LED powered by soldering iron - Page 1
Forum section: electronics / beginners

author: tszaboo
author_rank: Super Contributor
timestamp: May 18, 2017, 10:43:31 am
content: I was soldering an RGB LED the other day (Osram LTRB GFSF, not that it matters) The device has every cable disconnected from it. It is sitting on a ESD mat. It does not have a battery, or any large capacitor to store energy. I am soldering with a Metcal MX500, the tip is grounded, checked it twice. The LED starts emitting light when I touch the anode with the soldering iron.I dont really understand, what is going on tbh, and it puzzles me. I just dont see a clear current path. What am I missing?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 10:47:18 am
content: This doesn't surprise me. You're acting as an aerial, picking up electric fields, the current is passing through the LED to earth, via the soldering iron.Try connecting one side of the LED to another earthed object, such as the metal frame of an earthed appliance or a radiator and holding the other side of the LED in your hand. It will glow very dimly.

author: tszaboo
author_rank: Super Contributor
timestamp: May 18, 2017, 10:57:53 am
content: I have some 20mV RMS noise on the tip of the soldering iron between 10 and 100MHz.Now I wonder what is the deal with the ESD mat.

author: jonovid
author_rank: Super Contributor
timestamp: May 18, 2017, 11:41:48 am
content: try a neon bulb then a Fluorescent Tube if either flicker light then look at your soldering iron's power unit.you may have a High-Frequency EMF Noise problem in a switch mode power supply?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 12:08:28 pm
content: Quote from: NANDBlog on May 18, 2017, 10:57:53 amI misread your previous post.Still, it's highly likely your ESD mat is picking up EM from somewhere.

author: tszaboo
author_rank: Super Contributor
timestamp: May 18, 2017, 12:21:05 pm
content: Quote from: Hero999 on May 18, 2017, 12:08:28 pmNo surprises, the grounding plug was right next to the mains plug, and mat was completely floating.I'm not sure, if I left it that way, probably not, because why would I unplug the ESD mat in the office??? I have the feeling, no matter where I go working, one would need a separate office a lock and an armed guard. So people wont mess up my setup, or steal my cables or tools.Still, it puzzles me a bit, it supposed to be a high impedance source, is it not? I guess not at RF.

author: bktemp
author_rank: Super Contributor
timestamp: May 18, 2017, 12:26:42 pm
content: Modern LEDs can produce visible light at less than 1nA. So even the tiniest amount of leakage current will make the LED glow.This will also work with RF currents as long as there is enough voltage.

author: Cerebus
author_rank: Super Contributor
timestamp: May 18, 2017, 01:25:33 pm
content: The actual reason is pretty obvious if you know how the Metcal irons work.The Metcal soldering irons use RF to drive an induction heater by the tip and the tips have a curie point that sets the temperature that particular tip works at. In the case of the MX500 it's RF at around 13MHz. The exact frequency escapes me at the moment - checked, it's 13.560 MHzIt's hardly surprising that this is capable of inducing a current in the leads of the LED, which the LED, being a diode, is only too happy to rectify into a pulsed DC voltage. The tip of the iron is grounded so it connects one lead to ground while the other lead rattles around at 13MHz or a harmonic of 13MHz providing the LED with power.

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 06:05:03 pm
content: Quote from: Cerebus on May 18, 2017, 01:25:33 pmThat seems unlikely to happen, given the whole tip should be grounded which should be a good screen for the RF. I'll have to do some measurements with the Metal iron at work, when I've got time.

author: Cerebus
author_rank: Super Contributor
timestamp: May 18, 2017, 07:02:09 pm
content: Quote from: Hero999 on May 18, 2017, 06:05:03 pmThink again. Coil, millimetres away from the LED, with over 30 watts of RF being pumped into it, producing bothEandBfields. Youmightmanage to kill theEfield with a grounded shield but no way are you going to completely shield theBfield from leaking out. What does Mr. Faraday say happens to a conductor in a changingBfield?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 07:11:48 pm
content: Quote from: Cerebus on May 18, 2017, 07:02:09 pmBut the LED is connected to the tip with one leg and earth with the other which could cause it to be sensitive to the E field, rather than the B field. If a large number of turns were wound around the tip, then there's a possibility of tapping into the B field but that isn't the case.

