Engineering Reference
Diode Packages, Polarity and Markings Reference
Lookup common diode packages, cathode and polarity markings, SMD identification limits, and package-dependent thermal and assembly checks.
Package Family Lookup
| Package | Construction | Common use | Polarity / pinout | Engineering boundary |
|---|---|---|---|---|
| DO-35 | Axial glass | Small-signal and switching diodes | Band commonly marks cathode; verify part drawing | Lead length, copper area and ambient affect thermal behavior |
| DO-41 | Axial molded | General rectifier and protection diodes | Band commonly marks cathode | Current capability is part-specific, not package-only |
| SOD-123 / SOD-323 | Small SMD diode | Signals, Zener, Schottky and protection | Cathode line or package code; check drawing | Small thermal mass and land pattern require review |
| SMA / SMB / SMC | Molded power SMD | Rectifier and TVS applications | Cathode band common for rectifiers; TVS may be bidirectional | Power depends on PCB copper and transient waveform |
| SOT-23 | Three-lead SMD | Dual diodes, switching and reference circuits | No universal pinout | Identify exact suffix and internal schematic |
| TO-220 / TO-247 | Lead-mounted power package | High-current rectifiers and diode modules | Body symbol and datasheet define terminals | Tab may be electrically live; heatsink interface matters |
Polarity and Marking Rules
| Mark | Usual interpretation | Boundary |
|---|---|---|
| Cathode band | Commonly identifies K/cathode on single PN and Schottky diodes | Not a universal rule for every package or bidirectional device |
| Schematic bar | The bar side of the diode symbol is the cathode | Package orientation still needs the manufacturer drawing |
| Short SMD code | May identify a device family or assembly code | Codes are reused; package and manufacturer context are required |
| Zener label | Nominal voltage may appear in part number or ordering code | Voltage is defined at a test current and tolerance |
| TVS polarity | Unidirectional TVS parts have polarity; bidirectional parts often do not | Confirm internal schematic and marking diagram |
VF Context
Silicon PN, Schottky, Zener and wide-bandgap devices have different forward behavior, but no fixed VF range can identify a package or exact device. Compare VF only at stated current and junction temperature.
Assembly Checklist
- Match footprint and orientation drawing.
- Verify all pins and exposed tab connections.
- Use the specified PCB copper and thermal model.
- Check creepage, clearance and transient voltage.
- Confirm reflow or lead-forming limits.
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FAQ
Does the band on a diode mark the cathode?
For many single axial and molded diodes, yes. Verify the package drawing because bidirectional and special devices can use different conventions.
How do I identify an SMD diode?
Use the top code together with package dimensions, manufacturer context and circuit function. Short codes are not globally unique.
Is every SOT-23 diode pinout the same?
No. SOT-23 can contain single, dual and series-connected devices with different pin assignments.
Can package size determine diode current rating?
No. Die, leadframe, thermal conditions, waveform and PCB layout all matter.
Is the tab of a power diode isolated?
Not necessarily. The tab can be connected to a terminal, so check the datasheet before mounting it to a heatsink.
How is a bidirectional TVS marked?
Marking varies. Confirm the exact part number and internal schematic rather than relying on a cathode band.
Does forward-voltage range identify a diode?
No. VF varies with current, temperature and technology and is not a unique part identifier.
What must be checked before substituting a package?
Check pinout, polarity, footprint, creepage, voltage and current ratings, thermal path, surge behavior and assembly process.