SMD code 4120
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.
4120 on a chip resistor
412 Ω
412 Ω resistor, 1%
412 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 412 Ω within 1%. Measuring it in circuit reads the parallel combination of everything around it, which is almost always lower, never higher: well above 412 Ω on the board is a fault. Open, or well above 412 Ω, means it has failed: burnt, cracked or corroded. That does not change what to fit: the marking still says 412 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 (SOD-323), 4120 is a part code instead. Those parts are listed below.
The only part printed 4120 in our tables
BL4120WS
- What it is
- Switching diode
- Maker
- Galaxy Microelectronics
- Package
- SOD-323
- Pins
- 2
- Details
- 0.2A, 120V, Surface Mount Small Signal Switching Diodes
- Source
- Manufacturer datasheet
Is it dead?
Diode range, part lifted or one leg off the pad. Red probe on the anode.
Working, it reads
About 0.5 to 0.8 V anode to cathode, and open the other way. Small switching diodes read about 0.6 to 0.7 V; large rectifiers can read a little lower.
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: failed open.
- A reading the wrong way round as well: 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 three-pin body can hold two diodes. With a common pin they read two junctions from that pin, just like a transistor; in series they read two drops end to end. Test each pair the datasheet names.
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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