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Common Ground Loop

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Description

A 5 V controller and a 12 V motor driver each have their own supply. Predict what the driver's input reads with and without a shared ground wire, and watch the signal's current loop appear and vanish. Used in Chapter 3.

The signal current in a real circuit is tiny, so its dot speed is not to scale.

Lesson Plan

Grade level: Middle school and up (age 12+). Time: 10 to 15 minutes.

  1. Predict first. Is the ground wire connected? What does the controller send? What will the driver read?
  2. Follow the dots. With a ground wire, signal dots go out on the signal wire and come back on the ground wire.
  3. Break the loop. Remove the ground wire. The signal has no way home, and the input flickers between HIGH and LOW.
  4. Remember. Join the grounds, never the positive rails of two different supplies.

The picture is a schematic, drawn with the shared circuit-lib.js helpers. Orange dots show conventional current: they leave the + terminal of the supply, and their speed is proportional to the current.

Specification

The learning objective, evidence of mastery and content for this MicroSim are specified in Chapter 3: Electricity, Power, and Safety Basics.