Inductors and Transformers

Section Overview

The identification skills continue with the wound components — inductors and transformers — building on the inductor theory from Chapter 3. First you'll learn to recognize the common inductor forms (fixed inductors and chokes, ferrite beads, and the bulky power inductors of switch-mode supplies) and read their markings, including a three-digit code that works like a resistor's but yields microhenries. Then you'll meet the transformer: two or more windings sharing a magnetic core, coupled by the mutual induction you learned in Chapter 3, whose turns ratio sets the output voltage and steps it up or down. Finally you'll learn what winding isolation means — and why it makes the primary side of a mains transformer a genuinely dangerous, live part.

Why This Matters

Inductors and transformers are where identification meets real electrical danger. An inductor is usually a benign coil, but a transformer in mains equipment has a primary side sitting at full line voltage — one of the most hazardous parts on the board. Knowing how to identify a transformer, tell its primary from its secondary, and understand that its two sides are electrically isolated is both a repair skill and a safety skill. On the diagnostic side, transformers and inductors fail in the ways Chapter 3 described — most often an open winding — and a dead device with an open transformer primary is a common, findable fault. And the turns ratio explains the single most important thing a transformer does: convert one AC voltage to another. This section gives you the identification and the respect these components demand.

Required Prerequisites

What Is an Inductor? — the coil, inductance and the henry, and mutual induction; this section identifies inductors and extends the idea to coupled windings.
Inductor Behavior in Circuits — how inductors behave and the choke role, which the power inductors and beads here perform.
Capacitor and Inductor Failure Modes — the open-winding and shorted-turns failures you'll test for here.
Resistors — Types, Values, and Markings — the identification approach and the three-digit code, which the inductor code mirrors (in microhenries).

Recommended Consumables

No consumables required. Salvaged inductors, ferrite beads, and a small transformer or two make ideal, reusable practice material.

Recommended Practice Hardware

A few inductors (molded and bobbin types), a ferrite bead, and a small transformer or two — a low-voltage wall adapter's transformer or a salvaged signal transformer is ideal and safe
A magnifier and good light for the small, often-cryptic inductor markings
A multimeter with a continuity/resistance range to test windings — the primary diagnostic for open windings

Work only with de-energized components. A mains transformer removed from unplugged equipment is safe to examine; never probe a transformer in powered mains equipment.

Real-World Applications

Wound components are everywhere in power and signal paths. A switch-mode power supply is full of them: the main power inductor storing energy each cycle, small ferrite beads suppressing noise on supply lines, and a high-frequency transformer providing the isolation between the mains input and the low-voltage output. A traditional "wall wart" or appliance uses a mains-frequency transformer to step 120 or 230 V down to a safe low voltage. Audio equipment uses signal transformers for coupling and impedance matching. For a repair technician, these components are both diagnostic targets — an open transformer primary is a classic "completely dead" fault — and safety-critical: the transformer is often the boundary between dangerous mains voltage and the safe low-voltage circuitry, and knowing which side is which is essential.

Common Challenges

Cryptic inductor markings. Inductor codes are far less standardized than resistor codes. Some use a three-digit microhenry code, some a color code, some direct printing, and many small ones have no readable value at all — you measure or consult the schematic. Don't expect resistor-like consistency.
Reading the inductor code as ohms or picofarads. The three-digit code here yields microhenries, not ohms (resistors) or picofarads (ceramics). Same code format, third different unit.
Underestimating the transformer primary. On mains equipment the primary side is at full line voltage and lethal. It is easy to forget that a transformer, unlike most components, has a genuinely dangerous side — treat the primary of any mains transformer with the same respect as the mains itself.

Safety Notes

Risk Level: Medium. Identifying loose or de-energized components is low-risk, but transformers introduce mains voltage, and coils carry the inductive-kick hazard — so this section carries firm cautions.

Professional Tips Before Starting

Identify a transformer's primary versus secondary before touching it: on a mains transformer the primary usually has fewer, thinner wires and higher resistance, and the mains input connects to it; the secondary feeds the low-voltage circuitry. On a dead device, an open primary is a prime suspect — but confirm the equipment is unplugged first.
For inductors, don't fight the markings. If a small inductor's value isn't clearly printed or decodable, measure it or read it from the schematic — inductor labeling is genuinely inconsistent, and guessing wastes time.
Remember the isolation boundary. The transformer is often the line between dangerous mains and safe low voltage; keeping track of which side you're working on is a habit that keeps you safe.

