LED voltages

[rockc]

I am completely stupid when it comes to electronics. I am trying to set up an LED indicator light on a 120vac switch. Are there 120vac leds? Are there some sort of switch that senses 120volts and switches a 2.5v lead or pin?

I tried to do a search for led's and nothing came up.

Thanks

 

[duffy]

You can do it with a big-ass resistor and a diode. Get a 12k, 2W resistor and hook it in series with the LED. Put the diode in parallel with the led BACKWARDS to protect it.

 

[rockc]

Huh? I would like to re-iterate that I am completely stupid when it comes to electronics. I understand about the resistor, but what kind & size diode do I use? How would I put the diode in parallel with the led BACKWARDS? Do LED's have certain polarity?

If you don't mind, you might have to take me by the hand on this one.
Or should I rummage through the FAQ first to learn a few things?

Thanks.

 

[audioguru]

Since you say you are stupid to electricity then keep away from 120VAC that will electrocute you.
Light the LED with a 9V battery and a series current-limiting resistor.
Oh. Since you don't know about polarity then don't bother.

 

[Hero999]

LEDs are diodes so of course they have polarity.

A diode is a device which only conducts in one direction.

There is a maximum reverse voltage a diode can block before it breaks down and gets destroyed.

Unfortunately LEDs have a fairly low breakdown voltage, typically 5V.

You need a diode in reverse parallel, so the LED doesn't breakdown when it tries block the hundreds of volts generated by the mains is put across it. The diode diverts the reverse current around the LED so the reverse voltage is very low.

EDIT: audioguru is right, if you don't know about polarity then you clearly don't know enough to not be electrocuted when working with mains voltage.

 

[rockc]

HERO999, Thanks. That helps a lot.

AUDIOGURU, one can be quite educated in ELECTRICITY, but not in ELECTRONIC components. I am so sorry to take up so much of your valuable forum time to bother you with such a mundane post. How does one learn about electronics unless he challenges himself and requests knowledge from those who know. Did you get crapped out from where ever you came from with the absolute knowledge of electronics? I realize that I am a newbie, but if you can't be cordial, I wouldn't mind it at all if you would not respond to my posts.

 

[audioguru]

Haven't you heard of Google?
It shows that an LED has polarity and shows that 120VAC electricity can easily kill you.

Most kids learn about electicity in elementary school. Then learn about basic electronics in high school.

 

[Hero999]

You need to understand that we have know idea of a person's background when they first post here. In general, it's best to assume they know less rather than more when the topic of mains electricity is raised.

Generally when someone says that they don't know about polarity, it implies that they know absolutely nothing so it's safest to advise them not to carry out any electrical work in case they hurt themselves or others.

 

[Andy1845c]

Do you need it to be an LED? Neon indicator lights are common with 120v circuits and won't need the resistor or diode.

Just an example - Neon Indicator Lights

 

[Andy1845c]

Heres a picture of the LED circuit Duffy is talking about. Credit to RODALCO. Hes a member here.

DSC01066 230 Volts Leds on Flickr - Photo Sharing!

Heres the topic those pictures are from. RODALCO did exactly what you are asking about doing. Give the topic a look.
https://www.electro-tech-online.com/threads/120-volt-led-indicator-lamps.34892/

 

[duffy]

Neon indicator lights are common with 120v circuits and won't need the resistor or diode.
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Neon lights DO need a resistor! Sometimes it's built into the lamp housing, but it's got to be there or the light will explode.

 

[RODALCO]

Thanks Andy 1845c for the link and credit.

To run a led safely on the 120 Volts ac mains use 2 * 6k8 resistor 1 watt in series, the 1N4148 antiparrallel diode and a 1N4007 as can clearly be seen on the photo.

Most hi efficiency LED's give a great light output between 3 and 10 mA's for a light indication and don't always require 20 mA's, = high power resistor and more heat.

Andy, a neon on 120 Volts will blow up. It would need a 68 to 100 k.ohm resistor.
A neon strikes at 60 to 70 volts and a current limiting resistor IS required to limit the current below 1 mA.

 

[duffy]

That 1n4007 in RODALCO's post appears unwarranted, do you agree? The other diode across the LED will do the protection. Adding that 1n4007 would just increase the DC offset on the mains (a teeny amount, granted, but still...).

If it was high frequency, there might be an issue with turn-off times, but at 60hz each diode will conduct to spare the other.

If there's a high-frequency noise concern, a small cap across the diodes would be a better way to handle it.

 

[RODALCO]

That 1n4007 in RODALCO's post appears unwarranted, do you agree? The other diode across the LED will do the protection. Adding that 1n4007 would just increase the DC offset on the mains (a teeny amount, granted, but still...).

QUOTE]

I have fitted LED in mains applications for over 20 years with NO faillures with the extra 1N4007. for the sake of 5 cents i prefer reliability of the circuit.

It does reduce the power consumed in the series resistor by 0.7 hence less heat in it.

You can probably get away without it, but the 1N4007 will have 1 kV blocking voltage and protects the LED IMO.

 

[tkbits]

Neon lights DO need a resistor! Sometimes it's built into the lamp housing, but it's got to be there or the light will explode.

I remember smoking one (NE-2) because I didn't know that.

 

[Mike_2545]

Here check out this link...
Let me google that for you

 

[audioguru]

I was about 12 years old when I plugged an NE-2 into the 120VAC mains. It disappeared with a bang and I am lucky that my fingers and vision survived.

 

[Andy1845c]

Neon lights DO need a resistor! Sometimes it's built into the lamp housing, but it's got to be there or the light will explode.

Sorry for the bad advice..... I guess I have never cracked open one of those indicator lamp housings..... I learned something new today

Still, the premade neon with resistor indicator light would be simpler to use then the led.

 

[Hero999]

Neons also need no diode.

 

[rockc]

Andy1845c, duffy & RODALCO, I really appreciate the input. I found a neon light with a built-in resistor as you explained. It is a liitle larger than I want, but I believe it will do the trick. Do you know by chance how hot they get? If not, no big deal I'll run them for awhile to test whether I need to vent the housing I will have them in.

Thanks again guys for your help.