author: jmelson
author_rank: Super Contributor
timestamp: May 18, 2017, 07:47:50 pm
content: Metcal soldering irons use RF heating. They can radiate some amount of the RF from the tip. Possibly a ground wire in the cord is bad, or the soldering iron power supply is not grounded at the wall socket.Jon

author: Cerebus
author_rank: Super Contributor
timestamp: May 18, 2017, 07:56:43 pm
content: Quote from: Hero999 on May 18, 2017, 07:11:48 pmEh? According to the OP they saw this when soldering one side of the LED (thus earthing it via the tip) while the other side was effectively open (sitting on an ESD mat with >10 M to ground).Again, there's upwards of 30W of RF washing about at the tip of a Metcal iron, I have no difficulty believing that enough of that leaks to the component, asEfield orBfield or both, to induce the few 10's or 100's of uA necessary to visibly light up an LED when viewed from the distance one solders at. There's no need for loops and loops of wire for induction to occur, a simple lump of metal is all that is required. That is precisely how the iron tip gets hot in the first place, it's a slug of metal in a micro induction furnace that heats up from the eddy currents induced in it by the RF coil around it.

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2017, 10:33:39 pm
content: Quote from: Cerebus on May 18, 2017, 01:25:33 pmFascinating! I had wondered why they use coax, that explains it. Also explains why they're so expensive.

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 10:40:51 pm
content: Quote from: Cerebus on May 18, 2017, 07:56:43 pmNot quite:Quote from: NANDBlog on May 18, 2017, 12:21:05 pmAnd 20mV RMS isn't enough to turn on an LED.Quote from: NANDBlog on May 18, 2017, 10:57:53 amThe question is whether you experience the same with an ordinary soldering iron and if LED stops glowing when the ESD mat is properly earthed.

author: Muxr
author_rank: Super Contributor
timestamp: May 18, 2017, 10:41:44 pm
content: Quote from: james_s on May 18, 2017, 10:33:39 pmAlso why they can transfer heat quickly without having a lot of thermal mass, and why the handle can be so close to the tip for more precision. They are awesome soldering irons/systems.

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 10:46:38 pm
content: Quote from: Muxr on May 18, 2017, 10:41:44 pmI agree Metcal soldering irons are very good. I'd hope they would be very well screened, so shouldn't cause this kind of thing to happen.

author: tszaboo
author_rank: Super Contributor
timestamp: May 19, 2017, 09:27:17 am
content: Quote from: Hero999 on May 18, 2017, 10:40:51 pmOK, I tried reproducing it. The 20mV was when the iron was off.When the Metcal is heating, it produces 400-550mV peak to peak voltage measured on the tip by a scope probe. Frequancy is some 12-15MHz, seems to be changing, white noise-ish.The ESD mat picks up 50Hz noise, if it is not grounded properly. If I measure on the grounding connector, I have 38V peak to peak. That is (theoretical) 4uA through a 10MOhm resistor.If I place the board on the ESD mat, I measure about 8V peak to peak on a metal surface.And there seems to be enough power to light up the red LED If the board is on the ESD mat, and I place the LED between a floating GND and real GND. I measure 2uA.But, here comes the interesting part: The Metcal's 500mV plus the ESD mat floating together produces a voltage, which is higher than just the ESD mat. The LED is brighter, and I'm not equipped to measure uAs in that frequency range.OK, this was educational. I might DIY a device, which could detect the grounding disconnected. Or maybe redundant grounding the ESD mat could be easier? I have an idea!