Running a LED off of 240 Volts question

I'm no electronics expert but I have a situation whereby I want to run a LED off a 240Volt power supply [and before anyone asks, its got nothing to do with Empegs, docking stations or anything of this ilk ;-)) ].

Can you use a ordinary '5 volt' type LED or do you need special 'high voltage' versions to stop the LED shorting out?

And if so, then How big a resistance should the resistor be, and what sort of 'wattage' (1/4 Watt ok)
We are talking a regular (rectangular) Green LED here that runs fine on 3 volts.

WOW !! .....can you use a light vice an LED. That would have to be one hell of a resistor to drop the voltage that much. not to mention possiably alot of heat. Personally i think a 240V really small light would be a heck of alot easier. Are there spaceing issues or anything like that, is it 240 VAC or VDC. You have alot of options as to how to handle this.

12Kohm, 4.8W

But since you had to ask, this is definately not recommended. 240V is enough to kill, and should not be taken lightly.

Can you not use an enclosed class 2 power supply to derive a more suitable DC voltage - say around 5V, which you could then use with a 150ohm resistor in complete safety?

Does it have to be an LED?
Use a 240VAC Neon based indicator.

..and a 240V rectifier (power diode) in series, or otherwise the LED may "complain" about the 240V reverse voltage (it's AC..).

Do not do this!

No I decided to forgo this option and went with a 3 volt (battery) solution & a switch instead.

My original idea was prompted by the local electronics shop (JayCar) selling a small LED bezel with integral Red LED rated for connection to 240 Volts - my question was asking how do they fit whatever they require to make it 240 Volts 'safe' in such a small package [or maybe its not really safe - in which case why are they selling these things at all?].

The Bezel itself is big enough to hold a 3mm LED and little else - the anode and cathode leads are both insulated of course.

You know I've seen those things around for years, but never taken the time to find out how they work.

An inbuilt resistor and rectifier is not an option with heat generation of about 5W.

Maybe they really put about 120 chip LEDs in series in the package with a far smaller resistor? I'll have to take a look at one of those next time I'm in RatShack.

Are you sure the electronics store device was really an LED, and not just an incandescent light bulb with a red plastic lens over it?

As an experiment, in my younger days, I decided to find out what exactly would happen when you connect a normal low voltage LED to 120V AC (house) current.

It glows very, very, very brightly. So bright, it lights up the whole room. If you turn it on for short bursts (on, off, on off), it will last some time. If you leave it on, eventually it burns out. Believe it or not, it's true... I also played with blowing up other DC type items connected to AC current, such as peizo buzzers. Those things get very loud as well, before dying.

Be careful.

Calvin

LOL. By your presence here I trust you didn't try a capacitor!

No I didn't. :-) How big is the alleged explosion? Can the explosion be varyed by voltage, etc? I wonder.

I rigged these things with long wires and a knife switch that I threw at a distance.

Calvin

I'm glad you have a low voltage (safe) solution.

Then running an LED from 240V AC a small capacitor is normally used in place of the current limiting resistor and a diode connected in parallel with the LED with opposite polarity to the LED.

The capacitor MUST be rated for continual mains (AC) operation. I don't recommend you try this without experience in mains wiring which is why I'm not mentioning any capacitance values.

Rod

Running 240 Volts into a big capacitor in Physics class in school (think it was 1/2 Farad) our teacher got a pretty big explosion, but it wasn't so much a bang as a BIG PFFFT!! And some type of corrosive liquid covered one side of the room. Luckily we were all behind a glass screen, but it was very cool (as things like that are when you're 17)
Come to think of it, that teacher was a nutter - demonstrating the interesting reaction between chunks of Potassium and high molarity acids, showing how much gas you need to ignite to blow the door off a cupboard etc etc

They don't teach science like that anymore

You need to drop the voltage using a capacitor. I can't remember exactly what value you want, but 330nF sounds familiar. You can work it out easily enough, knowing you want to drop about 230V at 10 to 20mA (depending on your LED) at 50Hz. If you don't know the formulae involved, you probably shouldn't be working with the mains!

You could use an inductor instead, but capacitors are cheaper and more convenient. Don't use a resistor, unless you want to make a heater!

The other thing you need to remember is to put a diode (possibly another LED) back-to-back with your LED, but the opposite way around, so you don't get a big reverse voltage across it in the other half of the cycle. In fact, you can obtain a red LED back-to-back with a green one - these appear yellow with AC current.