Capacitors — Types, Ratings, and Identification

Section Overview

Having learned to read resistors, you'll now do the same for capacitors — and capacitors add two twists resistors don't have: they come in distinct physical types you must tell apart, and two of those types are polarized, meaning they can fail violently if installed backwards. This section teaches you to recognize the common types (ceramic, electrolytic, film, tantalum), to read capacitor markings — including the three-digit ceramic capacitor code, whose result is in picofarads — and to understand the ratings that matter: capacitance, the all-important voltage rating, tolerance, and temperature coefficient. It also covers polarity: which capacitors are polarized, how to find the negative lead, and why getting it right is a safety matter, not just a functional one.

Why This Matters

Capacitors are, after resistors, the components you'll identify and replace most often — and Chapter 3 already showed they're the single most common failure item in electronics. But replacing one correctly demands more than matching the capacitance. Fit an electrolytic with too low a voltage rating and it will overheat and rupture; install a polarized capacitor backwards and it can fail short, vent, or even burst. These aren't rare edge cases — they're the everyday hazards of capacitor work. Learning to read a capacitor's type, value, voltage rating, and polarity at a glance is what lets you choose a safe, correct replacement and understand the circuit you're repairing. This section turns the failure knowledge from Chapter 3 into the practical identification skill that repair depends on.

Required Prerequisites

What Is a Capacitor? — construction, the farad and its sub-units, and the stored-charge hazard; this section identifies those parts on a bench.
Capacitor and Inductor Failure Modes — why capacitors fail; here you learn to identify a replacement that won't repeat the failure.
Resistors — Types, Values, and Markings — the identification approach and the idea of tolerance and a numeric code, which the ceramic code parallels (with a crucial difference: the result is in picofarads).

Recommended Consumables

No consumables required. An assortment of capacitors — or a few pulled from a scrap board — makes ideal, reusable practice material.

Recommended Practice Hardware

A mix of capacitor types: ceramic discs, aluminum electrolytics (the cans with a stripe), a film capacitor or two, and a tantalum if you have one
A magnifier and good light — ceramic markings are tiny, and the polarity stripe and lead-length cues need a clear look
A multimeter with a capacitance range, if available, to compare a decoded value against a measurement (optional; basic meters may lack it)

Work only with loose or de-energized, discharged capacitors. Reiterating Chapter 3: a large capacitor can hold a dangerous charge — treat it as charged until you know otherwise.

Real-World Applications

Open any device and you'll see the capacitor types side by side: dozens of tiny ceramic chips decoupling the ICs, taller aluminum electrolytics smoothing the power rails, a film capacitor in a timing or audio path, perhaps a bright tantalum bead near a sensitive supply. A repair technician replacing a bulged electrolytic must read its capacitance and voltage rating and fit a replacement that matches the µF and at least the voltage, oriented the right way round. Choosing a decoupling capacitor, reading a value off a schematic, or identifying why a board failed all start with recognizing the type and reading the markings. And the polarity lesson is a safety staple: reversed electrolytics and tantalums are a classic cause of dramatic, smoky failures.

Common Challenges

Reading the ceramic code as if it were resistor ohms. The three-digit code looks just like a resistor's, but the answer is in picofarads, not ohms — "104" is 100,000 pF (100 nF, 0.1 µF), not 104 of anything. The unit is the trap.
Matching only capacitance, ignoring voltage. A replacement must also meet or exceed the original's voltage rating. The right µF at too low a voltage is a failure waiting to happen.
Forgetting polarity. Electrolytics and tantalums have a positive and a negative lead and must go in the right way; ceramic and film don't. Installing a polarized cap backwards is a violent-failure risk, not a minor slip.

Safety Notes

Risk Level: Low. This section is identifying and reading de-energized, discharged capacitors — but capacitors carry two real hazards you must respect even while learning to read them.

Professional Tips Before Starting

Identify the type first — it tells you most of what you need. Ceramic and film are non-polarized and low-value; electrolytic and tantalum are polarized and higher-value. Type narrows down markings, polarity, and typical use before you read a single number.
Always read the voltage rating, not just the capacitance. On electrolytics it's printed right there ("25V"); treat it as half of the specification — as important as the capacitance itself. A replacement's voltage must be at least the original's — going higher is fine, going lower is not.
On a polarized capacitor, find the negative lead before you do anything else: the stripe on an aluminum electrolytic marks the negative side, and on an unshortened part the longer lead is positive. Tantalums usually mark the positive end instead — read the part, don't assume.

