What Resistors to Buy?

Discussion in 'The Projects Forum' started by BR549, Mar 29, 2010.

  1. BR549

    Thread Starter New Member

    Mar 29, 2010
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    I have 4 LED lights that are too bright. Each light has 3 LEDs. The model number is RP-1445R. I want to use resistors to reduce the brightness of these lights to about 50%. The following shows these lights - http://www.amazon.com/RoadPro-RP-1445R-4P-Clearance-Marker/dp/B001JT1HOY

    The source is 12v. I'm an electrical novice, I know nothing more about these lights. What resistors should I buy? Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Can you open one up and take a photo of what it looks like inside?

    How much current does one of these use?
     
  3. BR549

    Thread Starter New Member

    Mar 29, 2010
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    No, unfortunately the lens seems permanently attached to the LEDs. I know nothing else about these lights. What's more, I know next to nothing about current, amps, Ohms, watts, voltage, etc.

    My research has determined that with certain specifications, a proper calculation can be done to determine the resistors needed. However, these specifications don't seem to be available.

    And so I'm hoping for a ball park idea of what resistors to buy. Otherwise I'll be left to try different ones and use a process of elimination.
     
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    With the risks it involves, could you get someone able to tell what resistors are limiting the current to those LEDs?
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Many built-up LED lamp units use active electronics, effectively a small switched-mode power supply, to drive the LEDs efficiently.

    These can not be dimmed with external resistors; either nothing happens or they turn off/flash when the power is too restricted.

    If however the item you show is simple LED+Resistor circuit already, then it can be dimmed with an additional external resistor.

    But: it's not possible to calculate a resistor value and power rating without knowing either the wattage and voltage of the light unit or the current it's drawing.

    We need either electrical measurements, specifications or internal photos to work with.
     
  6. BR549

    Thread Starter New Member

    Mar 29, 2010
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    Pretty much what I expected. Thanks
     
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    There is nothing to stop you experimenting to see if they can be dimmed.

    Try connecting them using various small 12V filament lamps bulbs in series with the LED as resistors, like 1W, 3W, 6W sizes or whatever you can find.

    You may find it dims the LEDs, or it may go out/flash/have no effect depending on the partucular lamp rating.

    Knowing what bulb size causes a reasonable dim light (from the LEDs) will at least give us a ballpark idea of the led lamp power and what range of resistor values may work.
     
  8. Blackbull

    Well-Known Member

    Jul 26, 2008
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    A 500 ohm pot. perhaps!
     
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