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I have a relay which is rated for 30 VDC @ 10 A or 28 VDC @ 10 A (which appears on this relay module) and I am running 12 V @ 6 A through the relay and this solenoid.

After a short amount of time (hundreds of triggers) I still hear the relay clicking but there is no longer continuity across the contacts. I have read that I can possibly put a non-polarized capacitor across the switch contacts to prevent sparking. Is it fair to assume the relay is failing due to sparking (even though it does not appear to be welding) and if so, what size capacitor should I implement?

Update 2018-03-19

Here is the schematic of how I'm currently running the circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

If I've understood answers correctly, here is what I should do to avoid sparking and damaging the relay:

schematic

simulate this circuit

I'm now considering using this diode.

Update 2018-03-22

I've placed a flyback diode as in the second schematic. When touching the connections in place of where the relay will be there is still some slight sparking but it significantly reduced. I may have to look for other ways to continue to reduce sparking.

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    \$\begingroup\$ A schematic would help. Do you have a flyback diode antiparallel with the solenoid? Can you remove the relay from the circuit and test it in isolation? \$\endgroup\$ – calcium3000 Mar 16 '18 at 21:43
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    \$\begingroup\$ The relay makes a nice arc generator without a power diode on 1 Henry \$\endgroup\$ – Sunnyskyguy EE75 Mar 16 '18 at 22:30
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From your description you have no diode across your solenoid coil.
That will cause a very large voltage to develop across the contacts of your relay as they open. This cause a spark and will degrade your solenoid contacts.

Simply place a diode across the solenoid to protect the contacts:

schematic

simulate this circuit – Schematic created using CircuitLab

Almost any general purpose diode such as a 1N400x series would work as it only has to tolerate the pulse power as the magnetic field decays.

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    \$\begingroup\$ If Solenoid uses 6A then Diode must also handle 6A without overheating for duration of L/R decay time. \$\endgroup\$ – Sunnyskyguy EE75 Mar 16 '18 at 22:29
  • \$\begingroup\$ As a side question, is it a requirement that the relay be on the negative side of the circuit? I originally placed the relay on the positive side of the circuit. I might update my post to show that. \$\endgroup\$ – Troy Weber Mar 19 '18 at 18:33
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    \$\begingroup\$ It makes no difference whether you use the driving relay for the solenoid in the high side or low side. \$\endgroup\$ – Jack Creasey Mar 19 '18 at 19:16
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The solenoid is highly inductive. It stores lots of energy. When you open the circuit the energy from the collapsing magnetic field makes a big voltage spike which wears out your relay contacts due to arcing. You must do something. You could start with a freewheel diode across the solenoid coil. You could use a RCD turn off snubber to limit the voltage spike. You could use a RC snubber to limit the rate of rise of the prospective volt spike so the contacts have time to get some distance between them and hence will not spark. You could use a Zener to limit voltage also. This problem is much worse with modern small relays. Large old school relays had much greater contact spacing. If you treat the relay as if it was a solid state device you won't have this failure.

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