What causes a relay contact to stick (weld)?
Contact welding happens when the arc formed at the moment of closing or opening, together with high inrush current, locally melts the contact surface. The most common causes: the load's inrush current exceeding the contact rating, selecting by AC rating for a DC load, no suppression component across the coil, and contact bounce under vibration due to a loose socket.
Welding is a thermal, not a mechanical event. As the contacts close, an arc forms across a very small gap just before full contact; the same happens on opening. That arc melts the metal at a point. When the molten surface cools, the two contacts weld together and the relay no longer releases. The result is dangerous: the load stays energised permanently and out of control.
Causes and countermeasures:
- High inrush current. Motor and lamp loads draw several times the continuous current at start. Select the contact according to the inrush current, not the continuous current.
- Wrong selection for a DC load. In DC the current does not pass through zero, so the arc does not extinguish and lasts longer. A contact selected by its AC rating wears out quickly in DC.
- Absence of a suppression component. With an inductive load, the interruption spike feeds the arc. A reverse diode across the coil or a suppression component suited to the load must be fitted.
- Poor contact and vibration. A loose socket or vibration makes the contact bounce, meaning more than one arc forms on every operation. Mount the relay rigidly.
- Frequent switching. A contact opening and closing at short intervals never gets a chance to cool.
Diagnose first: if the voltage measured across the contact while the load is running exceeds 0.5 V, the surface is already damaged and heading for welding. Do not repair a welded relay; replace it and eliminate the cause.
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