Identifying and Reading Capacitors

The Common Types

Four types cover almost everything you'll meet, and telling them apart is the first skill:

Ceramic — small discs or tiny surface-mount chips, inexpensive, non-polarized, and by far the most common. Used for decoupling and signal work, usually in the picofarad-to-microfarad range. They carry the three-digit code below (or none, on the smallest chips).
Electrolytic (aluminum) — the familiar cylindrical can with a stripe down one side, polarized, offering large capacitance (microfarads to thousands of microfarads). These are Chapter 3's failure star — they dry out, lose capacitance, gain ESR, and bulge. They print their value and voltage directly.
Film — plastic-film capacitors, often boxy, non-polarized, prized for stability and low loss; common in timing, filtering, and audio signal paths.
Tantalum — small, often bright-colored beads or chips with high capacitance for their size, polarized, and notable for failing shorted and sometimes dramatically (heat, smoke) if over-voltaged or reversed. They demand respect for polarity and voltage rating.

Reading the Markings

How a capacitor is marked depends on its size. Large capacitors — electrolytics especially — simply print the value and voltage: a can reading "470µF 25V" is 470 microfarads rated for 25 volts, no decoding needed.

Small ceramics are too small for that, so they use a three-digit code — the ceramic capacitor code — that works like a resistor's SMD code, with one essential difference: the result is in picofarads. The first two digits are significant, the third is the number of zeros (a power-of-ten multiplier), and the answer is pF. So:

"104" = 10 followed by 4 zeros = 100,000 pF = 100 nF = 0.1 µF (the ubiquitous decoupling capacitor).
"473" = 47 followed by 3 zeros = 47,000 pF = 47 nF.
"102" = 10 followed by 2 zeros = 1000 pF = 1 nF.
"221" = 22 followed by 1 zero = 220 pF.

A bare number with no multiplier digit is simply that many picofarads — "22" means 22 pF. You'll also see letter notation borrowed from resistors: an "n" or "µ" or an "R" acting as a decimal point (for instance "4n7" = 4.7 nF). Keep the unit ladder handy: 1 µF = 1000 nF = 1,000,000 pF. The single most common mistake is reading "104" as though it were a resistor value or a literal number — remember it lands in picofarads, then convert up.

The Ratings That Matter

A capacitor has several ratings, and a replacement must respect them:

Capacitance — the value in farads (µF/nF/pF), read from the marking or code.
Voltage rating — the maximum voltage the capacitor can safely withstand. This is critical: it must meet or exceed the voltage in the circuit, and a replacement's voltage rating must be equal to or greater than the original's. Under-rating it causes overheating and, for electrolytics and tantalums, rupture. A higher voltage rating is always safe (only physically larger).
Tolerance — the allowed difference from the marked capacitance (the same idea as a resistor's tolerance from Section 4.1), but usually looser — often ±10% or ±20%, since precise capacitance matters less in most uses.
Temperature coefficient — how much the capacitance drifts with temperature, mainly for ceramics. Codes name it: C0G/NP0 is very stable (for timing and precision), X7R is a general-purpose middle ground, and Y5V offers high capacitance but drifts a lot. You don't need the detail now — just know a ceramic's code hints at how stable it is.

Polarity

The dividing line that resistors never had: some capacitors are polarized. A polarized capacitor — every electrolytic and tantalum — has a positive and a negative lead and must be installed the right way round; ceramic and film capacitors are non-polarized and can go either way. Identifying the negative (or positive) lead is essential:

On an aluminum electrolytic, a printed stripe down the can marks the negative lead, and on a part with its original leads the longer lead is positive.
On a tantalum, the marking usually indicates the positive end instead — so read the specific part rather than assuming.

Get it wrong and the consequence is not subtle: a reversed electrolytic overheats and can vent or burst, and a reversed or over-voltaged tantalum can fail short and burn. Matching polarity is part of every polarized-capacitor replacement, alongside matching the capacitance and meeting the voltage rating.

Common Mistakes

Decoding the ceramic code into the wrong unit. "104" is 100,000 pF = 100 nF = 0.1 µF — picofarads first, then convert. It is not ohms and not a literal number.
Fitting a lower voltage rating. A replacement's voltage rating must be at least the original's; lower invites overheating and rupture. Higher is fine.
Installing a polarized capacitor backwards. The stripe on an electrolytic is the negative lead. Reversing a polarized cap risks venting or fire — it is a safety error.
Assuming every capacitor is polarized (or none is). Electrolytic and tantalum are polarized; ceramic and film are not. Know which you're holding.

Troubleshooting Guidance

Capacitor identification is the front end of capacitor repair. When you find a failed capacitor — a bulged electrolytic from Section 3.7, say — read its type, capacitance, and voltage rating so you can fit a correct replacement: same capacitance, a voltage rating at least as high, the same or a suitable type, and, if polarized, the right orientation. If the printing on a failed part is unreadable (charred or vented), get the value from an identical capacitor on the board, from the schematic, or from a matching unit, exactly as with resistors. Watch for the tell-tale signs of the type when diagnosing, too: electrolytics bulge and leak, tantalums fail short (often with scorching), and a reversed-polarity installation shows as an overheated or burst part. And never skip the voltage rating — a board that keeps destroying a capacitor may be feeding it more voltage than its rating allows, or the replacement may have been under-rated. Reading the part correctly is what turns "replace the capacitor" into replacing it with the right one.

