A diode is used with a relay primarily to protect the driving circuit (like an Arduino) from voltage spikes generated when the relay coil is de-energized.
Here's a breakdown of why this is crucial:
The Problem: Inductive Kickback (Flyback Voltage)
- Relays are inductive loads. The relay coil acts like an inductor. When current flows through an inductor, it stores energy in a magnetic field.
- When the current is switched off, the magnetic field collapses. This rapid change in magnetic flux induces a voltage in the coil itself, often in the opposite direction of the original voltage. This is called inductive kickback, flyback voltage, or back EMF (electromotive force).
- This voltage spike can be significant, potentially reaching hundreds of volts, even if the original voltage was just 5V.
- Without protection, this voltage spike can damage sensitive components like transistors in the Arduino or other microcontrollers used to control the relay.
The Solution: The Flyback Diode (also called a snubber diode)
- A diode is placed in parallel with the relay coil, with the cathode (the end with the band) connected to the positive side of the power supply and the anode connected to the negative side. This diode is reverse-biased during normal operation, meaning it doesn't conduct current.
- When the relay is de-energized, and the flyback voltage is generated, the diode becomes forward-biased.
- The diode provides a path for the flyback current to circulate around the relay coil, dissipating the stored energy. This clamps the voltage spike to a safe level (typically around 0.7V above the supply voltage).
Why This Matters: Practical Benefits
- Protection of the driving circuit: Prevents damage to microcontrollers, transistors, and other sensitive components.
- Increased reliability: Extends the lifespan of the control circuit by preventing voltage stress.
- Stable operation: Ensures the microcontroller operates reliably without spurious resets or malfunctions caused by voltage spikes.
Example Circuit
[Power Supply (+)] ----> [Diode (Cathode)]
^
|
[Relay Coil]
|
v
[Power Supply (-)] ----> [Diode (Anode)]
^
|
[Transistor (controlled by Arduino)]In this diagram:
- The diode is placed in reverse parallel across the relay coil.
- The transistor, controlled by the Arduino, switches the current to the relay coil.
Selecting the Right Diode
- Choose a diode with a reverse voltage rating significantly higher than the supply voltage. A common choice is the 1N4001 or 1N4004 diode.
- The diode's forward current rating should be equal to or greater than the relay coil's current.
By using a diode with a relay, you protect your circuit from potentially damaging voltage spikes and ensure reliable operation of your project.