Identifying Inductors and Transformers

Inductor Types

Inductors take several recognizable forms:

Fixed inductors and chokes — small coils, either molded into a body that looks much like a resistor (sometimes with a color code) or wound visibly on a bobbin or core. They provide a set inductance for filtering and tuning.
Ferrite beads — a ferrite bead is a small ferrite cylinder slipped over a wire (or a surface-mount block) that suppresses high-frequency noise. It behaves as a frequency-dependent choke and is one of the most common noise-suppression parts on modern boards.
Power inductors — the larger bobbin-wound or toroidal (doughnut-shaped) coils found in switch-mode power supplies, storing energy each switching cycle. They're physically substantial because they carry real current.
RF coils — small, sometimes air-cored or adjustable coils used in radio-frequency tuned circuits.

Reading Inductor Markings and Ratings

Inductor markings are less standardized than resistor markings, which is the first thing to know. Some small inductors use a three-digit code that works exactly like the resistor and ceramic codes — two significant digits and a zeros-count multiplier — but the result is in microhenries (µH):

"101" = 10 followed by 1 zero = 100 µH.
"220" = 22 followed by 0 zeros = 22 µH.
"470" = 47 followed by 0 zeros = 47 µH.
"102" = 10 followed by 2 zeros = 1000 µH = 1 mH.

Others use a color code like a resistor's, or simply print the value; and many tiny inductors carry no usable value marking at all, so you measure them or read the schematic. Beyond inductance, two ratings matter: the current rating — the current above which the core saturates or the coil overheats, so a replacement must handle the circuit's current — and the DC resistance of the winding, which is small but affects losses and is what a continuity check measures. The unit ladder mirrors the others: 1 H = 1000 mH = 1,000,000 µH.

What a Transformer Is

A transformer is two or more coils — windings — sharing a common magnetic core so that they are magnetically coupled. Drive an alternating current through one winding, the primary, and its changing magnetic field induces a voltage in the other winding, the secondary, by the mutual induction you met in Chapter 3. A crucial consequence: a transformer works on alternating current only — a steady DC in the primary produces no changing field and so no output. The primary receives the input; the secondary (there may be several) delivers the output.

Turns Ratio and Isolation

What makes a transformer useful is that the ratio of turns on its windings sets the ratio of voltages. This is the turns ratio:

Np / Ns = Vp / Vs

where Np and Ns are the number of turns on the primary and secondary, and Vp and Vs are their voltages. If the secondary has fewer turns than the primary, its voltage is lower — a step-down transformer; if it has more turns, its voltage is higher — a step-up. Two worked examples:

A 10:1 transformer (ten primary turns per secondary turn) fed 120 V on the primary gives 120 ÷ 10 = 12 V on the secondary — a step-down, the everyday mains-adapter case.
A 1:2 transformer fed 6 V on the primary gives 6 × 2 = 12 V on the secondary — a step-up.

The second key property is isolation: the primary and secondary windings are coupled only by their shared magnetic field, not by any electrical connection, so there is no direct copper path between them. Energy crosses the gap magnetically. This makes a transformer an isolation barrier — an isolation transformer exists specifically to provide it — which is a genuine safety function: it can separate a circuit from a direct connection to the mains. But note the flip side that the safety callout stressed: isolation protects the secondary side, while the primary remains connected to, and at, the mains.

Transformer Types and Ratings

A few types cover most cases: power (mains) transformers, the heavy iron-cored units that step the line voltage down (or up) at 50/60 Hz; signal/audio transformers, smaller units for coupling and impedance matching in signal paths; and switch-mode / high-frequency transformers, the compact ferrite-cored transformers in modern supplies that work at tens of kilohertz and provide the mains-to-output isolation. The ratings that matter are the primary and secondary voltages (e.g. "120 V : 12 V"), the VA (volt-ampere) or power rating — how much power it can handle — and the current rating of the secondary. A replacement must match the voltages and meet or exceed the power/current the circuit demands.

Common Mistakes

Decoding the inductor code into the wrong unit. The three-digit inductor code is microhenries — "101" is 100 µH, not 101 of anything, and not ohms or picofarads.
Getting step-up and step-down backwards. More secondary turns means higher secondary voltage (step-up); fewer means lower (step-down). Np/Ns = Vp/Vs keeps it straight.
Expecting a transformer to pass DC. Transformers work by a changing field — AC only. A steady DC on the primary gives no secondary output (and can overheat the primary).
Treating the primary side as safe. On mains equipment the primary is at line voltage and lethal; isolation protects the secondary, not you working on the primary.

Troubleshooting Guidance

Wound components are a rewarding diagnostic target because their most common failure — the open winding from Section 3.7 — is easy to find with a continuity or resistance check on a de-energized part. A transformer with an open primary produces no output at all and is a classic cause of a "completely dead" device: with the equipment unplugged, a continuity check across the primary that reads open confirms it. A good winding reads a finite, relatively low resistance (from a few ohms to a few hundred — and some small mains primaries higher still, into the low thousands); an open winding reads infinite. The key distinction is a finite reading versus an infinite (open) one, not the exact number of ohms. Shorted turns are subtler — the winding still conducts but the transformer runs hot or delivers the wrong voltage, as Section 3.7 described. When you identify a transformer for replacement, match its primary and secondary voltages, its power/current rating, and its type. And throughout, keep the safety order absolute: confirm the equipment is unplugged and discharged before you touch a mains transformer, because the primary side is deadly when live — no measurement is worth probing a powered mains primary.

