SMD marking

1Uin 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.

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.

The only part printed 1U in this package in our tables

MMBZ5V6CA

What it is: ESD protection diode
Maker: Diotec
Package: SOT-23
Pins: 3
Details: ESD Protection Diodes in SMD
Source: From the manufacturer's datasheet

Is it dead?

Diode range, from the protected line to ground, part lifted if you can.

Working, it reads

A one-way part reads about 0.6 to 0.8 V forward and open in reverse. A two-way part reads open both ways. Arrays with several lines read as the datasheet's diagram draws them.

Dead, it reads

From the line to ground, the same reading both ways, near 0 V with a continuous beep: shorted.
A reading both ways on a one-way part: leaky.
A one-way part open both ways: failed open.

On the board:: On the board, a line reading shorted to ground can be this part or anything else on that line. Lift it: if the short goes with it, it has failed.
Compare:: The same part on the next line or connector pin is the healthy reading to compare with.
Watch for:: A two-way part reads open both ways when it is good, exactly like one that has failed open. The meter cannot tell them apart; an identical part on the next line can.

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

Datasheet

Printed with a code easy to misread

These parts are printed with a character that is easy to misread as what you typed. The odd one out is marked on each.

AOSS21319Cprinted LUSOT-23P-channel MOSFET, 3 pins, Alpha & Omega SemiconductorP-channel MOSFET3 pinsAlpha & Omega Semiconductor
Printed LU on the part. Easy to misread as 1U. Check that character with a loupe before you order anything.
What it is
P-channel MOSFET
Maker
Alpha & Omega Semiconductor
Package
SOT-23
Pins
3
Details
P-channel MOSFET (8-60 V)
Source
From the manufacturer's datasheet
Is it dead?
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:
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.
Datasheetfor AOSS21319C, opens in a new tab
1 more part has no package on record. Nothing rules them out, so they stay.
- TLV803ED18DPWprinted IUVoltage supervisor, 5 pins, Texas InstrumentsVoltage supervisor5 pinsTexas Instruments
  Printed IU on the part. Easy to misread as 1U. Check that character with a loupe before you order anything.
  What it is
  Voltage supervisor
  Maker
  Texas Instruments
  Package
  Not recorded
  Pins
  5
  Details
  Low Power 250-nA IQ and Small Size Supply Voltage Supervisors
  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 TLV803ED18DPW, opens in a new tab

3 more parts have codes that begin with 1U

Codes that begin with 1U

For a marking with its end worn or burnt off. Each one shows its full printed code.

LDO regulator, all SOT-23, 3 pins, all Texas Instruments.

TPS7A0520PDBZprinted 1U8WSOT-23
Printed 1U8W. Only the start matches what you typed, so this fits only if the rest of the marking is worn off.
What it is
LDO regulator
Maker
Texas Instruments
Package
SOT-23
Pins
3
Details
1-µA Ultralow IQ, 200-mA, Low-Dropout Regulator in a Small-Size Package
Source
From the manufacturer's datasheet
Is it dead?
Voltage, board powered, black probe on ground. Then ohms with the power off.
Working, it reads
Input pin at the supply, output pin at its rated voltage, and the enable pin high if it has one.
Dead, it reads
Output equal to the input: the pass element has shorted. Everything after it may have taken the full input.
Input present, enable high, no output: failed open, or holding itself off into a shorted load. Power off and measure output to ground.
Power off, input to output near 0 Ω: shorted.
On the board:
An output at 0 V is often a shorted load with the regulator in current limit. Lift the output pin: if the short stays on the board, the regulator may be fine.
Compare:
The same regulator on another rail or another board shows the right voltages and the right resistances.
Watch for:
Makers put input, output and ground on different pins in the same package, so take them from this part's datasheet. Diode or ohms readings between its pins say little; check it powered.
A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.
Datasheetfor TPS7A0520PDBZ, opens in a new tab
TPS7A0522PDBZprinted 1U9WSOT-23
Printed 1U9W. Only the start matches what you typed, so this fits only if the rest of the marking is worn off.
What it is
LDO regulator
Maker
Texas Instruments
Package
SOT-23
Pins
3
Details
1-µA Ultralow IQ, 200-mA, Low-Dropout Regulator in a Small-Size Package
Source
From the manufacturer's datasheet
Is it dead?
Voltage, board powered, black probe on ground. Then ohms with the power off.
Working, it reads
Input pin at the supply, output pin at its rated voltage, and the enable pin high if it has one.
Dead, it reads
Output equal to the input: the pass element has shorted. Everything after it may have taken the full input.
Input present, enable high, no output: failed open, or holding itself off into a shorted load. Power off and measure output to ground.
Power off, input to output near 0 Ω: shorted.
On the board:
An output at 0 V is often a shorted load with the regulator in current limit. Lift the output pin: if the short stays on the board, the regulator may be fine.
Compare:
The same regulator on another rail or another board shows the right voltages and the right resistances.
Watch for:
Makers put input, output and ground on different pins in the same package, so take them from this part's datasheet. Diode or ohms readings between its pins say little; check it powered.
A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.
Datasheetfor TPS7A0522PDBZ, opens in a new tab
TPS7A0527PDBZprinted 1UAWSOT-23
Printed 1UAW. Only the start matches what you typed, so this fits only if the rest of the marking is worn off.
What it is
LDO regulator
Maker
Texas Instruments
Package
SOT-23
Pins
3
Details
1-µA Ultralow IQ, 200-mA, Low-Dropout Regulator in a Small-Size Package
Source
From the manufacturer's datasheet
Is it dead?
Voltage, board powered, black probe on ground. Then ohms with the power off.
Working, it reads
Input pin at the supply, output pin at its rated voltage, and the enable pin high if it has one.
Dead, it reads
Output equal to the input: the pass element has shorted. Everything after it may have taken the full input.
Input present, enable high, no output: failed open, or holding itself off into a shorted load. Power off and measure output to ground.
Power off, input to output near 0 Ω: shorted.
On the board:
An output at 0 V is often a shorted load with the regulator in current limit. Lift the output pin: if the short stays on the board, the regulator may be fine.
Compare:
The same regulator on another rail or another board shows the right voltages and the right resistances.
Watch for:
Makers put input, output and ground on different pins in the same package, so take them from this part's datasheet. Diode or ohms readings between its pins say little; check it powered.
A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.
Datasheetfor TPS7A0527PDBZ, opens in a new tab

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