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.

  • \$\begingroup\$ What's connected to them? \$\endgroup\$ – winny May 28 '18 at 11:00
  • 1
    \$\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

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.

  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.