What is a “sticking relay”?
A sticking relay occurs when the relay contacts inside the device remain closed even after an OFF command is sent, causing the connected load to stay powered.
This typically happens when the relay contacts become welded or mechanically stuck due to electrical stress.
Common symptoms include:
The load stays ON even after switching OFF
The device interface shows OFF but the load is still powered
The relay clicks but the load state does not change
The relay works intermittently or becomes stuck after switching a heavy load
Why does this happen?
The most common cause is high inrush current.
Some electrical devices draw a very large current for a short moment when they start, even if their normal operating power is low. This spike can damage or weld the relay contacts together.
Loads that commonly cause this include:
LED drivers and power supplies
Large LED lighting systems
Motors and pumps
Transformers
Capacitive loads
How to prevent relay sticking
If you are switching loads with high inrush current, additional protection is recommended.
Install an RC snubber
An RC snubber circuit helps suppress voltage spikes generated when switching inductive loads.
Benefits:
Reduces electrical stress on the relay contacts
Extends relay lifetime
Helps prevent contact welding
This solution is commonly used when switching motors, pumps, or transformers.
Use an inrush current limiter (NTC)
An NTC thermistor limits the startup current of devices that produce high inrush current.
Benefits:
Reduces the initial current spike
Protects relay contacts
Improves reliability when switching LED drivers or power supplies
Use a contactor
For larger loads, the recommended solution is to use a contactor.
In this setup:
The Shelly device controls the coil of the contactor
The contactor switches the actual load
Benefits:
The heavy load is handled by the contactor
The Shelly relay is protected from high current stress
Much higher switching durability