Electronics · Semester 1 — Analog · Lesson 6 · 55 min · build 2026.09.23-1644
The big idea
A capacitor stores charge — and filling or emptying it through a resistor takes time. Today, time becomes something your circuits control.
Kits closed, one minute: draw lesson 5's voltage divider from memory — the supply, two resistors, the tap, ground. Blanks and guesses are fine; this is a check, not a test. Then open the answer and fix your drawing.
Now build it from the schematic with R1 = R2 = 1 kΩ. Predict the tap voltage, then measure — black probe parked in the ground rail, red on the tap. Then one swap: R2 becomes 10 kΩ. Predict: does the tap go up or down? Measure. The bigger resistor takes the bigger share — that's the whole divider idea, and it comes back soon, twice. Keep the black probe parked: today it watches a voltage that moves.
Two metal plates, face to face, separated by an insulator. Charge can't cross the gap — so when the supply pushes, charge piles up on the plates instead. Think of it as a bucket for charge: this lesson fills it, empties it, and times both. From here on, the correct words: a capacitor charges and discharges, and how much it holds is its capacitance, measured in farads — ours are marked in µF (microfarads) and nF (nanofarads), because a whole farad is enormous.
Polarity — check it before every power-up
The can capacitors are electrolytic: they have a + and a − side, and they mean it. Wired backwards, an electrolytic slowly cooks itself and can burst its safety vent. House rule for this whole course: the striped leg always plugs toward ground — and you check the stripe before connecting power, every single build. The house rule works because ground is always our lowest voltage; the rule beneath it, for circuits beyond this course: striped leg to the lower-voltage side, and never more volts than the rating printed on the can. (The little ceramic disc has no polarity; either way round is fine.)
First contact: make light from a part with no battery in it. One capacitor, two paths — one to fill it, one to empty it — and only one is ever connected at a time.
The meter barely budged when the supply left — the charge was parked on the plates with nowhere to go. Then the touch: a flash, a fade, dark. Where did that light come from? From the energy you parked on the plates. The supply crowded charge onto the capacitor; when you opened a path, that charge flowed back around the loop, and the LED turned the stored energy into light and a little heat. The charge itself wasn't eaten — it moved. The energy is what got spent. That's a capacitor's whole career: store now, spend later.
Predict, then look
The LED has gone dark, still plugged into node A. What does the meter read there now? Write a number before you look.
The light dies, the charge doesn't
The meter still reads well over a volt. Write your number down — the table next to you will have a slightly different one, and both are right: no two LEDs give up at exactly the same point. The capacitor isn't empty; its voltage has just sunk to where the red LED passes too little current for your eyes to catch. Dark doesn't mean drained: one more reason the meter, not your eyes, gets the last word.
The reset. To really empty a capacitor, give the charge an easy way home: pull the LED chain out, then plug a bare 220 Ω — no LED — from node A to the ground rail, and watch the meter slide under 0.1 V. Two seconds does it. With no LED in the path, nothing stalls the drain. Then take the 220 back out. This is the reset: it starts every timed run today, and every capacitor goes into the box this way.
Charging through 220 Ω was too fast to see. Swap in a big resistor and the fill goes slow motion — slow enough to time:
Watch the numbers as it fills: fast at first, then slower and slower — the fuller it gets, the weaker the push across the resistor, the slower the pour. It creeps toward 5 V and never quite arrives. (Why 3.2 V is the fair finish line is a secret the math box spills.)
Predict, then race
Your 10 kΩ / 1000 µF time is the baseline. Before each row, predict in words first — faster or slower, and about how many times? — then a number if you have one (a word is a fine prediction). More resistance is a narrower pour; more capacitance is a bigger bucket. Reset before every run.
| Fill through… | Capacitor | Predict (s) | Measured (s) |
|---|---|---|---|
| 10 kΩ (baseline) | 1000 µF | — | |
| 1 kΩ | 1000 µF | ||
| 10 kΩ | both 1000 µF, side by side |
Side by side means parallel: both + legs in node A's row, both stripes to ground — two stripe checks.
The pattern: the clock follows both parts. A tenth of the resistance fills about ten times faster — so fast the meter can barely keep up, and "about a second" is a perfectly good measurement. Two buckets side by side hold twice as much and take about twice as long. Resistance and capacitance are the two levers on every timer in this course.
The fill circuit one more time — but now the schematic reads R = ▢, C = ▢. Pick them from your kit so the fill from 0 to 3.2 V takes as close to five seconds as you can get. Resistors in series and capacitors side by side are legal, and so is borrowing from a neighbor. Predict from your race table, reset before every attempt, build, time it, adjust. Closest table wins.
Go further — for the fast and the curious
1. The flash that ends with the power still on. Wire supply → 220 Ω → LED → capacitor → ground: the capacitor in the loop, stripe to ground. Predict: steady light, or something else? Then work out why — the answer is one of the most useful sentences about capacitors: a capacitor passes current only while its voltage is changing. (Reset it afterward.) 2. Borrow a neighbor's 10 kΩ and refill through 10 kΩ + 10 kΩ in series — predict the time first. Twice the squeeze, twice the…? 3. The pocket flashlight: charge a 1000 µF, pull it gently from the board, walk it across the room, and light a neighbor's LED. 4. Try the tiny 100 nF disc in the fill circuit. Nothing visible? It's 10,000× smaller than the big can — some buckets are thimbles. It'll get its real job beside a chip, soon. 5. Make alternating current with your bare hands: plug two LEDs in anti-parallel (second one reversed) behind one 220 Ω, and feed the pair through two jumpers from the rails. Now swap which jumper goes to + and which to ground, back and forth, steadily. The LEDs take turns — you're generating AC at about 1 Hz. One part is banned from this game: the electrolytics — a voltage that keeps reversing is exactly what the cans can't survive. (How capacitors and AC actually get along is on the How AC Works page, Under the hood.)
The math, for the curious — always optional
The secret of 3.2 V: multiply resistance by capacitance and you get seconds:
— and that product is exactly how long the fill takes to reach 63% of the supply. 63% of 5 V ≈ 3.2 V: your stopwatch was measuring τ all along. (Baseline landed at 8 or 12 seconds instead of 10? Blame the can, not your clock — electrolytics commonly run ±20% from their printed value.) The same product predicts your whole race table — 1 kΩ × 1000 µF = 1 s, 10 kΩ × 2000 µF = 20 s — so check it against your measurements, then read the τ = RC sheet on the math shelf for the whole story.
On your card: 1. draw the store-and-spend circuit from memory — both paths, stripe side marked. 2. One sentence: where did the energy for the light come from after the supply was gone? Hand it in at the door.
Then the teardown: the reset on every capacitor — bare 220 Ω from its + row to ground until the meter calls it empty. Parts go back in the box asleep, and the board goes back to bare rails.