Verification & Testing Methods

Check your understanding before moving on:

[ ] Identify the common inductor types (fixed/choke, ferrite bead, power inductor) and decode a three-digit microhenry code such as "101".
[ ] Use Np/Ns = Vp/Vs to find a secondary voltage and state whether the transformer steps up or down.
[ ] Explain what transformer isolation means and why the primary of a mains transformer is dangerous.
[ ] Describe how a continuity check finds an open transformer winding, and the safety step that comes first.

Then try the practice exercises below — identifying, decoding, and reasoning about de-energized parts only.

Practice Exercises

Identify and decode (10 minutes, with parts or images). Identify each of several wound components as a fixed inductor/choke, a ferrite bead, a power inductor, or a transformer. Then decode these inductor codes to microhenries: (a) "331"; (b) "100"; (c) "102". (Hint: watch the unit — it's µH.)
Turns ratio (10 minutes, pen and paper). For each, find the secondary voltage and say step-up or step-down: (a) a 1:10 transformer with 12 V on the primary; (b) an 8:1 transformer with 120 V on the primary. Use Np/Ns = Vp/Vs and show your working.
Primary or secondary? (5 minutes, reasoning). On a mains power transformer, describe how you'd tell the primary from the secondary, which side is dangerous when the equipment is powered, and what isolation does and does not protect.
Find the open winding (5 minutes, reasoning). A device is completely dead and you suspect its mains transformer. Describe the safe procedure to check for an open primary with a continuity test — including the first safety step — and state what an open reading versus a low-resistance reading tells you.

These core ideas — inductor types and the µH code, the turns ratio and step-up/step-down, isolation, and the mains-primary hazard — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

Inductors appear as fixed inductors/chokes, ferrite beads (slip-on high-frequency noise suppressors), power inductors (switch-mode coils), and RF coils; identify the form first.
Small inductors sometimes use a three-digit code like a resistor's, but the result is in microhenries ("101" = 100 µH); markings are less standardized than resistors, so often you measure or consult the schematic. Ratings: inductance, current rating, DC resistance.
A transformer is coupled primary and secondary windings on a shared core, working by mutual induction on AC only — a steady DC gives no output.
The turns ratio sets the voltage: Np/Ns = Vp/Vs; more secondary turns step the voltage up, fewer step it down (a 10:1 transformer takes 120 V to 12 V).
Isolation means the primary and secondary share only a magnetic field, with no electrical connection — a safety barrier that protects the secondary side, while the primary stays at the mains.
The primary of a mains transformer is live and lethal when powered; identification and testing are done unplugged and discharged, and an open winding (found by a continuity check) is the classic transformer failure.

Skills Learned

You can now recognize the common inductor types and read a three-digit microhenry code.
You can now use the turns ratio Np/Ns = Vp/Vs to find a secondary voltage and identify step-up versus step-down.
You can now explain transformer isolation and tell a primary from a secondary.
You can now apply the mains-primary safety rule and use a continuity check to find an open winding.
You can now connect Chapter 3's inductor theory and failure modes to identifying and testing real wound components.

Glossary Additions

transformer — two or more windings sharing a magnetic core, coupled by mutual induction, that transfers AC energy from a primary winding to one or more secondary windings; its turns ratio sets the voltage ratio, and its windings are electrically isolated from one another. It works on alternating current only.
turns ratio — the ratio of the number of turns on a transformer's primary and secondary windings, which equals the ratio of their voltages (Np/Ns = Vp/Vs); more secondary turns give a higher secondary voltage (step-up), fewer give a lower one (step-down).
isolation — the property that a transformer's primary and secondary windings are coupled only magnetically, with no direct electrical connection, so energy crosses between them without a copper path; it provides a safety barrier that separates the secondary side from a direct mains connection.
ferrite bead — a small ferrite component slipped over or in series with a wire that acts as a frequency-dependent choke, suppressing high-frequency noise while passing DC and low frequencies; one of the most common noise-suppression parts on circuit boards.

Suggested Next Sections

Must read next:

Diodes — Function and Types — the identification tour moves to the first semiconductor: the diode, a one-way valve for current, its types, and how to read and test it.

Recommended:

What Is an Inductor? — the inductance and mutual-induction theory this section identifies in real parts.
Capacitor and Inductor Failure Modes — the open-winding failure you now know how to test for.

What You Will Learn

What You Will Be Able To Do

Required Tools