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I have a NO DPDT relay, non latching, 12V coil controlled by transistor. One pole is switching fine. The other pole is stuck closed until I give it a few taps with a screwdriver. These are new production boards and the issue is seen on a handful of units.

I assume that the design is ok given that 1 pole is switching. Could this be a batch problem with the relay or a SMT process issue. I would be keen to hear if anyone has experience this before and what caused the issue. The relays are made by NEC and purchased through proper channels. I don't believe that the boards are washed meaning that no liquid is getting into the relays.

Thank you

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  • \$\begingroup\$ "Could this be a batch problem with the relay or a SMT process issue" yes. Quite possible. \$\endgroup\$ Nov 26, 2021 at 10:54
  • \$\begingroup\$ I have also ruled our contact welding as these relays are new off the production line and have not had any current through the switched side. \$\endgroup\$
    – RoyalAce
    Nov 26, 2021 at 10:58
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    \$\begingroup\$ Any evidence the relays were overheated during the SMT process? Is there a vent, and if so, was it possibly opened prematurely? \$\endgroup\$ Nov 26, 2021 at 11:21
  • \$\begingroup\$ Thanks Spehro - what signs would I be looking for? I dont see any vents on this relay or in the datasheet. \$\endgroup\$
    – RoyalAce
    Nov 26, 2021 at 14:36
  • \$\begingroup\$ Doesn't "NO" exclude a DT relay ? How can it be both ? \$\endgroup\$
    – tobalt
    Nov 26, 2021 at 17:26

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The relay contacts may stuck if the current flow at the moment the contacts are connected is quite large. This can be thought as some sort of welding.

If a contact pair in a DPDT relay is switching a sub-circuit which causes large inrush currents (e.g. capacitive loads such as an off-the-line device having a bridge rectifier and a large capacitor at its input) then it's quite normal to have the contacts stuck. You can use inrush-type relays for this purpose.

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  • \$\begingroup\$ No load has been connected. Hence, not contact welding. Thanks \$\endgroup\$
    – RoyalAce
    Nov 26, 2021 at 11:12

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