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SMD code U21 in SOT-23

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 and published code lists.

2 parts in SOT-23 are printed U21

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.

  • 2SC2759-21SOT-23RF transistor, 3 pins, NEC ElectronicsRF transistor3 pinsNEC Electronics
    What it is
    RF transistor
    Maker
    NEC Electronics
    Package
    SOT-23
    Pins
    3
    Details
    NPN transistor: RF-amp, 14V, 50mA, 150mW, B=40..80, 1.5GHz
    Source
    Published code list

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

  • 2SC2759U21SOT-233 pinsNot recorded3 pinsNot recorded
    What it is
    Not recorded
    Maker
    Not recorded
    Package
    SOT-23
    Pins
    3
    Source
    Published code list

    Is it dead? The meter test is below: This part.

  • 2 more parts have no package on record. Nothing rules them out, so they stay.
    • 2SC4185RenesasNot recordedNot recordedRenesas
      What it is
      Not recorded
      Maker
      Renesas
      Package
      Not recorded
      Pins
      Not recorded
      Details
      npn RF 10V fT2.4GHz hFE 40-80
      Source
      Published code list

      Is it dead? The meter test is below: This part.

    • QS5U21P-channel MOSFET, ROHMP-channel MOSFETNot recordedROHM
      What it is
      P-channel MOSFET
      Maker
      ROHM
      Package
      Not recorded
      Pins
      Not recorded
      Details
      P-Channel MOSFET + Schottky Barrier Diode
      Source
      Published code list

      Is it dead? The meter test is below: P-channel MOSFET.

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.

Bipolar transistor

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

Working, it reads

From the base, one probe colour 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.

This part

Diode range and ohms range, part lifted.

Working, it reads

Depends on what it is. The datasheet, or an identical part, says.

Dead, it reads

  • Between two pins, the same reading both ways, near 0 V with a continuous beep: shorted.
  • Open between every pair of pins: failed open, or a part that reads open when working.
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.

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

P-channel MOSFET

Diode range, part lifted. Test drain to source first, then short gate to source and test again.

Working, it reads

Red probe on the drain, black on the source: about 0.4 to 0.9 V, the body diode. Open the other way. The gate reads open to both other pins, both ways.

Dead, it reads

  • Drain to source the same both ways, near 0 V with a continuous beep, and still so after shorting gate to source: shorted. The classic failure.
  • The gate reads low to source or drain: gate shorted. Often all three pins end up shorted together.
  • No body diode at all: failed open.
On the board
the gate usually has a resistor to the source, so gate to source reads that resistor, which is not a fault. Lift a leg to be sure.
Compare:
If an identical part sits elsewhere on the board, measure it the same way: that is what this one should read.
Watch for:
The diode range can charge the gate enough to turn the part on, and then drain to source reads near 0 both ways on a good part. Short gate to source and test again before you condemn it.

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

Printed with a code easy to misread as U21

One character differs, and it is one that is easy to confuse under a lamp. Each code has a page of its own.

Codes that begin with U21

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

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