Run a string of LEDs from the mains

[mackrackit]

Hey Sparky

Why not buy a string of lights and save the possibility of letting your magic smoke out for another day?

[The Master]

Because that would be too simple

These LEDs are going to be on a PCB and used to make a really bright light. Ive been thinking about a few other projects that would need the same kind of thing so now would be a good time to get it right.

[mackrackit]

If you are serious about this we need to look at the data sheet for the LEDs.
You say 3-4 volt... That is quite a range.
Amps per LED? 0.025???
And does the mains fluctuate any? Here I get anything from 110 to 120.

[khoog]

Current limiting resistors limit current draw, not voltage. You will need the resistors.

[The Master]

The datasheet can be found here. They use 50mA so the whole string should only use 50mA.

I keep hearing all the time that things only take as much current as they need. So a 50mA string of LEDs will be fine on a 13A mains circuit. I read that they will only take too much current if i put too many volts into them.

The power in our house can be a bit strange. We are supposed to be getting only 230V but ive never seen it drop below 240 and quite often its as high as 250. Dips wont be a problem and spikes should be fine as i could put over 300V into the string and still be within the max voltage

[Melanie]

Don't do it.

As an experiment, put a string of LED's across your Car Battery without any form of Current Limiting - that's a reasonably constant voltage - then go rev your car engine up...

> Could i use 4 diodes to make a rectifier?

Yes

> The diodes are only meant for about 50V 1A.

Well that's kinda stupid to use them on 240v then!

> I found one thats 470N 250V. Would 470N be enough?

Depends on how much current you're pulling... but the 250v bit is totally inadequate, because by the time you full-wave rectify mains you'll have close to 340vDC.

Back to my first line... DON'T DO IT. You are running the risk of harming yourself, your property and putting at serious risk those around you.

[Ioannis]

I double the Melanies post.

But if you insist on doing it, PLEASE promiss to have a camera on and with direct link to internet, so we can all see what happens the moment you turn the power ON!

Direct link is needed as you may not be able to send the video file later!

Anyway, some notes on he LED.

1. 50mA is the Absolute Maximum. So according o D.S., 30mA is the working conditions.

2. Each LED is different so a range of 3-4 volts is logiacal. BUT, if you have a source of 10 volts, then you calculate the 3.5 Volts/.03A to find the resistor that each led should have in order to balance its current.

3. A slight increase in Voltage will dramatically increase the current through LED as its characteristics are not Linear. It is NOT a resistor! It is a diode.

Now if you still want to do it, I would suggest:

WARNING All voltages are lethal!

1. Have a 4x1N4004 or 4007 diodes as a rectifier, or even better a bridge rectifier (4 pins) for 500Volts/2-5Amps.

2. On the AC line, fuse and a small resistor for the in-rush currents, say 2-5 ohms.

3. After rectification a capacitor of 220uF/500 Volts.

4. Calculate maximum voltage as 250 Vac x 1,41=353 Volts DC on te capacitor.

5. Calculate the total drop of LEDs say 332.5 Volts. Calculate the number of LEDs 332.5/3,5= 95 LEDs.

6. Calculate the resistor to be in series as (353 - 332.5)/.03 = 680 Ohms

7. Calculate the power of the resistor .03 x .03 x 680= 620mW. I would go x4 to set it at 2,5Watts

OK. We finished.

Hope you wear safety glasses and gloves because:

1. LEDS may explode if they get high voltage by accident

2 Every where on the circuit are very lethal voltages!

If you still want to do it, don't forget the Video Camera!

Ioannis

[The Master]

Thanks for the info. I know the diodes are rated at 50V but my thinking was that the LEDs are rated at 3V and the diodes would be in series with them. Thats why i asked this seemingly silly question. I will make sure to use diodes suitable for mains.

It seems i give the impression that im willing to plug things into the mains without caring if it will explode or not. I do actually think about these things. I usually check here even if i know what im doing but i have a few doubts. I wont be plugging these LEDs right into the mains. I will start with lower voltages and step it up gradually. I wont be touching random parts of this circuit while its turned on. I have been working with mains voltages for many years and i know how painfull it can be. I will be as careful as i always am when dealing with high voltage