Resistor Help

Discussion in 'General Electronics Chat' started by OSearcy4, Mar 23, 2010.

  1. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    I'm partially familiar with resistors and I'm trying to find an expert.

    I bought a LED and I need a resistor for it but I'm not sure what size. The guy I bought it from said 5ohm 3watt but I can't find that resistor and I have also used some of the online resistance calculators and they didn't say to use that resistor.

    Can someone please help....

    Here is a link to the LED I bought.
    http://cgi.ebay.com/80Lm-3W-High-Po...emQQptZLH_DefaultDomain_0?hash=item19ba4e59be

    Any recommendations are welcome...

    I have thought possibly combining resistors but I'm not 100% clear if what I have in mind would work right. I just need to get the most out of this LED without burning it out.

    Oh I think one of the LED resistor calculators, http://led.linear1.org/1led.wiz, said use 12 ohm 5.9 watt but I can't find anything close to that either.

    Also I'm using 10v power, this LED is going in a car.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    What's it going to do in the car?
     
  3. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    light up when i open the doors
     
  4. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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  5. beenthere

    Retired Moderator

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  6. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    well the wire I tap into only got up to 10v with my meter.
     
  7. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    why 10 watt. is it ok to go that high? i'm not familiar with what the wattage affects
     
  8. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    ok correct me if i'm wrong...
    the ohms keeps the LED from going over its V limit like 12 ohms keeps the LED from exceeding 2.4 V

    What does the W do?
     
  9. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    i have found some 12 ohm 5 watt resistors... would they be better, (safely make LED brighter)?
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you would like to learn about selecting a properly sized resistor, have a look at this chapter of our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

    Your current limiting resistor has to dissipate over 5 watts. A 5 watt resistor will overheat and fail. The next step up is the 10 watt rating.
     
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  11. Wendy

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    Mar 24, 2008
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  12. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    Well I'm trying to stay as cheap as possible because i'm gonna be building this project in bulk, however can this buckpuck power multiple LEDs?

    I was thinking if I could hook this up directly at the power source then I could run all the LED powers to this buckpuck and I would only need one which wouldn't be that bad being that these, correct me if i'm wrong, adjust to changes in supply voltage???
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The BuckPucks automatically adjust the current to stay very close to their specified limits.

    You really do not want to operate those LEDs using plain resistors in an automotive environment. System voltage can be as low as 10v when cranking the engine, and over 40v during transients. Your LED's intensity will vary quite a bit even under normal operation. Additionally, power dissipated in the resistor(s) will be wasted.

    If you want something less expensive, look at building your own switcher using an IC. Texas Instruments, National Semiconductor, Linear Technology, and many other manufacturers make IC's specifically for LED current control.

    Off the top of my head, you might look at a National LM3404, LM3404HV or LM3405.
    Or, NXP's ZLD1366.
    There are many, many others.

    If you are considering selling these things, you won't have much of a market if they are not reliable and efficient.
     
  14. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    do you know if these drivers can power more than one LED? Might not be that bad if I can hook it directly to the power tap and then run the power to all the LEDs from that one source.
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
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    Yes they can. Just look at the datasheets for the products. Some are for arrays. Some are for flashlight type applications where 1 led is on for "dim" mode and 7 leds are on for "bright" mode. There are ka-zilions of combinations for led drivers and leds.
     
  16. OSearcy4

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    Mar 23, 2010
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  17. SgtWookie

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    Many regulators have multiple outputs, but you generally need all of the support components (ie: inductor, diode, etc) for each output.

    You may be able to operate a few of your LEDs in series, depending on their Vf at the current used. Paralleling LEDs is a poor idea; it can quickly lead to a melt-down.
     
  18. OSearcy4

    Thread Starter New Member

    Mar 23, 2010
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    what does that mean? So if I buy this current driver I can't just hook the LEDs directly to the driver??
     
  19. Audioguru

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    Dec 20, 2007
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    How will you cool that high power LED? Its life will be very short (maybe 10 seconds?) if you don't fasten it to a proper heatsink that has air circulation around it.
     
  20. OSearcy4

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    Mar 23, 2010
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