I want help on lighting 9 white LEDs from mains. These LEDs are fixed in parallel. First I lit them with 5V power supply and noted the current from DMM to be 500mA and they worked fine.

Now I want to light them from mains (220 V). I assumed a simple circuit with a resistor in series. I calculated that the value of the resistor should be 500 ohms. Finally I connected that resistor, the LEDs and a fuse to the mains. Suddenly, the resistor burned out then the LEDs exploded.

Can anyone tell me what am I doing wrong? why did this happen? How can I light LEDs from mains?

 

What you are trying to do is extremely unsafe, a transformer must be used to help you on this site. It is a core rule here.

Things that close your thread

I have an article on LEDs that might or not help.

LEDs, 555s, Flashers, and Light Chasers

If you want to discuss how to use high voltage AC through a transformer that is within the rules, but direct connect to mains can kill people , even if you think it is safe. This applies to power supplies too.

 

As Bill_Marsden said, you must use a transformer to isolate your circuit from mains power.

You can purchase "wall warts" aka "plug supplies" very inexpensively, in various voltage and current ratings. You probably already have more than one sitting around doing nothing in your home. Using a "wall wart" will keep you safely isolated from mains power.

 

Also you can you buy now a days a pre made LED controller - It comes with power supply and also you have the option of controlling multicolors if you needed.

Following is a good one that I used very recently...

RGB LED Controller 12/24VDC 5A

http://www.getprice.com.au/RGB-LED-Controller-12-24VDC-5A-Gpnc_640--42691634.htm

 

Where is the fun in that? LEDs are among the simplest devices you can build for, and visually gratifying to boot.

 

I agree with you Bill if you are making one for hobby ... But using it as commercial scale, then going for a ready made unit will be a good option.

 

LEDs run on DC, AC line mains are rated in RMS so the actual output peaks you get from them is +/- 1.414 x rated lines voltage.

 

LEDs run on DC, AC line mains are rated in RMS so the actual output peaks you get from them is +/- 1.414 x rated lines voltage.

Again, this is something we don't do here. What you say still applies to transformers. As long as the OP sticks with safe practice the thread stays open, but we have to follow the rules too. I have to admit I haven't always followed the rules, but I'm much better now.

I agree with you Bill if you are making one for hobby ... But using it as commercial scale, then going for a ready made unit will be a good option.

Agreed, but nothing the OP has said would hint that it is for commercial use.

 

... Now I want to light them from mains (220 V). I assumed a simple circuit with a resistor in series. I calculated that the value of the resistor should be 500 ohms.
Can anyone tell me what am I doing wrong? why did this happen? How can I light LEDs from mains?

Yeah, you calculated wrong.

If you aren't going to use a low-voltage isolated power supply (the sensible thing to do), at least go and get yourself one of the many made-for-purpose LED drivers. Also, use a step-down transformer (to make a low-voltage, isolated power supply).

Here's an example: (18 cents at quantity, free samples, etc) Many manufacturers have ready-made led drivers for lighting applications.

http://www.onsemi.com/PowerSolutions/product.do?id=NSI45020JZT1G

45 V, 20 mA ± 15%, 1.5 W package, Adjustable Constant Current Regulator & LED Driver, SOT-223

 

Yeah, you calculated wrong.

V = I*R + 3

220 = 0.5*R + 3

220-3 = 0.5*R

R = 217/0.5 = 434 ohm

Now I understand that it is better to step down the voltage to a safe value. But what is wrong in this calculation?

 

First of all 220V isn't 220V, see my previous post.

 

Hello,

We insist you use a step-down transformer to power the leds.

Bertus