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SMD code 2550

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.

2550 on a chip resistor

255 Ω

255 Ω resistor, 1%

255 followed by 0 zeros. Four digits is normally the 1% series.

Check it with a meter

Lift one end and measure across it. A good one reads 255 Ω within 1%. Measuring it in circuit reads the parallel combination of everything around it, which is almost always lower, never higher: well above 255 Ω on the board is a fault. Open, or well above 255 Ω, means it has failed: burnt, cracked or corroded. That does not change what to fit: the marking still says 255 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, SOT-89), 2550 is a part code instead. Those parts are listed below.

3 parts are printed 2550

Not sure which one you have?

Answer up to 4 quick questions and these 3 parts are ranked, with a meter test that settles it. It works on a dead part too.

PNP transistor, 3 pins. What differs:

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.

PNP transistor

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

Working, it reads

Black 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.

Did you find your part?

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