LED's light when soldering - Page 1
Forum section: electronics / general

author: GabYoung92
author_rank: Regular Contributor
timestamp: September 14, 2012, 05:06:07 am
content: Had something really strange happen.. I want to know what was going on.I was given some RGB strip lights that were faulty. The SMD resistors weren't soldered very well.As I was reflowing the solder, I noticed that the LED lights lit, very lightly. Only when I had the tip on the solder.Could my iron (basic wall plugged 40W) be inducting into the strip?

author: AndyC_772
author_rank: Super Contributor
timestamp: September 14, 2012, 06:00:23 am
content: Sounds like the tip of your soldering iron isn't properly isolated from the mains. Check for continuity between the tip of the iron and earth, and check for ac volts between the tip and earth when it's switched on. Don't use your best meter leads, obviously!

author: GabYoung92
author_rank: Regular Contributor
timestamp: September 14, 2012, 06:07:47 am
content: Instead of plugging it in, I tested it unplugged.tip - L: >10Mtip - N: >10Mtip - E: ~3ohmsSo the tip is shorted to earth. I was only soldering a 30cm strip which wasn't connected at all. I had it stuck down to my desk.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 14, 2012, 06:10:46 am
content: Can you also check tip-L and tip-E on a capacitance range? There might be some leakage at ac which doesn't show up in a dc resistance measurement.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: September 14, 2012, 06:13:36 am
content: QuoteDon't have to be lit that, my soldering iron is connected dead short to mains earth, hence you should be carefull if you soldering a circuit which is powered by a not floating voltage source.

author: David_AVD
author_rank: Super Contributor
timestamp: September 14, 2012, 06:29:45 am
content: I've had the same thing happen with 5050 package RGB LEDs. It made think "WTF?" the first time I saw it.The (earthed) iron tip is one conductor and your finger is the other. It's just stray voltage caused by your body's proximity to mains wiring.

author: G7PSK
author_rank: Super Contributor
timestamp: September 14, 2012, 07:20:02 am
content: I have seen led's glow slightly using a portable gas iron when the ambient light is low, I put it down to the heat itself making them light up but never bothered to investigate further at the time and forgot about it until just now, I will try heating a few led's tonight to find out, I will go outside well away from any stray inductance if it is not raining.

author: Psi
author_rank: Super Contributor
timestamp: September 14, 2012, 01:55:52 pm
content: Maybe it happens when the existing led wiring just so happens to be the right length/capacity to pickup some nearby RF energy.Grounding the other LED terminal would provide the 'other half' of the antenna.Just did a current test with one of my super efficient orange leds using a few volts and a 1meg resistor. I was able to see a tiny orange glow from the substrate with 0.003mA flowing through it.So it doesn't take much current to get a little light out of an efficient LED.

author: David_AVD
author_rank: Super Contributor
timestamp: September 14, 2012, 09:18:35 pm
content: In my case, the SMD LED glowed when I was holding it on the PCB with my finger (towards one side, touching pins) and touched the other side's pins with the earthed soldering iron. I'm 99.9% sure it would be mains being induced, not RF.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: September 14, 2012, 10:13:23 pm
content: Quote from: David_AVD on September 14, 2012, 09:18:35 pmRF in this context can mean interference picked up from, for example, a laptop power brick or other switching supply. Theoretically, there probably should't be enough energy for this to happen if everything meets electrical interference standards, butnot all PSUs do.

author: David_AVD
author_rank: Super Contributor
timestamp: September 15, 2012, 01:20:01 am
content: In my case, there was mains wiring within 100mm of where I was soldering (work bench). No switchmode power supplies near. I should try it again one day with the bench powered down.

author: Psi
author_rank: Super Contributor
timestamp: September 15, 2012, 02:10:51 am
content: I should try earthing one terminal of an led and holding the other near my tesla coil

author: amyk
author_rank: Super Contributor
timestamp: September 15, 2012, 11:06:17 am
content: http://www.aplomb.nl/SMPS_leakage/Doc_ie.htmlSMPS leakage current tests. Some of those can source tens of uA (one particularly bad one was over 100uA!) of induced AC to ground, and as Psi's experiment above showed, 3uA is enough to get light.

author: David_AVD
author_rank: Super Contributor
timestamp: September 15, 2012, 11:52:25 am
content: But that's leakage from power supply to ground though. We are (well, I am) talking about stray voltage picked up by your body.