How to reduce and eliminate sparking of electromagnetic relay contacts

On low power contacts electromagnetic relays rarely appears electric arcbut it often happens sincerely.

When quickly disconnecting a circuit that has inductance, has a significant EMF L (di / dt)which can exceed the breakdown voltage of the insulation gap between the contacts. This is especially dangerous with sensitive and fast-acting electromagnetic relays, where the contact gap is very small.

It increases sincerely when the contacts vibrate. It shortens the service life of electromagnetic relay contacts and can cause false alarms on high-speed control circuit devices or failure of semiconductor elements due to overvoltage.

To reduce the arcing of the relay contacts, special circuits are used that create an additional electrical circuit through which the current caused by EMF of self-induction… In this case, the electrical energy stored in the inductance of the involved circuit is released as heat in the resistors of the spark-suppressing circuit, thereby reducing the sparking energy.

When using direct current, the load is shunted by a diode. The moment the relay contacts open, a transient current occurs and energy is released across the active component of the load resistance.

Spark extinguishing schemes

When connecting the contacts of the relay with a circuit RshSsh, the energy of the magnetic field is released not only at the load, but also at the resistor Rsh. The value of the capacitance Csh in this circuit is equal to 0.5 — 2 μF and is finally selected when tuning the circuit. The resistance Rsh is determined by empirical formulas. For silver contacts, Rsh = Uc2/ 140, where Uc is the voltage drop capacitor… The resistance value Rsh in circuits of low-current electromagnetic relays is 100 — 500 Ohm.

All spark suppression schemes worsen the dynamic parameters of electromagnetic relays, increasing the time for their on or off.