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29508 Views 9 Replies Latest reply: Nov 3, 2014 6:08 PM by MFleisch RSS
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Sep 29, 2012 8:30 PM

Powering a standard LED with 120 VAC

I want to power a LED with 120 VAC. I set up a circuit like the one in the attached file. I sized the resistor with ohm's law. I wanted 20 ma so that means a 6.8 K  resistor with a minimum of 2.4 watts. So I used a 3 watt metal film resistor. even so it gets very hot. Is this normal?????????????? Where am I going wrong??????????? do people use a driver at that voltage????????? it would look goofy to use a 5 watt. please help.

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  • Renron Novice 1 posts since
    Sep 30, 2012
    Currently Being Moderated
    Sep 30, 2012 9:24 AM (in response to davidghofer)
    Powering a standard LED with 120 VAC

    Your best bet is to use a driver designed for 120V and correct wattage. Unless you want to string together Many , Many Leds like they do with Christmas lights.

    Your choice.

    Renron

      • MrSpidey Novice 11 posts since
        Sep 25, 2012
        Currently Being Moderated
        Sep 30, 2012 12:43 PM (in response to davidghofer)
        Powering a standard LED with 120 VAC

        D1 seems to be in the wrong place, it needs to be in series with the LED to block the reverse current, also in that config, it conducts every half cycle, loading the resistor.

         

        I would try about a 10k resistor to limit the total current to about 12ma. (you can still use the 6.8 but it will get somewhat warm) That will cut the power, and properly placing the diode will also help.

         

        Put the diode in series with the LED in reverse.

         

        You’re probably lucky the reverse voltage didn’t blow the LED.

          • MrSpidey Novice 11 posts since
            Sep 25, 2012
            Currently Being Moderated
            Sep 30, 2012 6:34 PM (in response to davidghofer)
            Re: Powering a standard LED with 120 VAC

            Yes you are correct, the diode belongs in series with the same polarity as the LED, and it’s there to drop most of the reverse voltage that some LEDs cannot tolerate. Sorry bout that.

             

            In your diagram the LED conducts for half a cycle, and then the diode conducts for the other half, shunting the current around the LED.

             

            I just bread boarded this circuit (in series) with a fairly new LED a 4003 diode and a 10k resistor. (1 watt resistor)

             

            I was getting 80-85 degrees F off of the resistor and the LED was very bright.

             

            I really don’t know why the diode is used as a shunt it that diagram, but it will burn more watts. Actually the diode may not even be needed for modern LEDs.

             

            Also to consider is the fact that running a LED from AC will cause the lifetime to be reduced because it goes on and off 60 times a second, without some kind of filter.

  • matt43 Novice 1 posts since
    Nov 1, 2014
    Currently Being Moderated
    Nov 1, 2014 11:00 PM (in response to davidghofer)
    Powering a standard LED with 120 VAC

      for the led i used a (22k 1/4 watt resistor with a diode) to make it work (does not get hot) i hope this helps ...

  • MFleisch Novice 1 posts since
    Nov 3, 2014
    Currently Being Moderated
    Nov 3, 2014 6:08 PM (in response to davidghofer)
    Re: Powering a standard LED with 120 VAC

    Better Idea: How do you know the most of the PIV will go across the series diode instead of the LED?  You don't!  Better plan:  Place the diode in parallel across the LED to short the reverse current around the the LED.  So, connect the diode anode to LED cathode and in parallel.  Still keep the series resister of 22k ohms to limit the current to about 7ma peak (120v X 1.414 / 22000).

     

    Matt43 sounds good to me.  Remember that, although a LED is a diode itself, they don't make them to take the Peak Inverse Voltage (PIV) of a 120 VAC power, which due to the high reverse voltage on the diode/led has to take pretty much the full peak voltage of 120 V X 1.414 or about 170 volts.  (Also remember the diode has to be connected to the LED with anode to cathode so they both allow current in the same direction).  LED should be too slow to show the 60 hz "flicker" you are powering with.

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