I have been trying to control the grid circuit (Germany, 230V, controls 3 50W compact florescent bulbs) using a relay and Arduino Nano 5v pin. It usually works as expected, however sometimes it sticks to the on state (Both when connected through NC or NO). Namely, sometimes if the lights on, they will never turn back off. So far, only waiting seems to be a working fix.

I don't know about the circuit diagram of the relay, but the product is hyperlinked. Basically, live and neutral of an extension cord was connected to the COM1-COM2 channels, the output was both either NC or NO. And the signal (white) sockets were connected to each other with another cable.

In the photo, the neutral was also connected directly (and no signal for the 2nd relay obviously), however the problem still exists in the both versions setup

So, I have limited electronics knowledge, and basically trying to understand what is happening and how to fix it.

Edit: Correction to voltage, specifications for bulbs + relay


  • 6
    \$\begingroup\$ Please post a diagram of your circuit....the devil is in the details. \$\endgroup\$
    – Aaron
    Apr 6 at 17:51
  • 2
    \$\begingroup\$ What kind of bulbs? Is it mechanical relay sticking or something else (ie. you tap the relay smartly and it opens or not)? \$\endgroup\$ Apr 6 at 17:56
  • \$\begingroup\$ Mechanical relays can weld themselves shut on surge currents. With incandescent bulbs, that might mean they have to be rated nearer the cold current than the hot (operating) current. \$\endgroup\$ Apr 6 at 20:17
  • \$\begingroup\$ My experience of Songle brand relays us that they are low quality Using them on mains voltages or at anything like rated current is risky. What you are experiencing is not surprising based in my experiences if them. \$\endgroup\$
    Apr 9 at 13:12

2 Answers 2


When standard electromagnetic relays are used to switch CFLs, their contacts would get welded on account of high switch-on surge currents.

Switch-on surge currents with some CFLs could be as high as 17 A.

You would need to select a relay, from 'high-inrush current' models, considering the TV rating mentioned in the data sheet.

The TV rating, as per UL and CSA standards, is an assessment of the inrush current withstanding capability of relay contacts as follows:

TV3 - 51A, TV4 - 65A, TV5 - 78A and TV8 - 117A, TV10 - 141A, TV15 - 191A.

TV3 would be the right choice for your application. Using a separate contact for each lamp would further improve relay life.


Using an AC relay on a DC circuit often causes it to latch on.

DC relays draw current limited only by the DC resistance of the coil, so have great pull-in power for the armature. AC relays rely on the weaker field limited by inductance and requiring a shading coil to prevent "chatter". A relay designed for 220 VAC would be very over-powered by 220 VDC, and the core would likely become permanently magnetized by the excessive current, before it burns out.


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