SMD code 121
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.
121 on a chip resistor
120 Ω
120 Ω resistor, 5%
12 followed by 1 zero. Three digits is normally the 5% series. The same marking also reads as 120 pF on a capacitor or 120 µH on an inductor, and nothing in the code says which; the body of the part does.
The same characters on another part
- 120 pF capacitor
- 120 µH inductor
Check it with a meter
Lift one end and measure across it. A good one reads 120 Ω within 5%. Measuring it in circuit reads the parallel combination of everything around it, which is almost always lower, never higher: well above 120 Ω on the board is a fault. Open, or well above 120 Ω, means it has failed: burnt, cracked or corroded. That does not change what to fit: the marking still says 120 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-323, WSON), 121 is a part code instead. Those parts are listed below.
2 parts are printed 121
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.
Digital transistor (NPN, resistors built in), all SOT-323, 3 pins, all ROHM.
DTC113ZU3SOT-323
- 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.
DTC113ZUBSOT-323
- 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.
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.
Printed with a code easy to misread as 121
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 121
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.