Why does my TS101 soldering iron turn on LEDs upon contact?

Question

1. An LED on my board turn on and stays on when I touch it with my soldering iron and it's freaking me out. This happens even though the board is just sitting on my (wooden) desk and not connected to anything. The current through the LED seems to have no way to complete a circuit!
2. Touching the negative terminal with my finger makes the LED shine much brighter.
3. Grounding my previously-un-grounded soldering iron doesn't seem to change anything. (Though, see update 1)

After grounding my TS101 soldering iron, tip-to-ground resistance is about 12 MΩ. I measured an AC voltage (with my cheap meter) between the iron tip and the grounding screw on the handle - it was around 100 mV. I measured DC voltage, about half that. That surprised me. I expected be just a few mV at most. Also, I didn't think that's enough to turn on a LED even a little.

Update #1:

4. With the grounded soldering iron, I no longer get continuous lighting of the LEDs. I do get flashes. But they flicker briefly and disappear (I'm guessing finite static charge moving around), and they don't appear only when making contact, but also when breaking contact, or otherwise moving the soldering iron about.

Update #2:

A proposed answer points to a small parasitic capacitance from the board to ground, which is enough to complete the circuit. And this suggests that the AC voltage on my iron's tip, together with this capacitance, is what's causing the LED to light. Which raises the question(s):

5. Why does my supposedly grounded iron tip, powered by a switching AC/DC supply, carry an AC voltage?
6. An LED is a diode. How can an AC voltage charge AND discharge the capacitance to ground through a diode? Is the tiny leakage current through an LED really enough to discharge the capacitor during each half cycle, for enough current to flow during the other half, so that the LED emits a visible amount of light?

Update #3 2023-06-20 (a month later): I've slowly made some progress figuring out the issue.

1. TS100 (yes, a previous gen) grounding was discussed on an EEVBlog forum thread. Following suggestions there, I checked and the power adapter that came with the TS101, though it uses a 3-prong connector, has a floating out, i.e. the DC- output has no DC path to the the earth ground prong. Discussion there suggests that results in AC leakage from the power brick finding its way to the iron tip. Apparently, some power bricks provide floating output and some don't. This one has a floating output. On the other hand, my laptop charger's DC- has continuity with the earth ground prong.

2. AFAICT, there is no actual Tip-to-ground(-screw) DC path in the TS101. Some posts on the thread suggest that the TS100 does, but i'm not sure those posting are not actually making a false assumption about the grounding terminal.

3. There's a reverse-engineered schematic from ralim/IronOS for the TS100. What it shows is that the ground screw is connnected only via a 100nF cap, to the DC- input. That is, if you connect the grounding screw to earth ground, you establish a path for AC noise on the DC input to be shunted to earth, instead of reaching the tip (IIUC, please correct me). Anyway, I think that means the ground screw has little to do with ESD-safety per-se, in the sense that it's not designed to provide a DC-path for draining electrostatic charge that has built up on the tip. It looks more like a way to compensate for the ungrounded output of the power brick.

4. I'm not sure if my TS101 is faulty but, AFAICT, the TS100 schematic does not match the TS101. In the TS100, the schamtic suggests that the ground screw is connected to DC- solely through C26, a 100nF cap. In the TS101, I measured continuity between DC- and the grounding screw, and C26 instead connects DC- to the barrel of the tip. It sits near the base of the tip and is connected to a flexible metal ring which makes contact with the tip barrel when it is inserted into the TS101's body.

So from what I can tell there's no way to ground the tip of a TS101 (nor TS100, for that matter) without modding the circuit. The grounding screw is there only to mitigate AC leakage from the power supply.

The forum thread suggests replacing C26 with a short, i.e. tying the grounding screw, the tip, and DC- together. I can't comment on that, but it seems the TS series of irons simply weren't designed for ESD safety. And even if they've made an attempt to provide a way to limit AC leakage to the tip, they've done a shitty job of communicating what's what with their iron.

I continue to be shocked by the TS101 soldering iron ;-) .

Answer 1

These LEDs are sensitive to electric fields in the air that even a small amount of current can pass through it and generate light if an AC voltage potential is applied on one end. The other open-end behaves like a capacitor sending the electric current into the nearest ground potential through air.

Here's an exciting video by Electroboom demonstrating this effect:
https://www.youtube.com/watch?v=_bgUy6zA0ts

Update: To answer your additional queries

5. Why does my supposedly grounded iron tip, powered by a switching AC/DC supply, carry an AC voltage?

Your soldering iron should be powered by AC, so the AC circuit is probably electrically connected to the heating element of your tip and thus carries an AC voltage potential that can leak through air and to any ground potential. Most of the current normally goes back to the neutral line of your AC, but if there's a stray capacitance (a.k.a. there's a stray ground potential nearby) it will act like a voltage divider that can drain some small currents to that capacitor and to the LED as well making it glow a little bit.

^{simulate this circuit – Schematic created using CircuitLab}

Take note that this parasitic capacitance also manifests in the solder iron without the LED but you have no way to see it, that is why it is possible to get a shock when you touch a solder iron that is not earth grounded because the iron itself is acting like a capacitor towards the earth and by touching it you inadvertently discharge this voltage potential from the iron through your body to the earth ground. :)

6. an LED is a Diode. How can an AC voltage charge AND discharge the capacitance to ground through a diode? is the tiny leakage current through an LED really enough to discharge the capacitor during each half cycle, for enough current to flow during the other half, so that the LED emits a visible amount of light?

The parasitic capacitance only manifests when there's an alternating current that's why it can leak current into the air, but with the diode you are right that it should only flow halfway of the time, at the rate of 0.5 * 50Hz or 60Hz. And some LEDs are sensitive enough to glow on these small currents as long as the required voltage difference is satisfied.