How to identify an SMD component
Most SMD parts are too small for their part number, so the maker prints a code of a few characters instead. Getting from that code back to the part takes four steps: read the code, note the package, look up every part with that code, and measure to tell them apart.
Read the code exactly
Use a loupe, or your phone's camera with the light coming in low from one side: laser marking on a black body often shows better as a change in sheen than in colour. Write down every character, small ones included.
Worn characters are easy to misread: 0 and O, 1 and I, 5 and S, 8 and B, 6 and G. If you are not sure, search both readings. WhatSMD's search also shows parts printed with a code that is easy to misread as the one you typed.
Makers often add a date or factory letter after the code, or a second line, which are not part of the code. A small letter after it, as in 1Ap, is often one of these. If a code makes no sense, try it the other way up: 6H upside down reads H9.
Note the package and count the pins
The same code means different parts in different packages, so the package narrows the list before anything else. Count the pins and measure the body: SOT-23 is about 2.9 by 1.3 mm with three pins, SOT-323 about 2.0 by 1.25 mm with three, SOD-123 about 2.7 by 1.6 mm with two, and SOD-323 about 1.7 by 1.25 mm with two.
A three-pin SOT-23 can hold a diode pair, a Zener, a MOSFET or a voltage reference as easily as a transistor, and a two-pin SOD body holds a diode of some kind.
Look up every part with that code
Codes are not standardised. There are only so many codes of two or three characters, and unrelated makers reuse them: in the standard code book, 6H on a SOT-23 is six parts from five makers. A lookup that returns one part for 6H has left out five.
Search the code with its package to see every part in our tables printed with it, with the columns that tell them apart, such as the kind of part and the maker. To browse instead, the online SMD code book lists every code by first character, part type and package.
The lists of transistor, MOSFET and diode codes give each code with only the parts of that type.
Confirm it with a multimeter
A meter reading separates parts that share a code. Use the diode range, with the part lifted or one leg off the pad: on the board the meter reads every path between the pins, and a reading can belong to a neighbour.
| Part | A working one reads |
|---|---|
| Silicon diode | About 0.5 to 0.8 V from anode to cathode, open the other way. |
| Schottky diode | About 0.15 to 0.45 V forward, open in reverse. |
| Zener diode | Like a plain diode on the meter. Its voltage shows only on a supply, through a resistor that sets about 5 mA. |
| Bipolar transistor | About 0.6 to 0.75 V from the base to the emitter and to the collector, red probe on the base for NPN, black for PNP, and open the other way round. Collector to emitter open both ways. |
| Digital transistor | On the ohms range, base to emitter reads its two built-in resistors in series, about 20 kΩ for a 10 k and 10 k part. |
| MOSFET | A body diode of about 0.4 to 0.9 V one way between drain and source, open the other way, and the gate open to both. Short gate to source before you judge it. |
Whatever the part, the same reading both ways, near 0 V with a continuous beep, is a short. The meter cannot tell a transistor from a pair of diodes with a common pin, which read alike, nor show a Zener's voltage above about 3 V: the datasheet's pinout settles the first, a bench supply the second.
If an identical part sits elsewhere on the board, measure it the same way: that is what yours should read.
When the code is in no table
Some makers publish no marking table, and some parts carry a code made for one customer. Then the board itself is the best reference: an identical working unit, the service manual's parts list, or the same part on the next channel.
Look up a code
Type the code and its package, or browse the online SMD code book.