23. Diode Visual Recognition

A diode is a two-terminal semiconductor device that allows current to flow in one direction (forward) and blocks it in the opposite direction (reverse).
It acts as an electrical one-way valve, protecting and controlling circuits.

Key Points:

• Polarity: Anode (+) and Cathode (–).

  

• Forward voltage drop (VF): Voltage needed for conduction. Forward conduction begins when VF reaches the threshold voltage:
• Silicon: 0.6–0.7 V
• Schottky: 0.15–0.45 V
• Germanium: 0.2–0.3 V

Diode Forward and Reverse Biased Connections

Diode Forward and Reverse Biased Voltage–Current (V–I) Curve

1. Rectification: Converting AC to DC (half-wave, full-wave, bridge).
2. Reverse Polarity Protection: Prevents damage if the supply is connected in reverse.
3. Voltage Regulation: Using Zener diodes to maintain a fixed voltage.
4. Signal Demodulation: Extracting audio from AM radio signals.
5. Waveform Shaping: Clipping and clamping circuits.
6. Freewheeling/Flyback Protection: Protects switching devices from inductive spikes.
7. Light Emission: LEDs, IR diodes, laser diodes.
8. ESD Protection: Protecting ICs from electrostatic discharge.
9. RF Mixing & Detection: Schottky diodes in RF circuits.
10. Overvoltage Suppression: TVS diodes in automotive or industrial electronics.

A. Based on Construction / Material

• Silicon – General use.
• Germanium – Low VF, RF detection.
• Gallium Arsenide – Optoelectronics, high-frequency.

B. Based on Function

1. Forward Voltage Drop (VF) – Voltage needed for conduction; lower = better efficiency.
   • Silicon: ~0.6 - 0.7V
   • Germanium: ~ 0.2 - 0.3V
   • Schottky: ~0.15 - 0.45V
   • LED: ~1.8 - 3.6V depending on color.
2. Maximum Reverse Voltage (VRRM) – The Highest reverse voltage a diode can block before breakdown.
3. Maximum Average Forward Current (IF(AV)) – Continuous current a diode can conduct safely.
4. Surge Forward Current (IFSM) – Short pulse capability, e.g., startup surge in rectifiers.
5. Reverse Leakage Current (IR) – small current flows when the Diode is in reverse bias. Higher in Schottky diodes.
6. Reverse Recovery Time (trr) – Time taken to stop conducting when switching from forward to reverse bias.
   • Standard: ~2–30 µs
   • Fast recovery: <500 ns
   • Schottky: ~ns range.
7. Junction Capacitance (CJ) –Acts like a capacitor in reverse bias; matters in RF/high-speed use
8. Thermal Resistance (RθJA / RθJC) – Indicates how well the diode dissipates heat. Lower values mean better cooling and higher reliability
9. Power Dissipation (PD) – Maximum heat the diode can safely handle before failure.
10. Temperature Coefficient – Shows how VF and breakdown voltage vary with temperature (VF decreases as temperature rises).

Example 1N4001 diode Specifications:

A. Through-Hole

B. SMD

C. Special

Cathode Identification Markings

• Polarity: Cathode marked with a band, stripe, or dot.
• SMD marking: Cathode side often marked with a line/bar.

Anode and Cathode Identification using Multimeter

• Use a multimeter in diode mode (forward direction shows ~VF, reverse shows OL).

  

SMD Diode Marking Table

Reminder: SMD diode markings are not globally standardized. Always confirm with the manufacturer’s datasheet when identifying unknown diodes.

Marking	Images	Diode Type / Part No.	Package	Notes
A1		BAV99 (dual switching diode)	SOT-23	100 V, 215 mA dual small-signal switching diode
A2		BAV70 (dual switching diode)	SOT-23	70–100 V, 200 mA dual small-signal switching diode
T4		1N4148W (fast switching diode)	SOD-123	100 V, 150–200 mA fast switching
SS14		Schottky rectifier	DO-214AC (SMA)	40 V, 1 A, low forward drop
SS16		Schottky rectifier	DO-214AC (SMA)	60 V, 1 A, higher VR version of SS14
S4		Alias for SS14 (Schottky)	DO-214AC (SMA)	Manufacturer-dependent code (not unique)

• Zener voltages: 3.3V, 4.7V, 5.1V, 6.2V, 9.1V, 12V, 15V, 24V, etc.
• LED VF: Red ~1.8V, Green ~2.0V, Blue/White ~3.0–3.6V.
• Common parts: 1N4148 (fast signal), BAT54 (Schottky), 1N5408 (high current).

1. Match the Application
   1. Rectifier: For AC–DC conversion (power supplies).
   2. Clamping/Protection: For voltage limiting or surge protection (TVS/Zener).
   3. Regulation: Zener diodes for fixed reference voltages.
   4. Light Emission: LEDs for indication or illumination.
   5. Sensing: Photodiodes for light detection.
2. Voltage Rating (VRRM)
   1. Choose at least 2× the peak reverse voltage expected in the circuit to ensure safe operation and margin against spikes.
3. Current Rating (IF(AV))
   1. Select ≥ 1.5× the maximum load current to handle continuous operation and occasional surges.
4. Switching Speed
   1. For low-frequency rectification, standard diodes (like 1N400x) are fine.
   2. For high-frequency circuits (SMPS, RF), use Schottky (very fast, low VF) or fast recovery diodes.
5. Thermal Considerations
   1. Check Power Dissipation (PD) and Thermal Resistance (RθJA / RθJC).
   2. Ensure proper heat sinking or package choice to avoid overheating.
6. Special Requirements
   1. Low Leakage: Critical in precision or low-power circuits.
   2. LEDs: Select by wavelength, brightness, and viewing angle.
   3. Zener: Pick the breakdown voltage that matches your regulation needs.

• Reverse Polarity Protection: Schottky in series with the supply in the IoT board.
• 5.1V Zener Regulation: Reset line clamp in MCU circuits.
• Flyback Diode: Across motor driver outputs.
• USB ESD Protection: Low-capacitance TVS diodes on D+/D– lines.
• Bridge Rectifier: 230VAC to DC conversion in SMPS.

• Shorted Diode: Caused by overcurrent or overvoltage.
• Open Circuit: Physical damage or overheating.
• Increased Leakage: Aging or heat damage.
• LED Dim/Fail: Overcurrent or thermal degradation.

• Always observe polarity when soldering.
• Use heatsinking for high-power diodes.
• ESD precautions for sensitive types.
• Avoid exceeding surge current ratings.