Verification & Testing Methods

Check your understanding before moving on:

[ ] Identify the four common capacitor types by sight and state which two are polarized.
[ ] Decode a three-digit ceramic code (for example "104") into picofarads and convert to nF and µF.
[ ] State the rule for a replacement's voltage rating and why under-rating is dangerous.
[ ] Identify the negative lead of an aluminum electrolytic and explain what happens if it is installed backwards.

Then try the practice exercises below — identifying and decoding loose, discharged capacitors.

Practice Exercises

Identify the types (10 minutes, with real parts or images). Sort a mixed handful of capacitors into ceramic, electrolytic, film, and tantalum. For each, state whether it is polarized and where you'd look for its value and (if present) voltage rating.
Decode the ceramic codes (5 minutes, pen and paper). Give the value in pF, then nF or µF, for: (a) "472"; (b) "224"; (c) "471"; and state what a bare "33" means.
Choose a replacement (10 minutes, reasoning). A bulged electrolytic reads "1000µF 16V." You have on hand 1000µF parts rated 10V, 16V, and 35V. Which are acceptable and which is not, and why? State every rating you'd match, including orientation.
Find the negative lead (5 minutes, with a real electrolytic). On an aluminum electrolytic, locate the polarity stripe and identify the negative lead, then the positive. Explain what would happen if you soldered it in reversed.

These core ideas — the types and which are polarized, the picofarad ceramic code, the voltage-rating rule, and polarity — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

The four common types are ceramic (small, non-polarized, most common), electrolytic (large µF, polarized, striped can, the usual failure item), film (stable, non-polarized), and tantalum (small, high µF, polarized, fails shorted) — knowing the type tells you most of what to check.
The three-digit ceramic capacitor code works like a resistor's but yields picofarads: "104" = 100,000 pF = 100 nF = 0.1 µF; keep the ladder 1 µF = 1000 nF = 1,000,000 pF.
The voltage rating is critical and must meet or exceed the circuit voltage; a replacement's rating must be equal to or greater than the original's, never lower — higher is safe.
Capacitor tolerance is looser than a resistor's (often ±10–20%), and a ceramic's temperature coefficient (C0G/NP0 stable, X7R general, Y5V drifty) hints at its stability.
A polarized capacitor — electrolytic and tantalum — must be installed the right way: the stripe marks the negative lead on an electrolytic (longer lead positive); reversing it risks venting, rupture, or fire.
Choosing a replacement means matching capacitance, meeting or exceeding the voltage rating, respecting polarity, and using a suitable type.

Skills Learned

You can now identify the common capacitor types by sight and state which are polarized.
You can now decode a three-digit ceramic code into picofarads and convert to nF or µF.
You can now state the ratings needed for a replacement and apply the voltage-rating rule.
You can now identify the negative lead of a polarized capacitor and explain the danger of reversing it.
You can now turn Chapter 3's capacitor failure knowledge into the identification skill needed to fit a correct, safe replacement.

Glossary Additions

voltage rating — the maximum voltage a capacitor can safely withstand; it must meet or exceed the circuit voltage, and a replacement's voltage rating must be equal to or greater than the original's — under-rating it causes overheating and, in electrolytics and tantalums, rupture.
polarized capacitor — a capacitor with a positive and a negative lead that must be installed the correct way round (electrolytic and tantalum types); the electrolytic's stripe marks the negative lead, and reversing a polarized capacitor risks venting, rupture, or fire. Ceramic and film capacitors are non-polarized.
ceramic capacitor code — the three-digit marking on a ceramic capacitor: two significant digits and a zeros-count multiplier, with the result in picofarads (e.g. "104" = 10 × 10⁴ = 100,000 pF = 100 nF = 0.1 µF); a bare number is that many picofarads.
temperature coefficient — a rating (mainly for ceramics) of how much a capacitor's value drifts with temperature, named by codes such as C0G/NP0 (very stable), X7R (general-purpose), and Y5V (high capacitance but large drift).

Suggested Next Sections

Must read next:

Inductors and Transformers — the identification skills continue with inductors and transformers: recognizing the types, reading their markings, and understanding what a transformer's ratings mean.

Recommended:

Capacitor and Inductor Failure Modes — the failures you're now equipped to identify and correctly replace.
Resistors — Types, Values, and Markings — the sibling identification section; compare the resistor and ceramic codes and their different units.