Capacitors

A capacitor stores electrical charge. Where a resistor dissipates energy as heat, a capacitor stores energy in an electric field and gives it back. That storing behaviour makes capacitors useful for smoothing, filtering, timing, and decoupling.

What "storing charge" does

A capacitor charges up when voltage is applied and discharges when the voltage drops. This gives it three practical behaviours:

1. It resists voltage changes. A capacitor "wants" to hold its voltage. When the supply voltage dips momentarily, the capacitor discharges to fill the gap; when it rises, the capacitor charges to absorb the surge. This smoothing is one of its main jobs.

2. It blocks DC, passes AC. A steady (DC) voltage charges the capacitor once and then nothing flows. A changing (AC) voltage continuously charges and discharges it, so current keeps flowing. This makes capacitors useful for filtering — separating steady from changing signals.

3. It stores energy for later. A charged capacitor holds energy that can be released quickly. Used in power smoothing, energy buffering, and timing.

The unit: farads

Capacitance is measured in farads (F) — but a full farad is enormous. Real capacitors are:

• Microfarads (µF): 10⁻⁶ F. Electrolytics for bulk smoothing (1µF to thousands of µF).
• Nanofarads (nF): 10⁻⁹ F. Ceramics for decoupling + filtering (1nF to ~1µF).
• Picofarads (pF): 10⁻¹² F. Small ceramics for high-frequency, RF, timing (1pF to ~1nF).

(L0's SI prefixes lesson covered µ, n, p — this is where you use them.)

Capacitor jobs in real circuits

Decoupling (the most common). Every microcontroller, every IC, has a small ceramic capacitor (typically 100nF) right next to its power pins. When the chip suddenly draws current (switching transistors inside), it needs that current instantly — faster than the power supply can deliver through the traces. The decoupling capacitor sits right there as a local energy reservoir, supplying the burst and keeping the power rail steady. Without it, the chip's power dips on every switch, causing glitches and resets.

Smoothing (power supplies). After rectifying AC to DC, the voltage is lumpy. A large electrolytic capacitor smooths it by charging on the peaks and discharging in the valleys, flattening the ripple into something usable.

Filtering (signals). Combined with resistors (RC circuits — Lesson 3), capacitors filter signals — passing some frequencies, blocking others. A low-pass filter smooths a noisy sensor reading; a high-pass filter removes a DC offset.

Timing. A resistor charging a capacitor takes a predictable time (the RC time constant). This sets delays, oscillation frequencies, and debounce timing.

The big practical warning: polarity

Some capacitors are polarised — they MUST be connected the right way round, and reversing them can destroy them, sometimes violently (an electrolytic can burst or vent). Others are non-polarised — they work either way.

This is the single most important practical fact about capacitors, and the next slide covers it in detail along with the capacitor types.

What you need to know about any capacitor

For any capacitor in a circuit:

1. Its capacitance (in F/µF/nF/pF) — how much charge it stores.
2. Its voltage rating — the maximum voltage it can handle (exceed it and it fails).
3. Whether it's polarised — and if so, which way round it goes.

The next slide covers the capacitor types (ceramic, electrolytic, film) — and which to use for which job.

Three capacitor types cover almost everything you'll use: ceramic, electrolytic, and film. Each suits different jobs, and the polarity difference between them is the most important practical distinction.

Ceramic capacitors

What they are: small, cheap, non-polarised capacitors. Usually small values (pF to ~1µF).

Use them for:

• Decoupling — the 100nF caps next to every IC's power pins.
• High-frequency filtering — ceramics respond fast.
• Timing in RC circuits.

Key traits:

• Non-polarised — connect either way round.
• Small physical size.
• Fast response (good at high frequency).
• Stable and reliable.

The 100nF ceramic decoupling cap is probably the single most-used capacitor in all of electronics. You'll place them constantly.

Electrolytic capacitors

What they are: larger, polarised capacitors with high capacitance (1µF to thousands of µF).

Use them for:

• Power smoothing — the big caps that flatten supply ripple.
• Bulk energy storage — buffering large current demands.
• Low-frequency filtering.

Key traits:

• POLARISED — must be connected correctly. The negative (cathode) lead is marked with a stripe, and is usually the shorter lead. Reverse it and it can fail, sometimes bursting or venting.
• High capacitance per size.
• Slower response than ceramics (less good at high frequency — which is why you pair a big electrolytic with a small ceramic).
• A voltage rating that must exceed your circuit voltage with margin.

The polarity warning, restated: an electrolytic installed backwards is a real hazard. At low voltage it might just fail quietly; at higher voltage it can heat up and burst. ALWAYS check the stripe (cathode) goes to the more-negative side of the circuit. This is the most common destructive beginner mistake.

Film capacitors

What they are: non-polarised capacitors, often medium values, known for stability and precision.

Use them for:

• Precision filtering + timing where stability matters.
• Audio circuits (low distortion).
• Higher-voltage applications.

