Powering high watt LEDs directly from 120v AC

Discussion in 'The Projects Forum' started by fust, May 4, 2012.

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  1. fust

    Thread Starter New Member

    May 2, 2012
    Hello, and thank you for all the great info you have provided on this site, and for all the help you all have provided to so many people!

    Total n00b, I know just enough to make myself look dumb. Please bear with my errors of judgment, information, and logic!

    I have a few of questions regarding a simple "AC Line Powered LEDs" circuit that has just two LEDs, one resistor, and one capacitor (see pic). I do grasp the basics of what the components are, but I am not sure I understand the actual purpose or action of each of them as they apply to this circuit, so I don't know if what I want to do will just instantly blow up or not.


    The basic question I am wondering is if this circuit could be used to safely power really high powered LEDs, like the 20w or 50w ones they have now, without making it much more complicated with mods. I assume there will be some, and really I hope they are limited to just selecting different capacitors and resistors to suit the LEDs' parameters.

    I like this circuit because it is compact and simple, both requirements for my project of converting some older 660W disco lights to LED. They have rotating heads inside a limited space housing, so I need something small and simple that runs on AC and can mount right on the head with the LEDs. What I mean to say is, "just go buy an LED driver" is not the type of answer I am looking for, because it would probably require more modification because of how the AC power gets to the spinning bulb sockets. I am also not inclined to entertain "just go buy a new LED disco light," because this project is mostly about reusing and modding old power hogging things to use fewer resources and give them new life. Throw in the fact that the old lights are way better built than the new ones, not to mention that they can be found for cheap since everyone is just going out and buying LED ones, and you have the why of it. Buying a whole new light at full retail completely defeats my purpose.

    The circuit comes from http://www.bowdenshobbycircuits.info/. There they state that:

    "The circuit below illustrates powering a LED (or two) from the 120 volt AC line using a capacitor to drop the voltage and a small resistor to limit the inrush current. Since the capacitor must pass current in both directions, a small diode is connected in parallel with the LED to provide a path for the negative half cycle and also to limit the reverse voltage across the LED. A second LED with the polarity reversed may be subsituted for the diode."

    So how would I know what values to use for the capacitor and the resistor for a high powered LED (if this will even work)?

    Here are some data on a high powered 20w LED, as an example:

    Emitting Colour: White
    Reverse Voltage: 5.0 V
    DC Forward Voltage: Typical: 13V Max: 15VDC
    DC Forward Current: 1400mA
    Luminous Intensity: 1800lm
    Viewing Angle: 120 Degree

    Another question, since the AC powers each LED alternately, is it true that you are really only getting the light output equivalent to one LED, since one or the other is always off?

    Next, I am struggling to understand the ramifications of reverse voltage. I am afraid this might be the Achilles heel of my idea. From what I understand, the reverse voltage rating is how much the LED can handle when hooked up backwards, which for this circuit, happens every other AC cycle. I notice from the specs above, that the forward voltage of that LED is 13 to 15v, but the reverse is 5v. So does that mean a circuit would blow the LEDs if I was to design it so that the capacitor and resistor used were correct for the forward voltage of 13v? Except that I am a bit confused, because the circuit description above says "a small diode is connected in parallel with the LED ... to limit the reverse voltage across the LED. A second LED with the polarity reversed may be subsituted for the diode." So does that save me and my circuit?

    Final question, slightly off topic, but couldn't you just put enough LEDs together in a series/parallel combination to eliminate the need for the capacitor and/or the resistor? So like for the example, you could do 9 of the above LEDs wired to one polarity, and 9 to the other, making 117v (13v * 9) that would draw approximately 12.6 (1400mA * 9) amps and light 9 of the LEDs at each phase of the AC, so you could just plug it straight into the wall... Or am I crazy, and that would start blowing things instantly, probably because of the reverse voltage limit getting the full 117v, when it can only handle 45v? Please say it ain't so... :)

    Thanks everyone for taking the time to help me out. I promise to read up some more so I have a better understanding of all this!
  2. Wendy


    Mar 24, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.

    6. Restricted topics. The following topics are regularly raised however are considered “off-topic” at all times and will results in Your thread being closed without question:

    • Any kind of over-unity devices and systems
    • Automotive modifications
    • Devices designed to electrocute or shock another person
    • LEDs to mains
    • Phone jammers
    • Rail guns and high-energy projectile devices
    • Transformer-less power supplies
    Please read our Terms of Service for more information.

    If you want information on how to power these LEDs safely please start another thread, but there is no way this can be done safely from power mains.

    This is not a punishment, but it is part of our rule set here. Feel free to use the site or ask questions for anything else.
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