Indicator LEDs On 120 Volts AC Functioning With PIV Exceeded? - Page 1
Forum section: electronics / projects

author: Bushougoma
author_rank: Regular Contributor
timestamp: October 28, 2017, 01:53:23 am
content: I have a Tripp Lite under monitor surge protector (the type with multiple switches for different components) and recently one of the green LEDs that indicate that a switch is on started to get extremely dim so I decided to replace all of the indicator LEDs.Based on the flicker I surmised they were feeding the these LEDs unfiltered DC so I decided to open it up and see how it was designed and boy was I shocked. I expected to see a PCB screwed to the front panel with the switches and LEDs soldered to it and instead I found a point to point wiring mess with LEDs hot snotted into the front panel. Not to mention some moron crushed the neutral wire on the line cord when they screwed the case together.To make matters even worse it turns out that the indicator LEDs are fed directly off the 120 volt AC mains with only a 33K resistor in series (see picture) and no that 1N4004 diode on the central PCB is for the wiring fault circuit and not the switch LED indicators.So they're using the LED itself as a rectifier no issues there but according to the many datasheets I've read indicator LEDs like this have a max PIV (peak reverse voltage) of 5 volts.How are these LEDs not becoming DED (dark emitting diodes) with -120V being applied to them? Is this the reason why a few of them have diminished in brightness?These are low intensity muted green 2V forward voltage LEDs and not the high intensity piercing green 3 volt forward voltage LEDs if that matters.So I need to restrip and resolder the line cord, replace the LEDs, and install an 1N4007 diode in reverse parallel with each LED so the new LEDs will be protected and see a max of -0.7 volts when they are reverse biased.

author: Old Don
author_rank: Regular Contributor
timestamp: October 28, 2017, 02:01:53 am
content: I'd button that back up and give it the float test (AKA - a long walk from a short pier) and try to forget seeing what was inside that beast!

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2017, 02:05:02 am
content: That looks like it gives excellent protection against flying Aardvarks, and not much else. +1 for float testing it.Seehttps://www.eevblog.com/forum/beginners/what-is-the-reverse-breakdown-voltage-of-a-red-or-green-5-mm-led/

author: Bushougoma
author_rank: Regular Contributor
timestamp: October 28, 2017, 02:11:42 am
content: Quote from: Old Don on October 28, 2017, 02:01:53 amBelieve me I'm considering it. It's absolutely abysmal that this is what 40 bucks gets you from a reputable manufacturer.Can't imagine what the poor person gets for hand assembling it.

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2017, 02:18:27 am
content: I have also found AC-driving LED's with a simple half-wave rectifier does not work reliably.Even on 24VAC HVAC systems, it seems a 1kV (1N4007) spike occurs easily and takes out the LED.Adding the reverse-diode fixes that, but remember the dropping resistor is now seeing full-wave across it, so double the power dissipation; 0.43W now vs 0.21W before.Ensure the resistor can take it, as these are on 24/7 and will age.

author: Nusa
author_rank: Super Contributor
timestamp: October 28, 2017, 02:26:24 am
content: I saw basically the same thing with a night-light I cracked open. Behind the plastic diffuser there were 3 LED's and 1 resistor in series, hooked up directly to the AC prongs.

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2017, 02:40:38 am
content: You can get bridge rectifiers in packages as small as SOT-143. (3mmx2.5mm) Using some to provide full wave DC to the LEDs would be a vast improvement.With the dropper resistor on the AC side of it it doesn't need a high voltage rating. Double the series resistor value as full wave doubles the duty cycle. Dissipation will be about the same as the half wave without antiparallel diode disaster, but its not abusing the LED (as long as its rated for >25mA If) and it wont flicker.

author: georges80
author_rank: Frequent Contributor
timestamp: October 28, 2017, 03:16:59 am
content: That POS have a real UL number on it?Looks like garbage made in China for the lowest possible price.cheers,george.

author: amyk
author_rank: Super Contributor
timestamp: October 28, 2017, 03:54:14 am
content: How long did it last, and were the other LEDs (not the one that failed) also on for approximately the same amount of time?The main problem with reverse breakdown is the current, and as you have noticed, they will survive just fine if the current isn't too high (in your case, that will be <0.4mA.) Replace with the same type of LED and the circuit will continue to work.In fact, a quick search shows that resistor+LED directly across mains is a pretty common solution.

author: Bushougoma
author_rank: Regular Contributor
timestamp: October 28, 2017, 04:29:58 am
content: Quote from: georges80 on October 28, 2017, 03:16:59 amYes right on the bottom cover. One of the square silver holographic ones. This is a name brand product so I doubt they would fake UL certification.

