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I am using a relay SRD-5VDC-SL-C(generic module made for diy projects) to control AC current going to my SMPS(120-300v AC to 12V, 2A DC) driving my LED strip. The relay is controlled by a Lolin NodeMCU v3 microcontroller. The line from power outlet is connected to COM and the line to SMPS is connected to NO. The switching happens perfectly in the start, but if I keep the relay energized for a long time then when I turn the relay off it still lets the current flow and doesn’t switch. I checked the continuity and it is indeed between NO and COM even after turning the relay off(control pin high). Knocking the relay solves the problem.

I am fairly new to electronics. For a load like SMPS do I need some additional circuitry along with the relay to avoid this or is it a bad relay?

Thank you in advance, my iot project is on hold because of this.

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    \$\begingroup\$ You should provide a link to the manufacturer's datasheets for the relay, for the power supply, and for the LED strips (not links to the vendor where you purchased these items). \$\endgroup\$ – Elliot Alderson Dec 1 '18 at 16:06
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Assuming your relay is rated appropriately for your LED strip (it would be helpful if you posted a link to the LED strip's datasheet), it might be the inductance of your load that is keeping your relay from turning off.

Inductance can make the voltage spike right as the contacts try to open. This can cause arcing and weld the contacts together. You can cure this with a snubber or bi-directional TVS diode, or perhaps a different relay.

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  • \$\begingroup\$ My relay is between my wall outlet and SMPS which takes maximum 300V and gives out 12v(constant) at maximum 2A and the whole setup is stable and works normally without a relay. Just to clarify the SMPS is an inductive load? If so then I will look into your suggestion of using a snubber. \$\endgroup\$ – Jugen Gawande Dec 1 '18 at 16:27
  • \$\begingroup\$ I didn't realize your relay was on the mains side, but yes, the SMPS is likely an inductive load. The relay you have is adequately sized, so I would look into using a snubber. \$\endgroup\$ – evildemonic Dec 1 '18 at 16:50
  • \$\begingroup\$ SMPS inputs usually are highly capacitive so they take very high inrush current which may weld your contacts. \$\endgroup\$ – Justme Dec 1 '18 at 21:33
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Most of the electronics problems are due to imbalanced current and voltage calculations. You gotta consider the Relay rating for operating load using a relay. Look if e.g. your load requires 2-ampere and 230-volts, then you gotta use a relay with more(~50%) capacity say 3-ampere and 450-volts.(be on safe side :) ) Also you need to consider the copper thickness from external wire to relay NC-NO-COM pin on relay board.

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