Turtleand Energy

Rung 8 · See fields and systems

Give inductor current a safe path

Watch an inductor oppose sudden current changes, then add a flyback diode.

intermediate35 minutes0 of 4 stages explored
Open blank FalstadReference options
A five volt switched resistor and inductor loop with a reverse-biased flyback diode across the inductive branch.
5 V source -> switch -> 100 millihenry inductor -> 100 ohm resistor, with flyback diode across the inductor branch

Knowledge this exercise builds on

Lesson 8: Conductors, Insulators, Grounding, and SafetyLesson 9: Generators and Electromagnetic Induction

Practice loop

Try each step before revealing its answer.

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ComponentsSee the parts and values before you start.
  • One 5 V DC voltage source
  • One switch
  • One 100 mH inductor
  • One 100 Ω resistor
  • One diode for the second run
  • Current and switch-voltage scopes
PredictCommit to an expected result before running the circuit.

Predict whether inductor current can stop instantly when the switch opens. Decide what voltage the inductor must create if no continuing path exists.

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Inductor current cannot stop instantly. Before opening, the long-term current approaches:

I = 5 V / 100 Ω = 50 mA

When the switch opens without another path, the inductor reverses its terminal voltage and can create a large transient to keep current moving.

BuildWire the circuit yourself from a blank canvas.

Build and run the series RL loop first. Close the switch long enough for current to rise, then open it. Add a reverse-biased diode across the inductive branch for the second run.

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The first path is a single series source, inductor, resistor, and switch loop. Place the diode across the inductive load path so it is reverse-biased while the source powers the loop and forward-biased only when the inductor reverses its terminal voltage.

MeasureSee what to inspect, compare, and change.

Compare inductor current and switch voltage when opening the switch without the diode and after adding it. Record the transient size and decay shape.

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Without the diode, switch voltage shows a large, sharp transient. With the diode, current circulates through the added path and decays more gently from about 50 mA. The series time constant is approximately:

τ = L / R = 0.1 H / 100 Ω = 1 ms
ExplainConnect the observation back to your model.

Explain why the diode is normally off, why it conducts after the switch opens, and where the inductor’s stored magnetic energy goes.

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The source polarity reverse-biases the diode during normal operation. Opening the switch makes the inductor reverse its terminal voltage to preserve current, which forward-biases the diode. The flyback path does not erase energy. It lets current decay while resistance dissipates the stored magnetic-field energy as heat.

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Safety boundary

This is a low-voltage educational model.

Do not translate it into mains construction, household wiring, battery-pack work, or real protection settings. Real electrical systems need rated components, proper protection, test equipment, and qualified safety judgment.