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LED bulb resistor??

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  • #31
    Originally posted by Mcgyver View Post
    A zero ohm resistor is a gold wire of infinite shortness
    At zero length, material or temp wouldnt matter.

    Fabrication issues issues might be tricky, though.

    Ed
    For just a little more, you can do it yourself!

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    • #32
      Originally posted by RichR View Post
      The bulb is for a night light so how bright could it possibly need to be?
      It's amazing how little light you really need to have in order to navigate a room IF your eyes are adapted to the dark.

      I have a cordless phone next to my bed with a red indicator light for "charging". It's not bright enough to have a reading on my light meter when the meter is held at the standard 3 feet. When it was moved to the other room I found that I was used to navigating by the light from that little LED when I got up in the middle of the night. Without it I was unable to see the shoes on the floor, or the door knob.

      Dan
      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.

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      • #33
        Originally posted by Seastar View Post
        How do you have anything of infinite shortness?
        Wouldn't that be "nothing"?
        Bill
        it was in jest, as the smilie if not the remark itself was intended to suggest ........like scientists improved on the zero ohm resistor and its now nothing. sort of like sending the new kid out to buy a bucket of steam.
        in Toronto Ontario - where are you?

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        • #34
          They are making jokes about my zero Ohm resistors. I wasn't. I was serious. I once purchased 1000 zero Ohm resistors when I needed a bunch for a device I was building. They looked almost exactly like ordinary 1/4 Watt resistors except they had only one colored band and it was black, for zero. When you broke the insulating material off, it was just a piece of solid wire.



          Originally posted by Seastar View Post
          How do you have anything of infinite shortness?
          Wouldn't that be "nothing"?
          Bill
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #35
            Isn't a "Zero Ohm Resister" a oxymoron? If it has no resistance it isn't a resister. I have a Great 5 mile long bridge I'll make you a smoking deal on

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            • #36
              While that may or may not be a high efficiency led, I've never seen one that was rated at less than 20 ma maximum. Most of the time they are in fact run at about that current level- except when a lower brightness was specified in the application. I can see it being reasonably bright still at only 5 ma, especially for a night light.

              When it comes to the wall wart, it certainly will have high output voltage under very low loading (and 5 ma is a pretty low current), so if it's a 12v rated wart, just consider that it's 15 volts for the sake of determining a resistor value. Leave out all the diodes and resistors that came with the led, and just for the sake of making it work, consider that the led needs 3 volts across it to light. The math is now easy- you want 5 ma to pass, and you are dropping 12volts across the series resistor. Ohms law says the resistor value is 2400 ohms. The closest standard values are 2200 and 2700 ohms- the led will be brighter with the 2200 and dimmer with the 2700. I'll take a guess here and suggest that for a night light brightness level with this 'high brightness' led, you would be choosing between 2700 and 4700 ohms for the series resistor.

              The actual voltage dropped by the led, and the voltage delivered by the wall wart are more or less irrelevant at this point as you are going to select a resistor according to what you perceive to be the right light output level. You could go as low as 680 ohms and still be under 20 ma through the led, which will still be safe for it, but I can almost guarantee that it will be way too bright. If you're looking for a maximum light output that the led can provide for years without failing, then maybe only go as low as 1000 ohms for the resistor. But I think 2700 ohms is pretty much what you want to use.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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