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SMD code search: identify a part by its marking

0.4 Ω resistor. 1 part in X2SON is printed R4.

R4 on a chip resistor

0.4 Ω

0.4 Ω resistor

The R stands in for the decimal point, which is how sub-ohm and fractional values are printed.

Check it with a meter

Lift one end and measure across it. A good one reads 0.4 Ω. Measuring it in circuit reads the parallel combination of everything around it, which is almost always lower, never higher: well above 0.4 Ω on the board is a fault. Open, or well above 0.4 Ω, means it has failed: burnt, cracked or corroded. That does not change what to fit: the marking still says 0.4 Ω. 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-23, SOD-123F, SOT-363), R4 is a part code instead. Those parts are listed below.

1 part in X2SON is printed R4

Not sure which one you have?

Answer up to 4 quick questions and these 2 parts are ranked, with a meter test that settles it. It works on a dead part too.

  • SN74LVC1G125DPWX2SONLogic gate, Texas Instruments
    What it is
    Logic gate
    Maker
    Texas Instruments
    Package
    X2SON
    Pins
    5
    Details
    Single Bus Buffer Gate With 3-State Output
    Source
    From the manufacturer's datasheet

    Is it dead?

    Voltage with the board powered; ohms from each pin to ground with it off.

    Working, it reads

    Supply pin at its voltage and the output doing what the datasheet says. Readings between pins depend on the part.

    Dead, it reads

    • A pin near 0 Ω to ground or to the supply pin where an identical part does not read that: failed short.
    • Supply present, output stuck: failed, or its load has.
    On the board:
    A pin shorted to ground on the board can be anything on that net. Lift the pin to be sure.
    Compare:
    The surest reference is the same IC on a working board, measured pin by pin.
    Watch for:
    Its protection diodes can read like junctions between pins, even like a transistor. That proves nothing either way.

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

    Datasheetfor SN74LVC1G125DPW, opens in a new tab
  • 1 more part has no package on record. Nothing rules it out, so it stays.
    • BFP640RF transistor, Infineon
      What it is
      RF transistor
      Maker
      Infineon
      Package
      Not recorded
      Pins
      Not recorded
      Source
      From the manufacturer's datasheet

      Is it dead?

      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.

      Datasheetfor BFP640, opens in a new tab

One code, several parts

Marking codes are not unique. In the standard code book, 6H on a SOT-23 is an NPN transistor, a varicap, an N-channel JFET, two digital transistors, a 3.6 V voltage detector: six parts from five makers. So a search shows the parts we hold under that code and under codes easily misread as it, with only the columns that tell them apart.

6Hon a SOT-23, in the code book
  • NPN transistor
  • Varicap
  • N-channel JFET
  • Digital transistor ×2
  • 3.6 V voltage detector

When more than one is left

Tap Help me choose. Up to 4 quick answers: whether the part has failed, then what separates the parts left. Those get ranked with the reasoning and a test that settles it. That uses one identification: 2 a month are free without an account, and 3 with a free account.

When you cannot read it

Tap the camera in the field. We read the characters, keep the ones we are unsure of, and search every reading. A photo too soft to read is turned back on your phone first; if it is the only one you can get, you can still send it.