure 2: Current and voltage data for the incandescent lamp and resistor.835×444 80.8 KB

Figure 2: Current and voltage data for the incandescent lamp and resistor.

Tech Tip: Be sure to order extra lamps as you are sure to have a few students who push things too far. About the same time as they exclaim. “Look how bright it is,” the bulb will burn out.

  1. Turn off the power supply.

  2. Replace the incandescent lamp with the 40 Ω 1 W resistor.

  3. Record the current for all voltages listed in the table.

  4. Turn off the power supply.

  5. Plot the results with voltage on the horizontal and current on the vertical axis. The results should be similar to Figure 3.

  6. Calculate the resistance for both the incandescent lamp and the resistor for every voltage value.

Figure 3: Current-Voltage Characteristics for the incandescent lamp and the resistor.
Figure 3: Current-Voltage Characteristics for the incandescent lamp and the resistor.2130×1264 88.1 KB

Figure 3: Current-Voltage Characteristics for the incandescent lamp and the resistor.

What did this tell us about Ohm’s Law?

Not all material follows the linear relationship. The incandescent lamp is non-Ohmic.

What is the significance of the straight line?

The Line is Ohm’s Law in action.

It demonstrates that the resistor has a linear relationship between current and voltage. Contrast this with the lamp which has a resistance that varies between 13 Ω to 41 Ω over the range of the experiment.

Describe the current voltage relationship for both the resistor and the bulb.

The resistor has a linear relationship between current and voltage as predicted by Ohm’s Law. The incandescent lamp has a nonlinear response with resistance increasing as a function of applied voltage. The cold resistance is about 13 Ω while the white-hot resistance is about 41 Ω.

Why was the resistor chosen?

The resistor was chosen to match the incandescent lamp’s white-hot resistance. At full rated voltage, the resistor’s current matches the lamp’s current.

In later lessons you will learn about power. In this case, the larger 1 W resistor is required to dissipate the nominal 0.6 W when operating at 5 VDC. Stated another way, the 7268 is a 0.6 W lamp, we a need resistor capable of dissipating the same power.

Would the resistor’s resistance change if it was glowing red hot?

Based on this experiment we would expect the resistance to increase. However, this may not be true as we do not know the thermal properties of the resistor. This may be a good exercise for the future especially when we encounter precision circuit that could be negatively impacted by temperature dependence.

BTW, if your resistor is glowing hot, resistance doesn’t matter as it will soon be infinite as the remaining resistive material burns up.

Next Steps

  • Repeat using a different incandescent lamp. For example, the common Christmas tree 2.5 V bulb can be used on a breadboard. You can also select other bulbs such as the 2.5 V 7732 or the 10 v 8095

  • Present your students with a random incandescent lamp and have them select the appropriate resistor.

  • Perform the experiment using two voltmeters including one to measure voltage across the lamp and another to measure current passing through the lamp.

  • Shift attention to the Wien bridge oscillator which features an incandescent lamp. Show that the change in lamp resistance is essential to stabilize the amplifier gain.

Tech Tip: The bulb glows allowing both learner and instructor to see the circuit’s operation.

The incandescent bulb has been the starting point for generation of technicians and engineers. Unfortunately, it has fallen out of favor as we tilt towards LEDs. My recommendation is to save LEDs for another day.

Intended Audience

This article is tailored for the 100–200 level STEM students beginning formal coursework in electronics. Educators looking for techniques or ideas to help students understand Ohm’s law.

Demonstrate Ohm’s Law Early in the Curriculum

This experiment makes an excellent first lab in electronics. The learner will:

  • Breadboard the circuit as shown in Figure 1.

  • Almost certainly make mistakes requiring circuit troubleshooting. This is more important than most people realize as it sets the learner up for success with complex circuits in the future.

  • Operate a power supply.

  • Read voltage and current from the power supply.

  • Plot the current to voltage relationship on graph paper. Yes, use graph paper to develop an intuitive analog understanding of the material.

Chalk Talk to be Given Before Conducting the Experiment

  • Demonstrate how to:

    • Set the power supply for a desired voltage such as 3.5 VDC.

    • Configure the power supply to display voltage and current.

  • Review Ohm’s law and demonstrate the cover-up method for easy-to-remember calculation.

  • Review the concept of the complete circuit. Mention that an incandescent lamp makes it easier to see the circuit in operation.

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About This Author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians.

Dahlen has returned to his Northern Minnesota home, completing a decades-long journey that began as a search for capacitors. Read his story here.