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RL Energizing / De-energizing Circuit MicroSim

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Description

This MicroSim visualizes the RL transient process — what happens when an inductor energizes through a resistor after a switch is closed (Position A), and how the inductor de-energizes through a second resistor when the switch moves to Position B.

Key Concepts Demonstrated:

Quantity Formula Behavior
Inductor Current (energize) \(I_L(t) = \frac{V_s}{R_1}(1 - e^{-t/\tau})\) Rises exponentially toward \(V_s/R_1\)
Inductor Voltage (energize) \(V_L(t) = V_s e^{-t/\tau}\) Falls exponentially toward zero
Inductor Current (de-energize) \(I_L(t) = I_0 e^{-t/\tau}\) Decays exponentially toward zero
Inductor Voltage (de-energize) \(V_L(t) = -I_0 R_2 e^{-t/\tau}\) Negative — flyback effect
Time Constant \(\tau = L/R\) Time to reach 63.2% of final current

Note on Flyback Voltage: During de-energizing, the inductor voltage is negative (shown in orange on the VL graph). If \(R_2 > R_1\), the flyback voltage can exceed \(V_s\) — this is the dangerous inductive kick that can damage switches and semiconductors.

Interactive Features:

  • Source Voltage Slider: Adjust \(V_s\) from 1V to 20V
  • R₁ Slider: Energizing resistance from 10Ω to 1000Ω
  • R₂ Slider: De-energizing resistance from 10Ω to 1000Ω
  • Inductance Slider: Adjust L from 10mH to 500mH
  • Switch Button: Flip between energizing (A) and de-energizing (B)
  • Animated Electrons: Flow speed proportional to current
  • Magnetic Field Glow: Blue glow inside coil grows with inductor current

How to Use

  1. Observe initial state: switch at A, inductor with no current
  2. Click Start to begin energizing
  3. Watch current rise and inductor voltage fall (mirror image of RC charging)
  4. Flip the switch to B to de-energize through R₂
  5. Observe the negative flyback voltage on the VL graph
  6. Try setting R₂ > R₁ to see how the flyback voltage exceeds Vs

Technical Notes

The simulation uses the first-order RL step response:

  1. Energizing: \(I_L(t) = \frac{V_s}{R_1}(1 - e^{-t/\tau_1})\) — current asymptotically approaches \(V_s/R_1\)
  2. De-energizing: \(I_L(t) = I_0 e^{-t/\tau_2}\) — current decays from initial value \(I_0\)
  3. Flyback voltage: \(V_L = -I_0 R_2 e^{-t/\tau_2}\) — can exceed supply voltage when \(R_2 > R_1\)
  4. Duality with RC: IL in RL ↔ Vc in RC; VL in RL ↔ I in RC

References