Neon Pilot light that's NOT and more..... - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: January 18, 2024, 04:31:09 pm
content: I ordered some "neon pilot lights" rated for 220Vac from AliExpress."Don't turn it on, take it apart" seemed like a good idea, partially because I wanted to assess suitability for 240V and also because I could see a 3mm LED through the lens.After spotting the LED I expected to find a diode in series with the LED for the reverse voltage aspect.Nope, just a LED and a 300k 1/4 watt resistorCurrent is about 816uA @245VTypical Vr for LED's like this is about 5V. Is this some sort of special LED with high Vr (built in diode?) or can a LED survive with Vr way over its rating if current is limited?

author: Gyro
author_rank: Super Contributor
timestamp: January 18, 2024, 04:42:20 pm
content: You can get LED packages that have two dies in inverse parallel. They give you twice the light output for a given AC current with no flicker or inverse voltage problems. I'm not saying itisone but it might well be. Of course, if it's Ali...P.S. Test the LED itself with DMM diode mode in both directions. That will clear it up.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2024, 04:54:28 pm
content: It could be two LEDs in the package connected back-to-back, but it might not be. Red LEDs aren't damaged by reverse current. As long as it's limited to a safe level, the diode will go into avalanche conduction and clamp the voltage. It's blue and pure green LEDs which are far more sensitive and will be damaged by reverse current.When I was a beginner, I didn't even know about using a diode to prevent reverse current through an LED, because I'd only ever used red LEDs on the mains before, so never used one and they worked fine. I do remember changing my tune after destroying anm expensive at the time, UV LED, when I tried it on the mains, with no diode.

author: Jester
author_rank: Frequent Contributor
timestamp: January 18, 2024, 05:28:47 pm
content: I received an agency certified version of one of these from Digi-Key this morning, looks very similar on the outside but it actually has a neon bulb and better insulation between the two legs.I have yet another (larger) AliExpress mains pilot LED, this one has a small PCB with a SMT LED and a couple of SOT23 devices I'm curious what the circuit is, but that will have to wait I have other things to do.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2024, 05:43:57 pm
content: Did you test the LED in both directions?There's nothing wrong with using an LED, rather than a neon lamp. I often use LEDs for mains power indicators. Nowadays I use a small SMT bridge rectifier, so the resistor can be a higher value and there's less flicker.

author: Jester
author_rank: Frequent Contributor
timestamp: January 18, 2024, 06:55:19 pm
content: Quote from: Zero999 on January 18, 2024, 05:43:57 pmMystery solved, there are actually two tiny LED'S in parallel inside the 3mm LED package, one direction one is on, and the other direction the other is on.Power in the resistor would be 200mW, and they used a 1/4W resistor, so it actually looks okay.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 18, 2024, 07:53:59 pm
content: I would be more concerned about the resistor voltage rating and the lack of insulation on the wires giving very poor creepage and clearances, that together with a thin ? plastic lens means I very much doubt this conforms to any safety standards. But then I guess it's a Chinese Export

author: floobydust
author_rank: Super Contributor
timestamp: January 18, 2024, 08:04:08 pm
content: I bought some tiny 8mm indicator lights from AliexpressDIANQI Storeand took one apart because it was so small, how did they pull it off- it is a dual-die 3mm LED and dropping resistor inside for 120VAC. I was happy with the store's quality.I've never seen dual-die LED listings or encountered them before.Anytime I've used a rectifier diode like 1N4007 in series with an LED, that prematurely fails. Found out it's -ve spikes exceeding the diode PIV rating, then that avalanche kills the LED. This is even on 24VAC systems. So I either use a shunt diode 1N4148 or tiny bridge rectifier if flicker is a concern, and dual resistors for mains.There is a safety issue if the indicator is mains powered that it has decent insulation and spacings to the housing or chassis. These things carry no approvals and lots of glue.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2024, 06:32:49 pm
content: Bipolar LEDs are quite common. The most common type has different colours, normally red and green/blue, but they're also available with both the same colour.https://www.tme.eu/Document/799880f1b6595caa87ca08014152ba82/OSXXX234E1A.pdfhttps://www.tme.eu/gb/details/osmmd234e1a/tht-leds-round/optosupply/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2024, 07:26:30 pm
content: LEDs are never rated for more than 5V -- at least, none that I've seen.They can typically handle more, quite a lot more. In my experience, blue/white fail suddenly at 20-30V; GaP green avalanche maybe 50-80V; red, 100-200V. Still not enough to handle mains -- but the latter handle at least enough avalanche to measure at all (0.1mA or so, I think it was?), so could potentially survive in such service. More likely (hopefully?) they're bidirectional as mentioned above. There are also occasionally series arrays, multiple chips stacked for better current efficiency; easily confirmed by peering into the lens (if clear) and measuring Vf.Incidentally, many illumination blue/white LEDs include ESD protection, either an antiparallel diode or zener -- the latter of which likely breaks down not far beyond the rating (so, 7V or so). I haven't measured one yet.Tim

author: Bud
author_rank: Super Contributor
timestamp: January 19, 2024, 08:48:24 pm
content: Quote from: T3sl4co1l on January 19, 2024, 07:26:30 pmHmmm... I have here a 120V Christmas Light LED string of 60 series LEDs with multiple LEDs failed...Almost all ones that failed are blue LEDs for some reason.

author: floobydust
author_rank: Super Contributor
timestamp: January 19, 2024, 09:43:15 pm
content: Quote from: Zero999 on January 19, 2024, 06:32:49 pm"AC LED" but Digi-Key and Mouser do not have that category. OK keyword is 'configuration "bidirectional" and there is only one KingbrightWP937GGDat Digi-Key. But not high efficiency green. Mouser has no category for them.I put LED indicators on HV buses for safety, in case I'm forgetting the cap charge etc. 350VDC, white 5mm LED and 220kΩ 1W for around 1.6mAWell, the first LED lasted for a few hours. Second one went dim. Third one worked but after a while would not light, then surprisingly light up after I'd touched my multimeter leads across it to measure what Vf was. Oh look a voodoo fest or a free SCR must be in there.At this point- it's a linear power supply with no inductive spikes on the output - figured there's some parasitics at play.Gave up and added a 1N914 and 0.1uF cap across the LED and it works fine, stopped the shenanigans.I would guess stray capacitance or inductance of the resistor caused the LED to behave not like an LED, or fail.Most are rated for max. 5uA reverse current.