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Lesson 2
A capacitor refuses to change its voltage suddenly; an inductor refuses to change its current. Switch off a relay coil and that refusal becomes a multi-kilovolt spike at your transistor. Commit to predictions first, then fire up the scope and see why.
Predict · 1 of 2 · the kick
An Omron G2RL relay's 12 V coil (2.47 H with its armature pulled in, 360 Ω) has been on long enough to carry its full 33.3 mA. The switch to ground opens, with nothing across the coil. The only thing at the switch node is 100 pF of switch and wiring capacitance. What is the peak voltage across the switch?
Result
Waiting on your call.
Make your mark, then lock it in. The answer, your error and the reason behind it land here, and the scope and every view below fire up.
Engine: in-browser MNA solver, 82,000 fixed steps for the cycle plus 2,000 for the zoom; switch and diode are piecewise-linear states, switched to one timestep. Checked against the closed forms by scripts/rl-check.mjs, and against SPICE: its numbers show once you're in the lab (they'd give the answers away).