author: Bushougoma
author_rank: Regular Contributor
timestamp: October 28, 2017, 04:33:30 am
content: Quote from: amyk on October 28, 2017, 03:54:14 amIt's been operating constantly since 2008. Wow just looked that up on Amazon didn't realize I had it that long. Two other LEDs were on for the same amount of time and were much brighter which is why I found it strange. Guess it could have just been a manufacturing defect in that LED.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 06:53:54 am
content: Red and green LEDs usually break down above 20V.The 5V absolute maximum rating, is customary for all LEDs, and doesn't mean anything about the actual breakdown value. For that, you would find "breakdown voltage @ Ir" in the characteristics section. You see this on many transistors and diodes, but not LEDs. They don't take the time to test it ($!).I don't recall having tested a red or green LED that avalanched below 20V. Some withstand almost 200V, and would survive just fine here (assuming the MOVs are selected for US 120V circuits, and haven't expired).It could be that they selected a preferred manufacturer, that they found has a high breakdown voltage. It could also be that you've got a bum lot, that's breaking down from repetitive avalanche damage.Replacing them with bidirectional LEDs would seem the best option (double the light, or use twice the resistor for the same light!), though I somehow doubt you can get monochrome AC LEDs (as opposed to the normal bi-color kind). So adding the reverse diode is the other option.Do replace the resistors, though. That looks like a 1/4W type, 1/2W at best if it's a metal oxide power resistor (preferably a fusible type as well!). You'll be dissipating 0.43W continuous, so a 1W resistor would be wise. Note, that means it was marginal to begin with (half wave --> half power, 0.22W). All the more reason to upgrade.You can also replace the LEDs with a superbright type (InGaN green) and use a larger resistor (100k or more?) with the same (1/4W) rating.Tim

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2017, 07:37:54 am
content: You do get dual die single colour LEDS, but in general the cost of the 4 diodes ( or one to cheap out and have flicker) is probably less than the difference. I tend to use 4 zener diodes there though, as I do have a box of them ( was a $1 buy), and this also acts as protection against spikes, though the spike after a 100k resistor will be greatly attenuated.Did a few as mains indicators, and after 20 years I guess they still are working in that lift room as power indicator, being potted in epoxy along with the resistors and diodes in a miniature circuit breaker blanking unit. If I do any more I will do like the last one, and take any old breaker and drill out the rivets to use the housing and keep the screw points instead, instead of having wire leads. Will still pot it though, and scrape off the current rating info on the case.

author: Bushougoma
author_rank: Regular Contributor
timestamp: October 28, 2017, 08:00:43 am
content: Quote from: T3sl4co1l on October 28, 2017, 06:53:54 amYeah 100K 1/4 watt is what I came up with when I did the math that should provide around 1 mA of current. The power dissipation would be too high at the existing 33K and would require at minimum a 1/2 watt resistor and at around 400 mW 1/2 watt doesn't leave enough safety margin for me.Plus I only have the high brightness (Gallium Nitride?) Green LEDs on hand which are absolutely blinding at higher currents. So it's a win win a smaller higher value resistor that runs cooler and no scorching my retinas.One thing I learned from experimenting Gallium Nitride LEDs sure don't like to be reverse biased without a diode in parallel I tried one out in the existing circuit (thinking they're both green LEDs it should be a drop in replacement) it didn't last long before failing.Thanks for all the input guys.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 08:49:46 am
content: Quote from: Bushougoma on October 28, 2017, 08:00:43 amNoticed that as well, blue LEDs (GaN varieties, including InGaN green, cyan, white and probably UV) break down at lower voltages (10-30V?) and tend to die even at low currents (~uA).Tim

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 08:53:21 am
content: Quote from: T3sl4co1l on October 28, 2017, 06:53:54 amMy experience with old red and green LEDs is that reverse breakdown is non-destructive. You'll probably find that the LEDs you tested for reverse breakdown, probably still work, as long as you limited the current to a safe level. I don't know whether reverse breakdown causes a little damage, resulting in reduced long term reliability or not.Newer high brightness green, blue and white LEDs suffer from destructive breakdown, which also makes them more ESD sensitive, than old LEDs.

author: IanB
author_rank: Super Contributor
timestamp: October 28, 2017, 08:57:49 am
content: Quote from: floobydust on October 28, 2017, 02:18:27 amCan you elaborate on this? I recently wired a red LED in series with a 1N4007 and a dropper resistor to replace a failed neon indicator and have been watching it to see how long it will last. Are you predicting it might fail?

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2017, 09:04:49 am
content: Use a resistor with an appropriate voltage rating though, or at least 2 in series. 1/4W resistors are not mains rated, they only are rated for 250VDC, and with mains transients they can flash over. There are resistors that are mains rated, which are not spiral cut, though the normal thing is to use 2 or more in series so the peak voltage is much lower.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 12:07:40 pm
content: Quote from: IanB on October 28, 2017, 08:57:49 amThe 1N4007 does leak some current. I wouldn't worry though, as even if the LED does avalanche, it won't be harmed, certainly not at such a low current. If it were a blue, white or high brightness green LED, then I'd be more concerned.

