SMD Inductor Marking: Codes 100, 4R7 and R47 Explained

On a repair bench you meet inductors in three forms: a power inductor in a DC-DC converter next to the PMIC, a tiny chip coil in an RF path, and a ferrite bead sitting in a supply rail. Their marking is either three characters or nothing at all. Here is how to read what is printed, and what to do when the part is blank.

Types of SMD inductors

Multilayer chip inductors

They look like any chip capacitor or resistor — a small block with metal end caps — and use the same case sizes: 0402, 0603, 0805, 1206. Values run from a few nanohenries to a few microhenries. Most of them carry no marking at all: some manufacturers even offer “no marking” as an ordering option, and the smallest parts may only get an orientation dot.

Wirewound on a ceramic core

Same footprints, but with real wire inside. You find them in Wi-Fi, Bluetooth and cellular front ends because of their high Q. Usually unmarked as well.

Power inductors

Square or round parts from about 2×2 mm up to 12×12 mm and larger: unshielded drum cores (you can see the winding), shielded types (the winding is enclosed in a ferrite or magnetic sleeve) and moulded composite parts. These are the ones that normally show a value code on top.

Ferrite beads

They look like a 0402–1206 chip, but they are noise absorbers rather than energy-storage inductors. Datasheets rate them by impedance in ohms at 100 MHz, not by inductance — Murata BLM18KG471, for example, is 470 Ω at 100 MHz. At DC a bead is almost a short.

Three-digit code in microhenries

The code works like a resistor code, but the unit is the microhenry (µH): the first two digits are significant, the third is the number of zeros to add.

Code	Calculation	Inductance
100	10 × 1	10 µH
220	22 × 1	22 µH
470	47 × 1	47 µH
101	10 × 10	100 µH
331	33 × 10	330 µH
102	10 × 100	1000 µH = 1 mH

The classic mistake is reading 100 as one hundred. The final zero is the multiplier, so it is a 10 µH part; 470 is 47 µH.

R as the decimal point

Values below 10 µH use the letter R in place of the decimal point: 4R7 = 4.7 µH, 2R2 = 2.2 µH, 1R0 = 1.0 µH. A leading R means less than one: R47 = 0.47 µH, R10 = 0.1 µH (100 nH).

Resistors use the same notation (4R7 = 4.7 Ω), so the code alone does not tell you what the part is. Look at the body and its surroundings: a converter inductor is a large square part next to the regulator IC and its capacitors.

N for nanohenries and tolerance letters

Very small values are written in nanohenries with N as the decimal point: 2N2 = 2.2 nH, 1N2 = 1.2 nH, 12N = 12 nH. You will see this in the part number (TDK, Murata) — on the reel label, in the BOM, in service documentation — not printed on the tiny chip itself.

The value in a part number is usually followed by a tolerance letter: M ±20 %, K ±10 %, J ±5 %. Bourns SRR1260-100M, for instance, is 10 µH ±20 %.

Inductor or resistor?

• Marking. Resistors 0603 and larger are almost always coded; chip inductors mostly are not.
• Colour. A resistor has a black or dark blue top over a white ceramic base; a multilayer inductor is usually the same colour all round, closer to a capacitor.
• Resistance. An inductor reads from a few hundredths of an ohm to a few ohms. A 0 Ω jumper (marked 0 or 000) and a ferrite bead also read near zero, though.
• Position. In series with a supply line near a connector or IC — likely a bead; next to an antenna — an RF chip coil.

The look-alikes are covered in SMD resistor marking and SMD capacitor marking.

Testing an inductor

Ohmmeter

Finds only an open winding. A good power inductor reads from tens of milliohms to a few ohms (around 0.045 Ω for a Würth 10 µH part, up to 1.7 Ω for a Bourns 1000 µH part). Open circuit means a broken winding. Shorted turns are invisible to an ohmmeter: resistance changes by a fraction while inductance drops sharply.

LCR meter

Remove the part and measure at the datasheet test frequency (1 kHz or 100 kHz for power inductors). Inductance well below nominal, or a collapsed Q, points to shorted turns. In-circuit readings are unreliable because of parallel capacitors and ICs.

Visual check

A cracked ferrite, a darkened winding or a chipped corner means replace it, even if it beeps through.

Saturation current: never replace by inductance alone

A power inductor has a saturation current (Isat) — the current at which inductance has dropped by a stated percentage — and a rated current based on temperature rise. Manufacturers define them differently: Bourns uses a 25 % drop, Würth lists Isat at 10 % and at 30 %. A smaller 10 µH part in a switching supply will saturate, the switch current spikes and the MOSFET or controller fails. Match inductance, saturation current, size and shielding — ideally the exact part number from the datasheet. For the switching transistor itself, see SMD transistor marking.

Common mistakes

• Reading 100 as 100 µH — it is 10 µH.
• Fitting a smaller part with the same inductance — it overheats and saturates.
• Swapping shielded for unshielded “because the value is the same” — an open core in a dense layout couples noise into neighbouring circuits.
• Replacing a ferrite bead with a jumper: the board works, the filter is gone.
• Mistaking a chip inductor for a capacitor in the same case — an ohmmeter settles it at once, a capacitor reads open.

In short

• Three digits on an inductor are µH: 100 = 10 µH, 101 = 100 µH.
• R is the decimal point: 4R7 = 4.7 µH, R47 = 0.47 µH.
• N is the decimal point in nH — in part numbers, not on the body.
• Small chip inductors and beads are usually unmarked; beads are rated in ohms at 100 MHz.
• An ohmmeter only finds open windings; shorted turns need an LCR meter.
• Replace by inductance, saturation current, size and shielding.

In ServiceCenterSoft an inductor taken from stock goes onto the repair estimate as a line item, and stock is kept as a movement log with quantities on hand and a minimum level, so parts that are running low show up straight away. More on that in spare parts tracking.