Teaching Node Voltage and Polarity with a Manual H-Bridge

[APDahlen]

The manually operated H-bridge shown in Figure 1 may be used to teach electrical polarity, reinforce the concept of node voltage, and demonstrate current paths. Students predict the voltage at each load node, trace the resulting current path, and verify which node is positive relative to the other. They are rewarded with the activation of the green or red sections of the dual-colored panel lamp. Some switch combinations leave the lamp off while others short out the power-supply output. Consequently, a current-limited power supply is required for this lab.

This structured activity is part of a sequence of industrial-control laboratories constructed from a shared, reusable parts kit. Earlier exercises use the same hardware to teach electrical nodes, switch logic, closed current paths, and voltage measurement. The manual H-bridge then recombines those familiar components to demonstrate that fixed node identities can assume different voltages as circuit topology changes.

Instructor Notes

These instructor notes document the educational intent behind the laboratory.

What new mental model is being developed?

Polarity is the observable objective of this lab.

However, the lab is working on a deeper level as students are asked to disassociate the concept of node from the voltage present on that node. They enter the lab assuming that a node’s identity is associated with a given voltage. They leave the lab understanding that a node may have different voltages depending on the state of the circuit. For example, the Left node may float or be connected to either 24 VDC or the 24 VDC return.

What is being reinforced?

This lab is a continuation of introductory industrial automation labs in which students learned:

• Wire color for industrial circuits
• Node labeling
• Polarity as associated with the power supply and with industrial relays

Please refer to the guided pathways for additional information concerning the lab sequence.

What observable behaviors demonstrate competence?

• Students can interpret the wire diagram to construct the H-bridge without assistance.
• Wire color is correct. Blue is used for all connections except for the two white-with-blue stripe wires that are permanently attached to the 24 VDC return.
• Each student can independently operate the H-bridge to control the dual-colored panel lamp.
• Students can describe the circuit’s operation in terms of switch states and voltage associated with the Left and Right nodes. More specifically, they demonstrate that load polarity is described in relative terms between the Left and Right node voltages.
• Students can describe the attributes of a floating node, a common node, and a short circuit.

Prerequisites

Before attempting this activity, students should have a working knowledge of:

• Electrical nodes with the ability to label the wires associated with a node
• The relationship between physical wires and the wire diagram abstraction
• Voltage measurement as a property measured across two nodes
• Logic gates with the ability to construct 2-input primitives using industrial switches

Chalk Talk

• Reason from nodes, not wires.

• A current-limited power supply is required because students will cause a short circuit condition via H-bridge shoot-through.

• Review KVL and KCL principles. Identification of the closed loop is a critical part of this lab.

• The fan is added directly across the power supply as an immediate notification that shoot-through has occurred.

  • It’s a crude bus-voltage annunciator that you can hear from across the room allowing you to observe the lab from a distance.
  • Be sure to use the term branch circuit to differentiate the fan from the H-bridge and the two bridge arms. A short on a bridge arm affects the entire circuit thereby stopping the fan.

• Node labels are absolutely required for optimal learning.

• Describe the purpose of the diode as well as the physical orientation.

• Require students to explain every observation in terms of node voltage and current path rather than switch combinations. Avoid statements such as “these two buttons make the green light turn on.” Instead ask:

  • Which node is positive and in relationship to what?
  • What is the voltage associated with the Left node? Requires a state-based answer.
  • What is the voltage associated with the Right node?
  • What current path produces that result?

Stumbling Points

• Resist the temptation to “rescue” students after an incorrect switch selection. A shoot-through event, an extinguished polarity lamp, or a slowing fan together provide formative feedback that encourages students to reason from node voltages and current paths instead of memorizing switch combinations. If you encounter hesitation, ask students to determine:

  • Which node is connected to +24 VDC?
  • Which node is connected to RET?
  • Which node is positive relative to the other?
  • What is the complete current path?
  • Which lamp color should illuminate?

