Inductors & transformers
An inductor is a coil of wire that stores energy in a magnetic field. Where a capacitor resists changes in voltage, an inductor resists changes in current. Steady DC passes through almost unopposed, while fast-changing current meets an opposition that grows with frequency. Inductance is measured in henries (H); practical parts are in µH and mH.
That current-holding behavior has teeth. Interrupt the current through an inductor abruptly and the collapsing field drives the voltage as high as it needs to go to keep current flowing for a moment. The resulting spike can kill a transistor. This is why every relay coil or motor driven by a transistor gets a flyback diode across it, to give the current somewhere safe to go.
A transformer is two coils sharing a magnetic core. Changing current in the primary induces a voltage in the secondary, scaled by the turns ratio: twice the turns, twice the voltage. Since only changing current induces anything, transformers are AC-only devices. They also isolate the two sides electrically, which is where mains safety isolation comes from.
Key points
- Inductors store energy in a magnetic field and resist sudden changes in current.
- They pass DC easily but oppose fast-changing AC, the opposite of a capacitor.
- Switching off an inductive load creates a voltage spike. A flyback diode tames it.
- Transformers scale AC voltage by their turns ratio and isolate the two sides.
- A transformer does nothing with steady DC; induction needs changing current.
Practice
0 of 2 answered · Not startedWrong answers just let you try again, and hints are there if you want them. Answering every question first time, without hints, is what earns mastery.
- 1
What happens when an inductor saturates?
- 2
Why does an inductor datasheet quote a DCR figure?
