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This is a board with 80 3535 LEDs on it just after completing the reflow cycle in my home-made reflow oven. Why are the bottom corner LEDs lit up? Rather, how is this happening?

LED board inside a reflow oven.

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  • \$\begingroup\$ What's connected to them? \$\endgroup\$ – winny May 28 '18 at 11:00
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    \$\begingroup\$ That's not a microwave oven, right? \$\endgroup\$ – Olin Lathrop May 28 '18 at 11:33
  • \$\begingroup\$ It’s not a microwave oven. Do microwave reflow ovens exist? Nothing is connected to them which is why the Q. \$\endgroup\$ – electrophile May 28 '18 at 13:35
  • \$\begingroup\$ @electrophile It would be an extraordinarily bad idea to try to heat a PCB in a microwave oven. \$\endgroup\$ – Spehro Pefhany May 28 '18 at 14:14
  • \$\begingroup\$ I agree. It’s a regular oven with resistive heating elements modified to be a reflow oven. \$\endgroup\$ – electrophile May 28 '18 at 14:20
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I'm going to guess that it has to do with the thermocouple I see touching the board vs. the metal insides of the oven (which are touching the bottom of the PCB).

Insulation conductivity increases exponentially with temperature so a small amount of leakage at room temperature can become quite significant at red heat.

It probably indicates that your oven interior is not properly earthed.

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  • \$\begingroup\$ There are through hole pads on the board that connect to the LED cathodes and anodes. The board is placed on an Alu plate which is again placed on a steel tray that came with the oven. There is a small earthing tab inside the oven which is connected to earth. I check the earthing of the oven with a ground fault tester and it showed no faults. How do I check the interior earthing? \$\endgroup\$ – electrophile May 28 '18 at 13:42
  • \$\begingroup\$ Check the resistance (at room temperature) between the aluminum plate and the earthing tab. It should be close to zero. That will steer any leakage current out the ground pin. \$\endgroup\$ – Spehro Pefhany May 28 '18 at 14:13
  • \$\begingroup\$ Yep that is zero. Now that I think about it, it might be some stray leakage current is flowing through the thermocouple probe through the LEDs. \$\endgroup\$ – electrophile May 28 '18 at 14:19
  • \$\begingroup\$ @electrophile That's another possibility, but I didn't want to go there first because it gets complicated- if you partially short the T/C you can have the oven grossly overheat. \$\endgroup\$ – Spehro Pefhany May 28 '18 at 14:21
  • \$\begingroup\$ Do you think that’s the case here? \$\endgroup\$ – electrophile May 28 '18 at 14:44

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