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I'd like to switch a power contactor (such as this one : https://www.legrand.fr/pro/catalogue/31925-bipolaires-250-v/contacteur-de-puissance-cx3-bobine-230v-sans-commande-manuelle-2p-250v-25a-contact-2f-1-module)

To switch the contactor, I use a classic SPDT relay connected on the arduino board, The spdt relay is switching 220 VAC, so the contactor is activated

There is no load (yet) on the contactor (but I assume the situation will be worse after!).

The problem is that switching the coil of the contactor generates lots of EMI with impact on near devices (glitches on screens, disconnections of USB devices aso)

We tried to put zero crossing (switching the SDPD relay when AC cross 0 Volts), which improve the situation on power-on but changes nothing (or almost) when swiching the relay off (I assume this is because the current on the relay is still present).

We also put a Metal Oxyde Varistor in parallel of the relay, which improve the situation, but EMI levels are still very high.

I think I undestand the phenomenon behind these EMI (coil tend to keep the current constant, so switching off produce very high voltage peaks), but how can we improve the situation?

Some documentation suggest using triacs instead of a spdt relay, but I don't understand how such component can improve the situation...

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  • \$\begingroup\$ "A contactor is typically controlled by a circuit, which has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch." - wiki. Why do you have to use another relay ? \$\endgroup\$ – Long Pham Aug 23 '18 at 9:32
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    \$\begingroup\$ Because I want to be able to use various kind of contactor which can be put on DIN rails. There is not so much 24VDC contactors (which will of course solve the issue with a freewheel diode). There are 24VAC contactors but it is far more expensive, and problem with remain the same (at a lower lever). \$\endgroup\$ – Golgot Aug 23 '18 at 11:06
  • \$\begingroup\$ That is useful for your question. Triacs help because they don't have any physical contacts (they are used in solid state relay). I'm not an expert but maybe you should try TVS diode instead (they are faster than MOVs) and a RC snubber to the relay. \$\endgroup\$ – Long Pham Aug 24 '18 at 15:16

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