Electronics
How does an inductor work?
An inductor is a coil of wire. Current through it builds a magnetic field, and the field stores energy. An inductor passes steady current like any wire, but it fights any change in its current, and if you cut that current suddenly it makes a large voltage spike.
Steady is easy, change is hard
Think of a heavy flywheel. It takes effort to spin it up, and once it's spinning it wants to keep going. A coil treats current the same way.
So at steady DC, a coil makes no voltage of its own: the only voltage across it is I × R from its wire's resistance. A 400 Ω relay coil on 12 V carries 30 mA, just as a 400 Ω resistor would. That's the opposite of a capacitor, which blocks steady current.
Live experiment
Switch off a coil
A 10 H coil on a 12 V battery, with a flywheel that spins with its current. Close the switch and watch the current build up. Open it with nothing across the coil, then with a 100 Ω resistor across it, then a diode, and compare the spikes.
“Inductors” is lesson 4 of 5 in Capacitor Caves, Electronics World 3. Free with an account. Lessons unlock in order, so you’ll do the 18 before it first.
V = L × ΔI ÷ Δt
The size of an inductor is its inductance, L, in henries (H). The voltage it makes depends on how fast its current changes:
Switch on, and the current can't jump: it builds up with a time constant τ = L ÷ R. Switch off, and the change is sudden. A 0.2 H coil carrying 100 mA, cut off in 10 µs, makes
That's kickback. The current is still flowing when the switch opens, so it jumps the gap as a spark, or takes any other path it can find.
Taming the spike
Give the current somewhere to go. With a resistor across the coil, the current flows on through it at switch-off, and the spike across the coil is just
With 220 Ω across that coil, the spike is 0.1 A × 220 Ω = 22 V instead of 2,000 V (the switch sees that plus the 12 V supply). A bigger resistor means a bigger spike but a quicker fade.
A flyback diode across the coil goes further: the current coasts round the coil and diode at about 0.7 V and dies away gently, with no spike at all. That's why relay boards have a diode beside every coil.
The energy in the field is E = ½ × L × I², the coil's match for a capacitor's ½ × C × V².
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Inductors” is lesson 4 of 5 in Capacitor Caves, Electronics World 3. Free with an account. Lessons unlock in order, so you’ll do the 18 before it first.
Or start from the very first lesson, no account neededWork it out
- RL time constant calculatorτ, the final current, the rise step by step, and the time to reach a current.
- Capacitive and inductive reactance calculatorA capacitor's or a coil's ohms at any frequency, the current, and the impedance with R.
- Capacitive and inductive reactance practice problemsCapacitor and coil reactance, AC current and impedance, with the working.
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: current. An inductor fights any change in its current. A capacitor is the one that fights changes in voltage.
- Why not “voltage”? That's a capacitor, whose voltage can't jump. A coil's voltage can leap, which is where the switch-off spike comes from: it's the current that can't change at once.
- Why not “resistance”? A coil's resistance is just that of its wire, and it doesn't fight changes in it. What a coil opposes is any change in the current through it.
Show the answer
Answer: The energy in its magnetic field has to go somewhere. The field's energy keeps the current flowing for a moment. With no path, the current jumps the opening switch as a spark.
- Why not “It holds extra charge”? A coil doesn't store charge: that's a capacitor's job. It stores energy in its magnetic field, and that energy drives the spark.
- Why not “Its resistance is zero”? Low resistance doesn't make a spark. The coil sparks because its magnetic field stores energy that keeps the current going when the switch opens.
Show the answer
Answer: 63%. The same rule as RC charging. In each τ it closes 63% of the gap to its final value.
- Why not “37%”? 37% is what's still missing after one τ, not what's been reached. The current closes 63% of the gap to its final value in each τ.
- Why not “100%”? The current doesn't reach its final value in one τ. It gets about 63% of the way, and about 99% after 5τ.
“Inductors” is lesson 4 of 5 in Capacitor Caves, Electronics World 3. Free with an account. Lessons unlock in order, so you’ll do the 18 before it first.
Questions people ask
What does an inductor do?
It stores energy in a magnetic field and resists changes in the current through it. Steady current passes as if through plain wire; a changing current makes a voltage that pushes against the change. That makes inductors useful in filters, power converters and relays.
Does an inductor block DC?
No, that's a capacitor. At steady DC an inductor is just a length of wire, with that wire's resistance. It fights changes in current, so it tends to pass low frequencies and hold back high ones.
Why does a relay or motor need a flyback diode?
Its coil's current can't stop instantly. When the switch opens, the coil makes whatever voltage it takes to keep the current flowing, often hundreds of volts, which can destroy the transistor driving it. A diode across the coil gives the current a path to circulate at about 0.7 V instead.
What is a henry?
The unit of inductance, L. A coil of 1 H makes 1 V when its current changes at 1 A per second. Small coils are measured in µH and mH; a relay coil might be around 0.5 H.
Related topics
- How does a capacitor work?Two plates and a gap: how a capacitor stores charge and energy, and why it blocks steady DC.
- RC time constant explainedτ = R × C: how long a capacitor takes to charge through a resistor, and the 63% rule.
- MOSFET as a switch, explainedA gate voltage, not a current, opens the channel: threshold, R_DS(on) and the flyback diode.