# ne2 neon,led drop in replacement?



## nitebrite (May 23, 2011)

hi,

is there a led that works on 120vac that is the same size as a ne2 neon lamp? it would be ok if i had to use a resistor. the ones at radioshack are too big.

thanks


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## Ken_McE (May 23, 2011)

> is there a led that works on 120vac


Not directly. They're all running off low voltage DC inside the emitter. Do you have room to stuff a power supply inside the housing somewhere?



> that is the same size as a ne2 neon lamp?


Err, how big is that in inches?


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## nitebrite (May 23, 2011)

i did not think it existed. the ne2 is like 1" long by 1/8" round(tiny). it has wire leads. the compartment was designed for it and there is no room to modify it. i think i will try busting open the rs led and see if i can make it smaller. otherwise i'll just replace the ne2. of all places, rs was the smallest 120vac led i could find.


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## Steve K (May 25, 2011)

Neon lamps are unique devices and there's not a simple replacement. A new neon bulb would be the easiest fix for a failed one. You could kludge together a red led in series with a power rectifier (to withstand the reverse voltage) and a series resistor. The resistor will be somewhat large, in order to handle the power dissipation resulting from conducting 10 or 20mA with 120v rms across it. Reducing the led current will let you make the resistor physically smaller, but give you less light.

Does someone actually sell an led that they say is rated for 120vac?

regards,
Steve K.


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## nitebrite (May 25, 2011)

i am an idiot i am sorry. those were infact neon bulbs at rs not led. i have no idea why i thought it said led.
so i just got a new ne2. i see that the power supply to run led's at 120vac is not tiny. i think i will take apart one of those light bulbs with the 5mm led's just to see what is in it though. however, i see that there are 120vac christmas leds. i am assuming those have a "power pack" at the end of the cord to run them on dc. otherwise those would fit. meanwhile the neon is fine for another few years or so. it is just a panel indicator anyways.


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## Steve K (May 25, 2011)

I'm not sure what's inside the led xmas lights. If I were to guess, I'd think they used a plain silicon rectifier diode to block the negative half of the AC wave, used 30 or so leds in series, and just put a resistor in series to limit the current. I did something conceptually similar to make a backlight for a stained glass item, although I did use a full wave rectifier and a two-transistor current limit circuit.

regards,
Steve K.


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## nitebrite (May 25, 2011)

so i see i could not just cut out one xmas light and use it. the ne2 is fine since it is all that would fit.

thanks for the info.


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## fyrstormer (May 25, 2011)

Steve K said:


> Neon lamps are unique devices and there's not a simple replacement. A new neon bulb would be the easiest fix for a failed one. You could kludge together a red led in series with a power rectifier (to withstand the reverse voltage) and a series resistor. The resistor will be somewhat large, in order to handle the power dissipation resulting from conducting 10 or 20mA with 120v rms across it. Reducing the led current will let you make the resistor physically smaller, but give you less light.
> 
> Does someone actually sell an led that they say is rated for 120vac?
> 
> ...


I think if you bridge the LED's wire leads with a normal diode pointing in the opposite direction, it will allow reverse current to flow through the circuit without burning out the LED.

Anyway, I did a quick search for "120V LED" and this was the first result: http://www.amazon.com/dp/B000K2IKUK/?tag=cpf0b6-20


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## Steve K (May 27, 2011)

fyrstormer said:


> I think if you bridge the LED's wire leads with a normal diode pointing in the opposite direction, it will allow reverse current to flow through the circuit without burning out the LED.



I saw somethink like that when I did a search for 120vac led.. and I can't figure out the benefit. So instead of using a series diode like I suggested and only burning up power in the resistor for half of the AC cycle, you can use a parallel diode and burn up power in the resistor for the full AC cycle?? I guess the benefit is that you can use a 1N4148 diode instead of a 1N4004. 

Seems like you could at least just use two leds in parallel instead of the led in parallel with a plain diode. That would allow you to cut the current in half, reducing power dissipation in the resistor by half (if you used my original proposal, or 3/4 with the led//diode idea), and still getting the same amount of light.



fyrstormer said:


> Anyway, I did a quick search for "120V LED" and this was the first result: http://www.amazon.com/dp/B000K2IKUK/?tag=cpf0b6-20


 
It says that it conducts 0.5mA, so the power dissipation is certainly reduced. Of course, so is the light produced. Doesn't seem all that useful, but I guess you'd have to compare it to the neon bulb to see which is brighter.

regards,

Steve K.


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## CKOD (May 27, 2011)

Steve, the anti-parallel diode arrangement is used when the reactance of a capacitor is used to drop the voltage from line voltage to that acceptible for the LED, instead of a resistor. If you didnt have the anti-parallel diode, the cap couldnt discharge, and current would stop flowing, and as you say, a LED could be used instead to use the other half of the waveform. 2 LEDs in anti-parallel and a capacitor in series could be used. If you used a high-efficiency red LED, you could get away with ~2mA instead of 5-10, and a ~40nF cap in series(if I did my math right  ), though stuff may not fair so well in the case of noise, or if whatever device you have it on gets hooked to a square wave inverter etc...


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## Steve K (May 27, 2011)

If a cap is used as a series impedance, then it would make more sense to use back to back diodes. There wasn't an explicit mention of what was being used to limit the current, although my search for "120vac led" did pull up a sketch that used a rectifier diode in anti-parallel with a led, and used a resistor to limit the current. This was what got me scratching my head, and I did infer the use of a resistor when I was discussing the power dissipation.

Do any commercial products use caps as the series impedance? My instinct tells me that the failure mode would be a short in the cap, probably, which could potentially cause something to overheat. A resistor doesn't have any way to short out (not counting a short on the circuit board), so it's probably a safer failure mode.

anyway... I guess there is some stuff on the market sold as a 120v led assembly, but I doubt that anything will be sold in a standard 5mm LED package that can tolerate 120vac. Or is there something special out there???

Steve K.


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## CKOD (May 27, 2011)

No good products would, 60 hz flicker would be annoying(well I guess no worse then the neon lamp..) , as you said, failure mode would be a problem. You could use an X capacitor, but an X capacitor of that size would be a bit large for the NE2 replacement. I have seen some products use a series cap for a low voltage power supply for a logic board in something like a washing machine IIRC, probably used with a bridge rectifier, storage cap and shunt regulator too. Like you said, with a resistor for voltage dropping, a series rectifier would definitely be the better way to go, as just that alone would halve your resistor dissipation vs the anti-parallel rectifier.


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