Relay Stuck ON

Your relay is permanently ON even when the control signal is OFF. The connected device stays powered continuously. This guide covers welded contacts, stuck armature, control circuit failures, and power supply issues.

Last updated: April 22, 2026 | 10 min read

Symptoms

• Connected device stays ON even when dashboard shows relay OFF
• Audible clicking stops but load remains powered
• Relay LED indicator OFF but output still active
• Relay stuck after switching high-current load
• Multiple ON/OFF cycles cause relay to eventually stick permanently
• Burnt smell or visible damage around relay contacts

Common Causes

1. Welded Contacts High inrush current arcs and welds contacts together. Most common with inductive loads
2. Exceeded Rated Current Load current exceeds relay rating
3. No Snubber Circuit Missing RC snubber or varistor across contacts allows arcing
4. Mechanical Failure Spring broken or armature stuck due to debris or corrosion
5. Control Transistor Failed Short MOSFET or BJT driving relay coil stuck in ON state
6. Relay Coil Voltage Too Low Coil not fully engaging, causing partial contact and arcing
7. DC Load Without Flyback Diode Inductive DC load without diode causes arcing across contacts

Relay Contact Ratings

Load Type	Max Current	Inrush Factor	Snubber Required
Resistive	10A	1x	Optional
Inductive AC	5A	5-10x	Required
Inductive DC	3A	3-5x	Diode required
Capacitive	5A	20-40x	Required

Inrush current can be 5-10x rated current. Always derate relay for inductive loads.

Step-by-Step Fixes

1. Test the Relay with Multimeter

First, verify if relay is truly stuck or if the problem is elsewhere:

// 1. Disconnect power completely
// 2. Set multimeter to resistance () mode
// 3. Measure across COM and NO terminals:
//    - Should read 0  when relay is OFF  STUCK
//    - Should read infinite  when relay is OFF  GOOD
// 4. Measure across COM and NC terminals:
//    - Should read 0 when relay is OFF  GOOD
//    - Should read infinite when relay is OFF  STUCK 

// For 5V relay modules, check coil resistance:
// Measure across IN and GND 
// Should read 50-150 depending on module

2. Check Control Signal

Verify ESP32/ESP8266 is sending correct signal:

// Add this diagnostic code
void testRelay() {
  Serial.println;
  pinMode;
  
  digitalWrite;  // Relay ON
  delay;
  digitalWrite;   // Relay OFF
  delay;
  Serial.println;
}

// Measure voltage at relay IN pin:
// - When OFF : Should read 0V  or 3.3V 
// - When ON : Should read 3.3V  or 0V 
// If voltage changes but relay stuck, problem is mechanical

3. Test the Relay Module in Isolation

Disconnect from ESP32 and test relay alone:

// Remove IN wire from ESP32
// Connect relay IN directly to GND  or 5V 
// Listen for click:
// - If relay clicks and releases  Control circuit is fine, problem elsewhere
// - If relay stays stuck  Replace relay module

4. Inspect for Welded Contacts

Physical damage to contacts is common with high inrush loads:

• Remove power and disconnect load from relay
• Listen for click when activating relay
• If relay clicks but load stays ON Contacts are welded
• Tap relay gently with screwdriver handle sometimes frees stuck contacts
• Replace relay module if contacts are welded

5. Add Snubber Circuit

Prevent arcing across contacts for AC inductive loads:

// RC Snubber circuit across relay contacts 
// For 230V AC loads:
// Resistor: 100, 1W
// Capacitor: 0.1F, 400V 

// For 12-24V AC loads:
// Resistor: 47, 0.5W
// Capacitor: 0.1F, 100V

// Wiring: Connect snubber directly across relay COM and NO terminals

6. Add Flyback Diode

For DC inductive loads :

// Connect flyback diode across the LOAD 
// Diode: 1N4007 or 1N4148
// Cathode   Positive side of load
// Anode  Negative side of load

// For solenoid valves, connect diode directly at solenoid terminals
// This prevents back-EMF from welding relay contacts

7. Replace the Relay Module

If relay is mechanically stuck, replacement is the only solution:

• Purchase same relay module
• Disconnect power and remove old module
• Connect new module with same pinout
• Test with simple blink sketch before connecting load
• Consider higher current rating relay for same load

Prevention Tips

• Always derate relay by 50% for inductive loads
• Add RC snubber for AC motors, compressors, pumps
• Add flyback diode for DC solenoids, relays, valves
• Use solid-state relay for frequently switched loads
• Add soft-start circuit for motors to reduce inrush current
• Inspect relay contacts periodically if blackened or pitted, replace

Related Issues

• Relay Not Clicking Control signal or power issue
• Relay Clicking But No Power Wiring or load problem
• Relay Stuck OFF Opposite issue
• Power Supply Not Enough Current Coil voltage drop

Frequently Asked Questions

Still having issues? Contact Support or return to the Troubleshooting Hub.