Run a string of LEDs from the mains

[mackrackit]

You should see a color change when running at the peak voltage.

Just curious.... What does RMS stand for and how is applied to electronics?

[The Master]

With 4 LEDs running on 12V they are all running at their lowest voltage. I have accidentally put too many volts into some of these LEDs before. They dont change color they just get slightly brighter and stop working after a few days.

Do you mean root mean square? I think someone has mentioned this in one of my previous threads but i didnt quite understand it

[Ioannis]

1. your typical 240 Vac when rectified will be 240 x 1.41 =338.4 Volts DC. But if line goes up to 250 then 250 x 1.41 = 352.5!

So watch out.

2. RMS is that AC voltage that represents the same thermal effect when an equal DC voltage is applied to the load. That is if your line is 240Vac and you put a resistor on the line, that resistor will get hot. If you then apply DC voltage, you have to apply exactly 240 Volts to have the same thermal effect.

BUT note that 240Vac is not the maximum level that the line reaches. Is the peak of the sinus wave form and is 240x1,41.

Ioannis

[boroko]

The explanations that people are giving you are good but maybe a different one will help.

A LED will pull WAY more current than is healthy for it if it is not restricted in some way. The color is one indication that things are going badly and in short order. The resistor is sized so that it limits the current to a safe level using Ohm's law. Limiting it to MAX will reduce the potential life of the LED. Common practice it to limit it to less than max, hence, the suggestion of 30 mA instead of 50 mA. The voltage rating is what is needed to light the particular material (they are different because they use different materials to give different colors).

A quick look at a Digikey catalog gives a few examples:
Color,forward voltage, max current
Red, 2.0vdc, 25mA
Green , 2.2vdc, 25mA
Blue , 3.65vdc, 30mA

You can run any of these off of a 12 v supply because there is more than enough to overcome the "bandgap" of the material, but you still need to limit the current to around 20 mA. (less than the max listed).

The calculations are this:
Red: 12.0v - 2.0v = 10v: 10v / .020A = 500 ohm
Green: 12.0v - 2.2v = 9.8v: 9.8v / .020A = 490 ohm
Blue: 12.0v - 3.65v = 8.35v: 8.35 / .020A = 417.5 ohm

Common practice it to find the common value that is closest, but higher value, or if close, but lower (more current) recalculate and see how close you are to max and determine if you can live with the reduction in life.

On a side note, if you put a 1n917 diode accidentally across 120v ac mains, they release their smoke explosively! ie: glass shards many feet away.

Reading between the lines of your questions, prudent advise would be that lower voltage is much more likely to give you the chance to live a long and fruitful life.

I will admit that the idea did intrigue me when I first read it. Outdoor IR illumination came to mind, but after mulling it over, even with 25 years of working around the "magic lightning" I decided I'm going to pass.

Good luck
Bo

The little LED thumb sized flashlights are an interesting case in design. You open it and wonder why they don't need a resistor. The batteries are coin cells that are limited in the amount of current that they can supply even if shorted, so the current limit is a function of the batteries. If you supplied the same voltage from a power supply, the LED would smoke in short order.