author: StillTrying
author_rank: Super Contributor
timestamp: October 28, 2017, 12:48:51 pm
content: Use a 2 colour back to back LED.I don't think there's any AC LEDs with 2 of the same colour back to back.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 01:05:28 pm
content: Quote from: StillTrying on October 28, 2017, 12:48:51 pmNo, they do exist. Kingbright make them, see the data sheet linked below.https://www.rapidonline.com/kingbright-l-57ggd-5mm-2-2v-green-led-bipolar-10mcd-56-1260They're available from various distributors.http://uk.rs-online.com/web/p/visible-leds/2285821/https://www.rapidonline.com/kingbright-l-57ggd-5mm-2-2v-green-led-bipolar-10mcd-56-1260

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2017, 04:21:48 pm
content: OK here are circuits discussed.I was surprised to see (GaN) LED's keep failing at 24VAC, 1N4007, 4k7 resistor.HVAC 24VAC loads like zone valve motors, solenoids etc. have large inductance. Easy to make a kV spark. If a tech shorted power by accident, the power LED was guaranteed blown. Otherwise they failed about once twice a year. So I added a shunt diode and zero problems for years now.Speculating, it could be the reverse recovery, junction capacitance, or as OP deduced, rectifier avalanche. 1N4007 is a slow part.GaN LED's really can't tolerate any reverse current.edit:fixed wrongly drawn bridge rectifieredit2:added second resistor to bidirectional LED

author: IanB
author_rank: Super Contributor
timestamp: October 28, 2017, 04:56:05 pm
content: Your bridge rectifier symbol triggered my OCD tendencies

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 05:24:10 pm
content: Quote from: IanB on October 28, 2017, 04:56:05 pmIf you had used the built in symbol, it would've at least been correctTim

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2017, 08:27:33 pm
content: Fixed, apologies to the 18 people who had to see the horror

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2017, 08:38:48 pm
content: Yes, that abortive hybrid between a bridge and a ring modulator was not your finest effort!Now go back and increase the resistor (or add another one) for the LED with antiparallel dies. Like the bridge, its effectively fullwave so needs twice the resistance.

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2017, 08:57:52 pm
content: OK updated the sch again, hopefully good enough.I start with resistor wattage and work backwards to get a resistance value. It's about how much heat can they dissipate, instead of the usual LED current desired.Added several 3mm GaN green/white to mains extension cords for car block heater, as at -30C car won't start unless plugged in, and you never know if there is power at the cord end or a breaker tripped.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 09:15:02 pm
content: Another resistor in parallel with the LED is another possible solution. The tiny current passed by the diode's leakage/capacitance, should be reduced to a tiny voltage, well below the LED's reverse breakdown voltage, by the resistor.

author: Bushougoma
author_rank: Regular Contributor
timestamp: October 29, 2017, 03:13:27 am
content: I played around with resistor values 100K (1 mA) was still way too bright man these higher intensity green LEDs are efficient I finally settled on 2M of total resistance.I put 1 Meg between the line and anode and the other 1 Meg between the cathode and neutral because it made the point to point wiring neater with a 1N4007 in reverse across the LED. I cycled the power numerous times and let it run for awhile the LED seems happy. Around 60 uA current flow 7mW dissipation total or 3.5 mW through each resistor those 1/4 watters aren't even breaking a sweat.I considered using 1N4148 high speed switching diodes as floobydust posted I have plenty but the 100V peak reverse voltage kind of bugged me I know it wouldn't come into play since when it is reverse biased the LED would be forward biased dropping around 3 volts but I figured if the LED were to fail open circuit the 4148 would see the full mains voltage and exceed the it's peak reverse rating.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 29, 2017, 03:55:20 am
content: Quote from: Hero999 on October 28, 2017, 09:15:02 pmJust put a small electrolytic cap across the LED. Then it will also eliminate flicker.

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2017, 04:10:07 am
content: If the LED dies open circuit, does it really matter if it takes out the antiparallel 1N4148?Anyway the 1N4148 will probably go into avalanche breakdown somewhere up around 120V, and with the 2x 1Meg resistors limiting the current at the AC peak to about 25uA, there is very little chance of it failing.Seehttps://2n3904blog.com/1n4148-reverse-breakdown-voltage/Of more concern is the LED mounting, especially in the original single resistor configuration. A bare LED poking through a panel, powered from a supply of over 50V is generally not acceptable, any more than a bare neon poking through in a rubber grommet is.If the dropper resistor flashes over due to a surge, it may fuse the LED and blow the front off it leaving the leadframe exposed to touch. The proposed circuit with a 1Meg resistor in each leg is safe under those conditions, because even if the package is compromised, the 1Meg resistors will limit the touch current to a non-hazardous level, but for lower resistor values, if the possible touch current exceeds 0.25mA, it should be impossible to touch the lead frame if the LED has ruptured. i.e. use a hole that at the front is smaller than the lens diameter so the lens is captive, use a lightpipe or use a panel mount that fully encloses the front of the LED.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 29, 2017, 05:15:42 am
content: Since nobody else here has mentioned it:BigClive did a nice video on the specific topic: Using LEDs as mains indicator lights on 120V and 230VAnd with live, on-camera demonstrations. Recommended