How to light LEDs

Discussion in 'General Electronics Chat' started by meijaz, Sep 26, 2010.

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  1. meijaz

    Thread Starter Member

    Nov 20, 2009
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    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?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    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.
     
    Last edited: Sep 26, 2010
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    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.
     
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  4. majsyd2010

    Member

    Aug 29, 2010
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  5. Wendy

    Moderator

    Mar 24, 2008
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    Where is the fun in that? LEDs are among the simplest devices you can build for, and visually gratifying to boot.
     
  6. majsyd2010

    Member

    Aug 29, 2010
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    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.
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    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.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    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.
    ;):rolleyes::D

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

    Member

    Feb 26, 2010
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    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
     
  10. meijaz

    Thread Starter Member

    Nov 20, 2009
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    0

    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?
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    First of all 220V isn't 220V, see my previous post.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

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

    Bertus
     
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