SMD code 55S
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.
55S on a chip resistor
36.5 Ω
36.5 Ω resistor, 1%
EIA-96 precision code: 55 selects 3.65 from the E96 series and S multiplies by 10. These are 1% parts. The trailing S is also used as a decimal point, so this could instead be 55 Ω. Measure it before fitting a replacement. If it has failed and reads open or high, that settles nothing: measure the same position on a working board.
Check it with a meter
Lift one end and measure across it. A good one reads 36.5 Ω within 1%. Measuring it in circuit reads the parallel combination of everything around it, which is almost always lower, never higher: well above 36.5 Ω on the board is a fault. Open, or well above 36.5 Ω, means it has failed: burnt, cracked or corroded. That does not change what to fit: the marking still says 36.5 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 (SMA, SMC, SMB), 55S is a part code instead. Those parts are listed below.
6 parts are printed 55S
Not sure which one you have?
Answer up to 4 quick questions and these 6 parts are ranked, with a meter test that settles it. It works on a dead part too.
BAT17-05SOT-23Schottky diode, 3 pins, InfineonSchottky diodeSOT-233 pinsInfineon
- What it is
- Schottky diode
- Maker
- Infineon
- Package
- SOT-23
- Pins
- 3
- Source
- Published code list
Is it dead? The meter test is below: Schottky diode.
BAT17-05WSOT-323Schottky diode, 3 pins, InfineonSchottky diodeSOT-3233 pinsInfineon
- What it is
- Schottky diode
- Maker
- Infineon
- Package
- SOT-323
- Pins
- 3
- Details
- dual ca BAT17
- Source
- Published code list
Is it dead? The meter test is below: Schottky diode.
MMBZ5255SPTSOT-23Zener diode, 3 pins, Chenmko EnterpriseZener diodeSOT-233 pinsChenmko Enterprise
- What it is
- Zener diode
- Maker
- Chenmko Enterprise
- Package
- SOT-23
- Pins
- 3
- Details
- Zener diode: 27.44..28.56V, Izt=5mA, Zzt=44 , 225mW
- Source
- Published code list
Is it dead? The meter test is below: Zener diode.
MMCZ5255SPTSOT-23Zener diode, 3 pins, Chenmko EnterpriseZener diodeSOT-233 pinsChenmko Enterprise
- What it is
- Zener diode
- Maker
- Chenmko Enterprise
- Package
- SOT-23
- Pins
- 3
- Details
- Zener diode: 27.44..28.56V, Izt=5mA, Zzt=44 , 225mW
- Source
- Published code list
Is it dead? The meter test is below: Zener diode.
MMHZ5255SPTSOD-123Zener diode, 2 pins, Chenmko EnterpriseZener diodeSOD-1232 pinsChenmko Enterprise
- What it is
- Zener diode
- Maker
- Chenmko Enterprise
- Package
- SOD-123
- Pins
- 2
- Details
- Zener diode: 27.44..28.56V, Izt=5mA, Zzt=44, 225mW
- Source
- Published code list
Is it dead? The meter test is below: Zener diode.
UM14240SSOT-23LDO regulator, 3 pins, Union SemiconductorLDO regulatorSOT-233 pinsUnion Semiconductor
- What it is
- LDO regulator
- Maker
- Union Semiconductor
- Package
- SOT-23
- Pins
- 3
- Details
- Linear voltage regulator IC: LDO, 4.0V±2%, 300mA
- Source
- Published code list
Is it dead? The meter test is below: Voltage regulator.
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.
Schottky diode
Diode range, part lifted or one leg off the pad. Red probe on the anode.
Working, it reads
About 0.15 to 0.45 V anode to cathode, roughly half a silicon diode, and open the other way.
Dead, it reads
- The same reading both ways round, near 0 V with a continuous beep or the same middling value: shorted.
- A reading the wrong way round as well: leaky.
- Open both ways: failed open.
- 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 large Schottky can read about 0.1 V forward and beep like a short, and leaks a little the wrong way when good, more when warm. It is shorted only if it reads the same both ways. Measure it cold, beside an identical cold one.
A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.
Zener diode
Diode range for the forward check, part lifted, red probe on the anode. A bench supply for the Zener voltage.
Working, it reads
About 0.6 to 0.8 V forward and open in reverse on the diode range. Its Zener voltage shows only on a supply: feed it through a resistor that sets about 5 mA, cathode to positive, read quickly, and it should hold near its rated voltage.
Dead, it reads
- The same reading both ways round, near 0 V with a continuous beep or the same middling value: shorted.
- Open both ways, forward too: failed open.
- Holds well off its rated voltage on the supply test: drifted.
- A soft knee, or a voltage that creeps on the supply test: 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 meter's diode range stops at about 2 to 3 V, so almost every Zener reads open in reverse on it, good or bad; a 3 V meter may just show a 2.4 or 2.7 V part. Above about 3 V the meter cannot tell a Zener from a plain diode.
A dead part still carries its marking. A short or an open says it has failed, not that it is a different part.
Voltage regulator
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.
Printed with a code easy to misread as 55S
One character differs, and it is one that is easy to confuse under a lamp. Each code has a page of its own.
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