Identifying an SMD diode from its code
SMD diodes come in two-lead bodies with a band, and in three-lead SOT-23 bodies that look like transistors. The code names the part, and its band or pinout and a meter reading confirm it.
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Find the cathode
On a two-lead body, such as SOD-123, SOD-323, SMA or the round MELF types, a band at one end marks the cathode. A bidirectional TVS diode has no polarity and often no band.
A SOT-23 has no band. With the single lead at the top, pin 1 is at the bottom left and pin 2 at the bottom right, and the datasheet's diagram shows which pins are anodes and which cathodes.
Four ways a SOT-23 holds diodes
A three-lead diode body holds one diode or two, and two can share their anode, share their cathode or sit in series. Each way has its own part numbers and its own codes. The codes here are the ones most makers in our tables print in SOT-23:
On the Schottky side the last letter of the part number names the arrangement: BAT54A has a common anode, BAT54C a common cathode and BAT54S two diodes in series. The code changes with it.
On the meter, a common-anode pair reads like an NPN transistor, two junctions from pin 3 with the red probe on it, and a common-cathode pair like a PNP. A series pair reads a junction from pin 1 to pin 3 and from pin 3 to pin 2, and roughly twice that from pin 1 to pin 2.
Zener codes and the voltage
A Zener's code does not give its voltage; its part number does. The BZX84C series in SOT-23 has one part number for each voltage, and a code for each. The table gives the codes most makers in our tables print, from 2.4 to 39 V.
| Voltage | Part | Codes, the most makers first |
|---|---|---|
| 2.4 V | BZX84C2V4 | Z11C8W1 |
| 2.7 V | BZX84C2V7 | Z12D8W2 |
| 3 V | BZX84C3V0 | Z13E8HZD |
| 3.3 V | BZX84C3V3 | Z14F8W4 |
| 3.6 V | BZX84C3V6 | Z15H8W5 |
| 3.9 V | BZX84C3V9 | Z16J8W6 |
| 4.3 V | BZX84C4V3 | Z17K8W7 |
| 4.7 V | BZX84C4V7 | Z1M8W8 |
| 5.1 V | BZX84C5V1 | Z2N8W9 |
| 5.6 V | BZX84C5V6 | Z3P8WA |
| 6.2 V | BZX84C6V2 | Z4R8WB |
| 6.8 V | BZX84C6V8 | Z5X8WC |
| 7.5 V | BZX84C7V5 | Z6Y8WD |
| 8.2 V | BZX84C8V2 | Z7Z8WE |
| 9.1 V | BZX84C9V1 | Z8A9WF |
| Voltage | Part | Codes, the most makers first |
|---|---|---|
| 10 V | BZX84C10 | Z9B9WG |
| 11 V | BZX84C11 | Y1C9WH |
| 12 V | BZX84C12 | Y2D9WI |
| 13 V | BZX84C13 | Y3E9WK |
| 15 V | BZX84C15 | Y4F9WL |
| 16 V | BZX84C16 | Y5H9WM |
| 18 V | BZX84C18 | Y6J9WN |
| 20 V | BZX84C20 | Y7K9WO |
| 22 V | BZX84C22 | Y8M9WP |
| 24 V | BZX84C24 | Y9N9WR |
| 27 V | BZX84C27 | Y10P9WS |
| 30 V | BZX84C30 | Y11R9WT |
| 33 V | BZX84C33 | Y12X9WU |
| 36 V | BZX84C36 | Y13Y9WW |
| 39 V | BZX84C39 | Y14WXZ9 |
The C in the part number is the tolerance, ±5 %. BZX84B parts, at ±2 %, carry codes of their own from some makers and the C part's code from others.
A meter cannot show the voltage of most Zeners, because its diode range stops at about 2 to 3 V. Feed the Zener from a bench supply through a resistor that sets about 5 mA, cathode to positive, and read the voltage across it.
Silicon, Schottky or TVS on the meter
On the diode range, with the part lifted and the red probe on the anode, a silicon diode reads about 0.5 to 0.8 V and a Schottky about 0.15 to 0.45 V; both read open the other way. A Zener reads like a silicon diode. A one-way TVS diode reads about 0.6 to 0.8 V forward. A two-way one reads open both ways when it is good, just like one that has failed open, so compare it with the same part on the next line.
The same reading both ways, near 0 V with a continuous beep, is a short whatever the diode. Every reading, and what a failed part shows, is in Testing SMD parts with a multimeter.
Try it
Search L43, a BAT54C code from the table above, to see every part printed with it and how to test each.