Soldering iron is giving off usable power

[BigBrother]

I tried to solder some Cree MX6 LEDs just now and I noticed that a couple of them lit up, oh so faintly. They are on an MCPCB (Metal Core PCB).

The soldering iron in question is an M-Line Mark V from Micro-Measurements.

[SeanB]

Soldering iron tip is not connected to ground properly.

[dog80]

Was the PCB powered or plugged in while you soldered it?
 
Since the tip of the solder iron is ground referenced you might complete a path to ground by touching it to the LED.

[BigBrother]

Was the PCB powered or plugged in while you soldered it?
 
Since the tip of the solder iron is ground referenced you might complete a path to ground by touching it to the LED.

No, in fact it was on my plastic-lined desk.

Soldering iron tip is not connected to ground properly.

How do I ground it? it uses a 2-pin connector to the base.

[IanB]

How do I ground it? it uses a 2-pin connector to the base.

In that case it's probably best not to use the iron for any sensitive electronic work. Irons suitable for electronics will be marked "ESD safe" in the product literature and will have a three pin mains plug with ground connected to all exposed metal parts.

[David_AVD]

I'd guess that his soldering iron is properly grounded.  I've seen exactly the same thing happen when hand soldering 5050 RGB LEDs with a Hakko 936 iron.

I suspect that the stray field from the 240VAC running under my bench coupled via me (holding one side of LED) and returned via the grounded tip.

It only takes microamps to make the LED glow and if the room lighting is low enough you'll see it.

[Psi]

I've seen leds glow with one leg connected to an oscillating MCU pin and the other leg floating in air with no connection what-so-ever.

The only thing i could think of was some sort of radio receiver effect.
(Like a crystal radio set using only the energy from the radio waves.)
Perhaps the LED leg just so happened to be the right length to pick up something.

[amyk]

I've seen leds glow with one leg connected to an oscillating MCU pin and the other leg floating in air with no connection what-so-ever.

The only thing i could think of was some sort of radio receiver effect.
(Like a crystal radio set using only the energy from the radio waves.)
Perhaps the LED leg just so happened to be the right length to pick up something.

Sounds more like it's transmitting, actually.

[ftransform]

 

[Psi]

I've seen leds glow with one leg connected to an oscillating MCU pin and the other leg floating in air with no connection what-so-ever.

The only thing i could think of was some sort of radio receiver effect.
(Like a crystal radio set using only the energy from the radio waves.)
Perhaps the LED leg just so happened to be the right length to pick up something.

Sounds more like it's transmitting, actually.

Interesting, yeah. That didnt occur to me but makes more sense.

[BigBrother]

what's the final say? Will the soldering iron hurt the LEDs in some way and can this damage other electronics?

[David_AVD]

No apparent damage to any of the countless LEDs I've soldered and seen this effect on.

The solder iron is not "giving off usable power".  It's just completing a circuit.

Linger to marvel at the mysterious light then move onto the next component I say.   

[Ketturi]

Ive noticed this too with white smd leds. Often if iron is grounded but you aren't and vice versa, there is floating AC current around, which is often half of local line voltage. Currents are minimalistic and are caused from devices that have capacitors between line and ground (for meeting emi standard or so). Even very small potential differences from longer ground wires can cause leds to light up. White leds have often lowest current threshold to light up partially, and can be powered actually with RF power from wlan antenna connectors or leaking microwave oven etc. if leads are long enough to  make antenna.

This leakage current can also ruin some smd leds and active parts, so using ground wristband connected to near soldering irons ground point is good move. (ESD also!)

[Praxis]

I've seen leds glow with one leg connected to an oscillating MCU pin and the other leg floating in air with no connection what-so-ever.

The only thing i could think of was some sort of radio receiver effect.
(Like a crystal radio set using only the energy from the radio waves.)
Perhaps the LED leg just so happened to be the right length to pick up something.

I had an LED connected to ground in a breadboard battery-powered circuit and it'd light up when I touched the other leg with my finger.  I figured it had to be some kinda RF pickup being rectified by the diode.  I made sure I was electrically insulated against any kinda direct conduction.