SMD code R21M
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.
R21M is an inductor marking
0.21 µH
0.21 µH inductor, 20%
The R stands in for the decimal point, in microhenries: 0.21 µH. SMD power inductors below 10 µH are printed this way. M is the tolerance, ±20%.
Check it with a meter
Measure it on an LCR meter, out of circuit. A good one reads close to 0.21 µH. An ohmmeter only shows the winding resistance, which is a fraction of an ohm on a good part and says nothing about the value; open circuit means the winding has failed. Shorted turns still read near 0 Ω on an ohmmeter, so only the LCR meter, or an identical part beside it, shows them. A failed reading does not change what to fit: the marking still says 0.21 µH.
On a transistor or diode body (SOT-89-5), R21M is a part code instead. Those parts are listed below.
The only part printed R21M in our tables
R5510H021M
- What it is
- Not recorded
- Maker
- Nisshinbo Micro Devices
- Package
- SOT-89-5
- Pins
- 5
- Details
- CMOS-IC: LDO VR, 4,5V±2%, 300mA and VDet, -Reset ODO
- Source
- Published code list
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.
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