Choosing The Right Relay Triggering Mechanism For Your Microcontroller Application

Understanding Relay Triggering Mechanisms

Electromagnetic Relays

Electromagnetic relays contain a coil that activates an electromagnet when current flows through it. The electromagnet generates a magnetic field that pulls a contact armature and switches the relay contacts. The coil current can be controlled from a microcontroller by toggling a digital output pin high or low. Example microcontroller code to control an electromagnetic relay is shown below:

int relayPin = 8;  // Relay connected to digital pin 8

void setup() {
  pinMode(relayPin, OUTPUT); // Set pin as output
}

void loop() {
  digitalWrite(relayPin, HIGH); // Turn relay on
  delay(1000); // Wait 1 second
  digitalWrite(relayPin, LOW); // Turn relay off 
  delay(1000); // Wait 1 second
}

Solid State Relays

Solid state relays contain no moving parts. Instead they use transistors, silicon controlled rectifiers (SCRs), triacs or other semiconductor switching devices to control the relay contacts. This makes them faster, silent, and more reliable than electromagnetic relays. Example microcontroller code to control a solid state relay is shown below:

int relayPin = 8; // SSR connected to pin 8

void setup() {
  pinMode(relayPin, OUTPUT); // Set pin as output
}

void loop() {
  digitalWrite(relayPin, HIGH); // Turn SSR on
  delay(500); // Wait 0.5 secs
  digitalWrite(relayPin, LOW); // Turn SSR off
  delay(500); // Wait 0.5 secs  
}

Matching Relay Characteristics to Your Application

When selecting a relay, it is important to match its electrical ratings and contact configuration to your specific microcontroller application’s requirements.

Table of Contents

Consider Voltage and Current Ratings

Make sure to choose a relay that has voltage and current ratings higher than what your application demands. Check the pickup voltage and sensitivity for electromagnetic relays. Also confirm the maximum switching voltage and current for solid state relays. Overload conditions can damage relay contacts.

Pay Attention to Contact Configuration

Relay contacts come in various configurations like NO (normally open), NC (normally closed) and changeover contacts. Choose contact types that suit your microcontroller circuit for appropriate open/closed logic.

Choose Between Electromechanical and Solid State Relays

Consider size, lifespan, switching speed and isolation requirements when deciding between electromechanical and solid state relays. Electromechanical relays are more versatile but solid state relays are more reliable and better for fast switching logic.

Example Code for Relay Configuration Checks

// Check relay voltage ratings
if (relayVoltage <= 5V) {
  // Use 5V relay
}
else {
  // Use appropriate higher voltage relay  
}

// Check relay current rating 
if (relayLoadCurrent >= 2A) {
  // Use relay with > 2A current rating
}
else {
  // Use relay with lower current rating
} 

// Check normally open/closed contacts
if (circuitNeedsNO) {
  // Use NO relay
}
else {
  // Use NC relay
}

Integrating Relays with Microcontrollers Safely

When interfacing relays with microcontrollers, proper protective measures should be taken to avoid damaging sensitive controller electronics.

Protecting Digital I/O Pins from Back EMF with Flyback Diodes

Flyback diodes are connected across relay coils to give induced back EMF voltage spikes a safe path to ground instead of microcontroller pins. Add diodes in parallel with inductive DC loads like relays.

Adding Fuses or Current Limiting Resistors

Fuses and resistors limit the current flow through relay coils to prevent microcontroller pin overcurrent damage. They act as protective measures for the controller side circuitry.

Isolating High Voltage Relays

Optocouplers and optoisolators can isolate low voltage microcontroller circuits from higher voltage relay contact circuits for improved safety and prevented electrical noise issues.

Example Integration Schematic

             5V               _______________
            ____              |             |
           |    |             |   Relay     |
Microcontroller -|>|-----+->---|Coil         |
                       |     |             |
                       -->|--|Flyback Diode|
                           |_______________|

Choosing the Optimal Relay Triggering Method

Microcontroller output pins can directly drive small signal relays, but higher power relays require external components for appropriate current and isolation needs.

Direct Driving from a Digital Pin

Low current electromagnetic or solid state relays can be triggered simply by connecting to a microcontroller I/O pin configured as a digital output.

Using a Transistor for Higher Current Loads

BJT, MOSFET or darlington transistors amplify current from pins to drive relays requiring more power. Base/gate activation is still controlled via code.

Employing a Relay Driver IC for Multiple Relays

Relay driver ICs integrate transistor arrays to independently control multiple relay coils while isolating pins and handling larger currents.

Example Circuits for Relay Triggering Methods

// Direct drive
digitalWrite(relayPin, HIGH); 

// With transistor
digitalWrite(transistorBase, HIGH);

// Using driver IC 
relayDriver.setRelay(1, ON);
relayDriver.setRelay(2, OFF);