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SMD code 123

Marking codes are not standardised, and the same code is reused by unrelated makers for unrelated parts. Below is every part in our tables printed with it, and any printed with a code that is easy to misread as it, from the manufacturers' own marking tables.

123 on a chip resistor

12 kΩ

12 kΩ resistor, 5%

12 followed by 3 zeros. Three digits is normally the 5% series. The same marking also reads as 12 nF on a capacitor or 12 mH on an inductor, and nothing in the code says which; the body of the part does.

The same characters on another part

  • 12 nF capacitor
  • 12 mH inductor

Check it with a meter

Lift one end and measure across it. A good one reads 12 kΩ within 5%. Measuring it in circuit reads the parallel combination of everything around it, which is almost always lower, never higher: well above 12 kΩ on the board is a fault. Open, or well above 12 kΩ, means it has failed: burnt, cracked or corroded. That does not change what to fit: the marking still says 12 kΩ. A burnt resistor usually means the part it feeds drew too much, so find that before fitting a new one.

On a transistor or diode body (SOT-323, SOT-23), 123 is a part code instead. Those parts are listed below.

4 parts are printed 123

Not sure which one you have?

Answer up to 4 quick questions and these 4 parts are ranked, with a meter test that settles it. It works on a dead part too.

3 pins. What differs:

  • DTC143ZU3SOT-323Digital transistor (NPN, resistors built in), ROHM
    What it is
    Digital transistor (NPN, resistors built in)
    Maker
    ROHM
    Package
    SOT-323
    Pins
    3
    Details
    NPN 50V, 100mA digital transistor (bias resistor built-in)
    Source
    From the manufacturer's datasheet

    Is it dead? The meter test is below: Digital transistor.

    Datasheetfor DTC143ZU3, opens in a new tab
  • DTC143ZUASOT-323Digital transistor (NPN, resistors built in), ROHM
    What it is
    Digital transistor (NPN, resistors built in)
    Maker
    ROHM
    Package
    SOT-323
    Pins
    3
    Details
    Digital transistor (bias resistors built in), NPN
    Source
    From the manufacturer's datasheet

    Is it dead? The meter test is below: Digital transistor.

    Datasheetfor DTC143ZUA, opens in a new tab
  • DTC143ZUBSOT-323Digital transistor (NPN, resistors built in), ROHM
    What it is
    Digital transistor (NPN, resistors built in)
    Maker
    ROHM
    Package
    SOT-323
    Pins
    3
    Details
    NPN digital transistor (bias resistor built-in); R1 = 4.7kOhm, R2 = 47kOhm
    Source
    From the manufacturer's datasheet

    Is it dead? The meter test is below: Digital transistor.

    Datasheetfor DTC143ZUB, opens in a new tab
  • STD123SSOT-23NPN transistor, JSCJ
    What it is
    NPN transistor
    Maker
    JSCJ
    Package
    SOT-23
    Pins
    3
    Details
    Bipolar transistor, NPN, 15 V, 1 A
    Source
    From the manufacturer's datasheet

    Is it dead? The meter test is below: NPN transistor.

    Datasheetfor STD123S, opens in a new tab

Is it dead? One meter test per kind

Parts of the same kind are tested the same way, so each test is here once. Each part above points to its own.

Digital transistor

Ohms range, part lifted. The datasheet gives the built-in resistors: R1 in series with the base, R2 from base to emitter.

Working, it reads

Base to emitter reads about R1 plus R2 either way round, about 20 kΩ on a 10 k and 10 k part, give or take their wide tolerance. Collector to emitter reads open. A part with no R2 reads a junction behind R1 one way instead.

Dead, it reads

  • Collector to emitter the same both ways, near 0 Ω with a continuous beep: shorted.
  • From the base, open or far above R1 plus R2: R1 has failed. Static damage shows here most often.
  • Base to emitter near 0 Ω: failed short.
On the board
this is a first look only: the meter reads every path between those pins, and a short can be a neighbour. Lift one leg before you condemn it.
Compare:
The readings depend on the resistor values, so an identical part elsewhere on the board is the surest reference. Measure it the same way.
Watch for:
On the diode range a good part with 10 kΩ resistors reads open on every pair, so a good part looks dead there. Use the ohms range.

A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.

NPN transistor

Diode range, part lifted or its base leg off the pad.

Working, it reads

Red probe on the base reads about 0.6 to 0.75 V to the emitter and to the collector, and open the other way round. Collector to emitter reads open both ways.

Dead, it reads

  • Collector to emitter the same both ways, near 0 V with a continuous beep: shorted. The commonest failure.
  • The base the same both ways to another pin, near 0 V or a middling value: a shorted junction.
  • Only one junction reads, or none: a junction has failed open. It then looks like a diode.
  • A reading the wrong way round, where it should be open: leaky.
On the board
this is a first look only: the meter reads every path between those pins, and a short can be a neighbour. Lift one leg before you condemn it.
Compare:
If an identical part sits elsewhere on the board, measure it the same way: that is what this one should read.
Watch for:
A pair of diodes with a common pin reads just like a transistor, and some transistors carry a built-in diode or resistor. The datasheet says which.

A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.

Codes that begin with 123

For a marking with its end worn or burnt off. Each code has a page of its own.

Open this in the search to correct a character or try another package.