Identifying the Anode and Cathode

A standard diode has two terminals:

• Anode (A)
• Cathode (K)

On many through-hole diodes, the cathode is identified by a stripe or band around the body.

        Cathode
           K
           |
───────|>|───
       ↑
     Anode

Always verify the marking against the manufacturer's datasheet when the physical marking is unclear.

Forward-Bias Connection

A diode is forward biased when the anode is at a higher potential than the cathode by a sufficient amount for the diode to conduct.

+V
 |
 |
 |   Anode       Cathode
 +─────|>|───────────+
                     |
                    Load
                     |
                    GND

In this configuration, conventional current can flow through the diode and the load when the applied voltage is sufficient.

Reverse-Bias Connection

A diode is reverse biased when the cathode is at a higher potential than the anode.

+V
 |
 |
 |   Cathode       Anode
 +─────|<|───────────+
                     |
                    Load
                     |
                    GND

A conventional diode normally blocks current in this condition, apart from a small leakage current.

The reverse voltage must remain within the diode's specified rating.

Using a Diode for Reverse-Polarity Protection

A diode can be placed in series with a power input to protect a circuit from accidental reverse connection.

Supply + ───|>|──── Circuit +
Supply - ────────── Circuit -

With correct polarity, the diode conducts. If the supply is connected backwards, the diode blocks the current path.

The disadvantage is that the diode introduces a forward-voltage drop. A Schottky diode may be useful in some low-voltage applications because of its lower forward voltage.

Diode Across a Relay Coil

A diode is commonly connected across a DC relay coil to suppress the voltage transient produced when the coil current is switched off.

        +V
         |
       Relay
       Coil
         |
         +────────|<|────────+
         |         Diode     |
         |                   |
        GND──────────────────+

The diode is normally reverse biased while the relay is energized. When the coil is switched off, the collapsing magnetic field produces a voltage transient and the diode provides a path for the coil current to decay.

The diode polarity is critical. Reversing it can create a short circuit across the supply while the relay is energized.

Bridge Rectifier Wiring

Four diodes can be connected as a bridge rectifier to convert an AC input into pulsating DC.

              + DC
               |
          ┌────|>|────┐
          │           │
 AC ~ ─────┤           ├──── + DC
          │           │
 AC ~ ─────┤           ├──── - DC
          │           │
          └────|<|────┘
               |
              - DC

In a practical bridge rectifier, two diodes conduct during each half-cycle of the AC waveform.

Commercial bridge-rectifier modules contain the four diodes internally and normally provide clearly marked AC, positive, and negative terminals.

Half-Wave Rectifier

The simplest rectifier uses a single diode.

AC ─────|>|──── Load ────
```


The diode conducts during one polarity of the AC waveform and blocks the
other polarity.



This produces a pulsating DC output rather than smooth DC. A capacitor can
be added to reduce the ripple in a suitable power-supply design.

Zener Diode Wiring

A Zener diode is commonly connected in reverse bias when used as a simple voltage reference or shunt regulator.

        +V
         |
       Resistor
         |
         +────── VOUT
         |
        ─|<|─
        Zener
         |
        GND

The series resistor is important because it limits current through the Zener.

The resistor and Zener must be selected so that the current and power remain within their ratings.

LED Wiring

An LED is a diode and therefore has polarity.

• Longer lead on many through-hole LEDs → anode
• Shorter lead → cathode
• Flat edge on many LED packages → cathode

However, package markings can vary, so the datasheet should be used when polarity is uncertain.

Current-Limiting Resistor

An LED normally requires a suitable current-limiting method. A common approach is a series resistor.

+V ─── Resistor ───|>|─── GND
                   LED

A simple resistor estimate is:

R = (VS − VF) / IF

where:

• VS = supply voltage
• VF = LED forward voltage
• IF = desired LED current

Schottky Diode Wiring

Schottky diodes are wired in the same basic polarity arrangement as conventional diodes.

They are commonly used where low forward voltage and fast switching are important.

Typical applications include:

• Switching power supplies
• DC-DC converters
• Rectifiers
• Reverse-polarity protection

Check the reverse-voltage and current ratings carefully because Schottky diodes are available in many different configurations.

TVS Diode Wiring

A TVS diode is commonly connected across the circuit or power line that needs transient protection.

+ Supply ─────────────── Circuit
    |
   TVS
    |
GND ─────────────────── Circuit

The exact connection depends on whether the TVS device is unidirectional or bidirectional and on the circuit being protected.

The TVS must be selected according to the normal operating voltage and the transient conditions it is intended to suppress.

Diode Orientation on a PCB

When replacing a diode on a PCB, look for the polarity marking on the board. Common markings include:

• A diode symbol printed on the PCB
• A band or line marking the cathode
• A "+" marking for certain specialized circuits
• Component reference designators such as D1, D2, D3

Always compare the PCB marking with the schematic and the original component when possible.

SMD Diode Polarity

SMD diode polarity markings vary considerably between manufacturers and package types.

A line, stripe, dot, or other marking may identify the cathode, but the exact meaning depends on the component.

Do not assume that every SMD diode uses the same marking convention. Use the part number and datasheet whenever possible.

Testing Wiring with a Multimeter

A multimeter's diode-test mode can help verify whether a diode is correctly connected and functioning.

Forward Test

• Red probe → anode
• Black probe → cathode
• A forward-voltage reading should normally appear

Reverse Test

• Reverse the probes
• A conventional diode should normally indicate an open or very high resistance condition

Some specialized diodes may produce different readings. Always consider the diode type before interpreting the measurement.

Common Wiring Mistakes

• Reversing the diode polarity
• Ignoring the cathode marking
• Using a diode with insufficient reverse-voltage rating
• Using a diode with insufficient current rating
• Connecting an LED without suitable current limiting
• Installing a relay suppression diode with incorrect polarity
• Using a slow diode in a high-frequency switching circuit
• Failing to check the PCB polarity marking

Important Safety Notes

• Disconnect power before changing diode wiring.
• Discharge capacitors before servicing power-supply circuits.
• Be particularly careful with mains-powered equipment.
• Verify diode polarity before applying power.
• Check the diode's voltage, current, and power ratings.

Quick Diode Wiring Checklist

• ✔ Identify the diode type
• ✔ Identify anode and cathode
• ✔ Check the schematic polarity
• ✔ Check the PCB marking
• ✔ Verify voltage rating
• ✔ Verify current rating
• ✔ Check power dissipation
• ✔ Consider switching speed where applicable
• ✔ Check the circuit before applying power

Key Points

• Diodes are polarized components.
• The cathode is commonly identified by a band on through-hole diodes.
• Correct polarity is essential for rectification and protection circuits.
• LEDs require appropriate current limiting.
• Zener diodes require suitable current limiting when used in shunt circuits.
• Relay-coil suppression diodes must be connected with the correct polarity.
• Always verify SMD diode markings using the component datasheet when necessary.