Learning Objectives

Students completing this exercise should be able to:

• Identify electrical nodes
• Measure voltage between two nodes
• Predict current paths
• Explain polarity as the voltage difference between two nodes
• Explain how an H-bridge reverses load polarity

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This article is part of the DigiKey Field Guide for Industrial Automation

Location: Understand It → Relay Logic → Fundamental
Learning Pathway: Industrial Automation Technician → Fundamentals
Difficulty: Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 14 Oct 2026

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Figure 1: Finished H-bridge in operation on the author’s workbench.

Required Materials

The required materials are outlined in an introductory article which identifies components for Outfitting an Industrial Automation Lab. The parts checklist for this portion of the project includes:

• 4 ea industrial normally open (green NO) switch blocks
• 2 ea industrial terminal blocks with integral diode (essentially a 1N4007 in a terminal block)
• 1 ea red-green panel lamp (not polarity sensitive)
• 1 ea 24 VDC fan
• Variety of pre-cut and ferruled wires
• 3D printed wire markers

In addition, a current-limited 24 VDC lab-style power supply is required such as the B&K 1670 shown in Figure 1.

Tech Tip: A current-limited power supply must be used as students will cause short circuits as part of the learning process. The technical name for this type of H-bridge short circuit is “shoot-through.” It occurs when two same-side pushbuttons are closed. With respect the Figure 2, the short occurs when either the PB1 to PB2 or the PB3 to PB4 switches are closed.

Bridge Construction

Construct the bridge as shown in Figure 1.

• Maintain the same physical orientation as shown on the Figure 2 wire diagram.
• Label every wire with its node designation using the 3D printed wire labels. In total, 11 wire labels are required.
• Pay close attention to the orientation of the diodes. The diode should “point” to the red (X1) input of the dual colored lamp. In this case the cathode (banded end) of the diode is connected directly to the red terminal of the panel lamp.

This assembly is an important part of the lab as it reinforces the relationship between the physical components and the wire diagram. You are encouraged to rebuild the circuit. Each person in the lab should be able to perform the work without assistance.

Tech Tip: When complete, every wire shall have a label. Note that some nodes have multiple wires. For example, the Left node has three wires including a connection between PB1 and PB2 as well as wires to the diodes. The distinction between wire and node labels is explored in this article.

Figure 2: Wire diagram for the H-bridge.

Tech Tip: Figure 2 presents the H-bridge in ladder form with the +24 VDC rail on the left and the return on the right rail. While this is technically correct, the H-bridge is normally rotated 90 degrees clockwise so that the positive rail is on the top. Note that the Left and Right node designators assume the conventional H-bridge configuration. Each vertical stacking of switches is called a bridge arm. In this case the bridge has two bridge arms. Three-phase systems will have three bridge arms.

Operate the H-Bridge

Connect the fan and the bridge circuit to the power supply as shown in Figure 1. Set the current-limit to about 0.5 A and the voltage to 24 VDC.

Control the panel lamp via the pushbuttons. There are 16 possible button press combinations. Identify:

• Combinations that produce a short circuit.
• The combination to activate the green section of the panel lamp.
• The combination to activate the red section of the panel lamp.

Reinforce Spatial Reasoning

Practice identifying the switches by disconnecting the bridge from the power supply, rolling the assembly into a ball (Figure 3), and then reorienting the bridge into either the ladder logic form or the conventional H-bridge form. This is harder than it sounds, especially if your partner twists the assembly by looping one side through the other like a pretzel.

The exercise is difficult, but worth the time to train your mind to spatially orient the switches with the wire diagram.

Figure 3: Circuit balled up and ready for the next student.

Measure the H-Bridge

Place a multimeter across the Left and Right nodes. Operate the bridge, observe the multimeter readings, and record your results. As part of this exercise, you will need to identify the valid switch states. Hint: A voltmeter lead connected to a “nothing node” does not provide a valid reading.

Explain the H-bridge

Explain the operation of the H-bridge to your instructor. It’s not enough to say “close PB1 and PB4 to activate the red lamp.” Instead,

• Describe the voltage associate with the Left and Right nodes.
• Describe the flow of current from the +24 VDC terminal of the power supply, through the load, and back to the return terminal.
• Describe the Left node voltage when both PB1 and PB2 are pressed.

About This Author

Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.

Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.

Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.