Ac / dc led?

Discussion in 'Homework Help' started by MingJae, Mar 3, 2010.

  1. MingJae

    Thread Starter New Member

    Feb 19, 2010
    I want to lit up to 10 LEDs in one circuit (red,green,blue,white,and orange) using
    house supply (220v AC), is it still possible if I only use :
    ( (220v-1.7v)/ 0.02mA ) ±10.915Ω resistor? or I need different components in AC circuits? thx b4... :D
  2. bertus


    Apr 5, 2008

    Isolation must be provided.
    Use a transformer to create a lower voltage.

  3. MingJae

    Thread Starter New Member

    Feb 19, 2010
    hello too...

    do u mean electricity tape? with black colour right?
    what is a transformer? is it still possible if I use only resistors?
  4. JDT

    Well-Known Member

    Feb 12, 2009
    Use a transformer to electrically isolate the LED circuit from the mains supply.

    You will still need a resistor to control the LED current. You will also need a rectifier to convert from AC to DC.

    The transformer has two advantages:- Not only does it provide safety isolation but it reduces the voltage. This means that far less power will be lost in your resistor.

    To work out what the value of your resistor will be, find out what the forward voltage of your LEDs are at the current you want to use. A small red LED at 20mA (0.02A) will drop about 1.2V. Other colours often have a higher voltage drop. Blue ones are about 3V.

    Connect all your LEDs in series. Add up the voltage drops. Choose a transformer secondary voltage a few volts higher than this. Calculate your resistor to give this voltage drop at your current. R=V/I

    Calculate the power loss in your resistor P=I^2/R Make sure your resistor can handle the power.

    Fit a bridge rectifier on the secondary of your transformer to convert from AC to DC.
  5. LoganFife

    New Member

    Feb 7, 2010
    hi, what you are proposing is very dangerous, please don't wire up Leds to the wall socket.

    i think your best bet is to get a pre-made DC wall wart, those chunky black boxes for powering small electrical appliances. maybe an old phone charger or something? or pick one up from your local DIY for a couple of quid.

    that will give you a low voltage smooth DC output from the mains supply, perfect for what you need and a hundred times less deadly.:D