Resistor needed for 24V -> 12V ?

Discussion in 'General Electronics Chat' started by Nicholas, Jan 10, 2013.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Hi all!

    If I need to take 24V down to (approx.) 12V, what size resistor
    would I need? The 12V is to be used for an LED at short bursts.

    I know there's a lot missing, like internal resistance etc. but I tried
    an online circuit maker, and it seems 10 ohm will do it. If so, I should
    probably also know the wattage of the resistor.

    Any advice?

    Thanks
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Yes, there is alot of information missing. If you want to connect 24V to a LED, there is no need to convert it to 12V. Just a series resistor of 500 0hms or so wll do. If you give me the specs for the LED, I can show you how to calcualte the exact resistor value.
     
  3. jfish

    New Member

    Mar 6, 2011
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    I would say 600 ohm (24V - 12V = 12V/.02 = 600) but 12V is way too much for a single LED. Wattage is probably 1/8.
     
  4. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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  5. Brownout

    Well-Known Member

    Jan 10, 2012
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    Looks like is has some integrated circuitry. In that case, you'll need 12V to run it. I wouldn't suggest using a resistor to get the 12V. Instead, I'd use a 12V regulator, such as the 7812.
     
  6. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Ok, why is that? (just seems the easy way:))

    The LED will only flash here and there
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,137
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    Because a resistive divider is no good at dealing with variable loads. When the LED turns ON it wont be running at 12 volts any more because it loads down the resistive divider.

    Maybe you should provide a schematic of what you are planning. The three terminal regulator can also be configured as a constant current source independent of voltage which will give constant and consistent brightness.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Looks to me like "H6W" is the base type and not the power... so who knows how much power (and thus current) this thing takes?

    If you already have one, give it 12V and see how much current it takes. Then the resistor is 12V divided by that current (in ohms). What size wattage is again that current times 12V (in watts), and you can cheat that number since it will not always be on.

    By just giving it 12V from a fixed regulator you can bypass the measurement.
     
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  9. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Hi!

    I just measured it. At 12V it's at 0.13 A

    So,

    12V/0,13 A = 92 ohm

    12V * 0,13 A = 1.56 W

    Thanks a lot!
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    ALWAYS multiply the wattage times 2 (or more) for safety.. You don't want a 1W resistor dissipating 1W unless you want a seriously hot resistor.
     
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  11. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Ok, I thought about that!! Good tip!
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    That LED light bulb is illegal to use in North America because its brightness is not specified.

    Too many kids in Canada used cheapo Chinese LEDs from E-Bay for car lighting that could not be seen. Then the government banned modifying car lighting.
     
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