120VAC Step Down Help

Discussion in 'The Projects Forum' started by Brutis1220, Nov 17, 2008.

  1. Brutis1220

    Thread Starter New Member

    Nov 17, 2008
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    I am working on a school project of creating a LED bulb that works on standard 110-120 VAC. The bulb should serve as a viable replacement for a 60 incandescent bulb. The plan was to run the 120VAC through a 2:1 transformer, then through a rectifier so that the output would be around 60VDC(this will serve as the input to various components and the LED driver). I am having trouble finding a transformer that will meet the desired space requirements (very compact) and still meet the voltage and current requirements (60VDC & around 1.5-2 amps). I suppose i could just place a large resistor in line before the rectifier, but this seems to be very wasteful. Can you help me find another method to step down the voltage efficiently.

    Thanks in advance:D
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    They don't use transformers for LED bulbs, they actually power these from the mains. This is clearly dangerous if you don't know what you're doing!

    Steve
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You shouldn't be trying to replicate the power consumption of an incandescent 60W bulb; you should be trying to replicate the light output using far less current.

    So, find the most efficient white LEDs that you can, and then figure out how many of them it would take to equal the light output of a standard 60w bulb. Then you can work out what you really need in the way of a power supply.

    Keep in mind that many LED manufacturers inflate their numbers by using lenses with a very narrow focus. A standard 60w incandescent lamp radiates almost equally in every direction, whereas most LEDs are focused. You'll have to emulate the coverage of a 60w incandescent by overlapping the light patterns.
     
    Last edited: Nov 17, 2008
  4. Brutis1220

    Thread Starter New Member

    Nov 17, 2008
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    SgtWookie,

    Thanks for the info. I should have worded the post above differently. I realize that i am trying to replicate the luminous output of the incandescant. I have a few leds in mind ranging from ones that produce 60 lumens to some that produce 240 lumens (obviously not taken into account the loss from various sources such a heat etc...) and these have a forward voltage ranging from 3.4 to nearly 12 V, and a forward current ranging from 350mA to 1 A.

    The average 60W bulb produces aprrox 900 lumens, so if i use the higher of the LEDs i would need 4 (240*15=900), which would required 48V if connected in series correct?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK, but you must keep in mind the focal pattern of the LEDs. I don't know of ANY commercial LEDs offhand that have an illumination pattern similar to that of an incandescent lamp. If they had a really broad pattern, one might construct them in something like an inverted pyramid shape and place it within a translucent envelope to diffuse the light. Direct viewing of either high-intensity LED or incandescent light will rapidly lead to permanent vision damage and blindness.

    I'm not following your math; how did you come up with 240*15=900? Perhaps you meant 60*15=900, which does work.

    Let's look at some numbers.

    If the 240 lumen LEDs require 12V @1A, then each requires 12W. (Power in Watts=EI, or Voltage * Current)
    900/240=3.75. So, you'd need 12W x 3.75 = 45W to equal a 60W incandescent bulb. It's a net savings of 25% power; if stepping down to the necessary voltage/current were 100% efficient. However, the best you're going to get there is around 93%, even with a pretty sophisticated design.

    If the 60 lumen LEDs require 3.4v @ 350mA, each one requires 1.19W. Since you need 15 of them to equal the 900 lumens, your LED power requirement is then 17.85W. If you could run them all in series, you'd need 51V @ 350mA. With a very efficient converter, you might get 93% efficiency for around 19.2W power consumption, or about 3x more efficient than a standard 60w incandescent lamp.

    But now you're having to deal with more components. Assembly/test is always a large portion of the cost of producing an item. Increased component count decreases reliability.
     
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