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I was watching one of my seniors soldering a few SMD LEDs on one of his power converters. He touched the iron on the anode of one of the LEDs and it glowed. I think it is because of thermal generation.

Later, I heard him talking about the LED not glowing if a current limiting resistor is present.

Am I correct in thinking that thermal generation of electrons might be the cause here? Also, I probably saw one of the LEDs glowing when the iron was touched on its cathode. Is this possible?

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Yes it is possible but unlikely from thermal generation.

Much more likely cause is having mains leakage current via soldering iron, or the iron just closing a circuit for current to flow.

The circuit has to complete via some route so simply the soldering iron itself does not explain the phenomenon.

The soldering iron may be the terminal for providing the leakage or providing the ground for leakage, the other terminal is unknown.

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  • \$\begingroup\$ Is the iron a safety hazard or is it safe to use? It seemed to me that the iron is old. From what I can remember, it had ESD safe written on it. Will such an iron be ESD safe if it leaks current? \$\endgroup\$ Dec 6, 2023 at 11:02
  • \$\begingroup\$ @Jonathan_the_seagull We can't analyze that. If it is ESD safe it might be intentional that it has a grounded tip, so of course it will complete a circuit if leakage comes from somewhere else. So we can't possibly know if your soldering iron has been degraded and if it is a safety hazard, or are everything else you are connecting to degraded or safety hazard. \$\endgroup\$
    – Justme
    Dec 6, 2023 at 11:12
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I think it is because of thermal generation.

It can be because of the ungrounded tip.

Some soldering irons have grounded tips, some don't. Here's a possible scenario:

schematic

simulate this circuit – Schematic created using CircuitLab

In the overly simplified diagram above, Parasitic Res represents a parasitic resistance between the LED and the operator's non-isolated body part (hand, finger, etc). When the operator is touching the board or the LED's one leg a path may be formed for a current to flow through the LED.

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  • \$\begingroup\$ But, the LED doesn't glow if a current limiting resistor is used. The resistance of the human body is much higher. Why is that? \$\endgroup\$ Dec 6, 2023 at 10:44

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