Key traits:

• Non-polarised.
• Very stable + accurate.
• Larger than ceramics for the same value.
• Less common in beginner digital circuits; more in analog/audio/precision work.

Choosing the type for the job

A common pattern: a big electrolytic + a small ceramic in parallel on a power rail. The electrolytic handles bulk smoothing (slow, large); the ceramic handles fast transients (quick, small). Together they keep the rail clean across all frequencies.

The voltage rating

Every capacitor has a maximum voltage rating. Exceed it and the dielectric breaks down — the capacitor fails, possibly short-circuiting.

Rule: pick a voltage rating comfortably above your circuit voltage. For a 5V circuit, a 16V or 25V capacitor is sensible (headroom for transients). Running a 6.3V-rated capacitor on a 5V rail leaves almost no margin — a small surge could exceed it.

For electrolytics especially, voltage derating extends life: running a capacitor well below its rated voltage makes it last much longer than running it near the limit.

The non-polarised vs polarised summary

The single most important type distinction:

• Ceramic + film = non-polarised. Connect either way. Forgiving.
• Electrolytic = polarised. Connect correctly (stripe/cathode to negative) or risk destroying it.

When you pick up a capacitor, the first question is "is it polarised?" If it has a stripe and a long/short lead, it's an electrolytic — mind the polarity. If it's a small disc or block with no polarity marking, it's a ceramic — connect freely.

The next slide covers reading capacitor values from their markings — and reinforces the polarity warning one more time.

Capacitor markings are less standardized than resistor colour bands, but a few conventions cover most cases. And the polarity marking — for electrolytics — is the one you absolutely must read correctly.

Reading ceramic capacitor codes

Small ceramic capacitors use a 3-digit code, just like SMD resistors — but the result is in picofarads (pF):

• First two digits = significant figures.
• Third digit = number of zeros.
• Result is in pF.

Examples:

• 104 → 10 + 4 zeros = 100,000 pF = 100nF (0.1µF) — the ubiquitous decoupling cap.
• 103 → 10 + 3 zeros = 10,000 pF = 10nF.
• 220 → 22 + 0 zeros = 22 pF.
• 471 → 47 + 1 zero = 470 pF = 0.47nF.

The trick: it's pF, so 104 is NOT 104 of anything — it's 10 followed by 4 zeros = 100,000pF = 100nF. Converting pF → nF → µF (each step is ÷1000) is where people slip; practice the conversions.

Reading electrolytic capacitor markings

Electrolytics are usually large enough to print the value directly:

• 100µF 25V — 100 microfarads, rated 25 volts.
• 470µF 16V — 470 microfarads, rated 16 volts.

Both the capacitance AND the voltage rating are printed. Always note both — the voltage rating must exceed your circuit voltage.

Reading film capacitors

Film capacitors often print the value directly (e.g., 0.1µF or 100nF) or use a code similar to ceramics. Some use a letter for tolerance. They're generally legible enough to read the value off the body.

The polarity marking — read it every time

For electrolytic (polarised) capacitors, the polarity marking is critical:

The stripe. A vertical stripe down one side of the can marks the negative (cathode) lead. The stripe usually has minus signs (−) printed on it.

The lead length. On a fresh radial electrolytic, the longer lead is positive (anode), the shorter is negative (cathode). (Once leads are trimmed, this clue is gone — rely on the stripe.)

On a PCB: the silkscreen usually marks the positive pad (a +) or shades the negative half. Match the capacitor's stripe (negative) to the unshaded/un-plus side.

The rule: the stripe (cathode, negative) goes to the MORE NEGATIVE side of the circuit. In a simple power-smoothing setup, that means the stripe goes toward ground.

Why polarity is worth this much attention

Reversing an electrolytic is the most common destructive mistake in beginner electronics — and unlike a wrong resistor value (which just makes the circuit behave oddly), a reversed electrolytic can fail violently:

1. At low voltage: it may just leak current and fail quietly, or get warm.
2. At higher voltage: the electrolyte heats, builds pressure, and the can can burst or vent — hot electrolyte, a loud pop, and a ruined component (and possibly a damaged circuit + a startle).

So before powering any circuit with electrolytics: check every one. Stripe to negative. Every time. It takes two seconds and prevents the most common bench accident.

The capacitor identification skill

Putting it together, identifying a capacitor means reading:

1. The type — ceramic (small disc/block, non-polarised), electrolytic (can with a stripe, polarised), or film.
2. The value — from the 3-digit code (ceramics, in pF) or the printed value (electrolytics/film).
3. The voltage rating — printed on electrolytics, must exceed circuit voltage.
4. The polarity — for electrolytics, find the stripe (negative).

The P-Check for this module includes a capacitor with a readable marking — demonstrating you can identify its value. Combined with the resistor identification, it's the practical "read passives by their markings" skill that L1 certifies.

The next lesson covers inductors (briefly — they're less common in beginner circuits) and the passive circuits that combine resistors and capacitors into useful building blocks: voltage dividers, RC filters, and pull-up/pull-down resistors.