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Thread: Wattage on resistors

  1. #1

    Default Wattage on resistors

    So I recently finished wiring up a RGrB Tri-Cree up to my US Apprentice LE v4, and while I was, I had to, of course, hook up resistors to the Red and Royal Blue to make purple. My dad helped me make it, and with his extensive knowledge of electronics and related material, he told me to not use a .47ohm 1/2 watt resistor on my blue LED. He said that the wattage was too low, and told me to get a .47ohm 5 watt resistor. Herein lies my question: could I have used the 1/2 watt resistor? I was using a 4 AAA battery pack (rechargeable, so a bit less than 6 volts). I like my current setup, with the 5 watt resistor, so I'm just asking out of curiosity.
    Last edited by Cjhpkns; 06-10-2018 at 06:39 PM. Reason: Mistake

  2. #2
    Sith Warrior darth_chasm's Avatar
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    What you're saying doesn't make sense unless I'm reading wrong. Your father said a 1/2 watt resistor was too low then suggested an even lower wattage? 0.5 > 0.47... Then you say you have a 5 watt resistor...

    For a 6 volt set up you need at least a 2.7 ohm 3+ watt resistor for the blue. 0.47 wont cut it. And 3.9 ohm 5+ watt resistor for red.
    Last edited by darth_chasm; 06-10-2018 at 05:40 PM.
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  3. #3

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    You're right. I meant 5 watt. It was a .47 ohm resistor. Sorry about that. So I used a .47ohm, 5 watt resistor on the blue, then a 4.7ohm 5 watt resistor on the red, and I got the purple I wanted.

    Define "won't cut it"; what do you mean?

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  5. #5

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    Quote Originally Posted by jbkuma View Post
    Rapid unplanned disassembly.
    What?

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    Sith Warrior darth_chasm's Avatar
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    Quote Originally Posted by Cjhpkns View Post
    You're right. I meant 5 watt. It was a .47 ohm resistor. Sorry about that. So I used a .47ohm, 5 watt resistor on the blue, then a 4.7ohm 5 watt resistor on the red, and I got the purple I wanted.

    Define "won't cut it"; what do you mean?
    You need a minimum resistance that will protect your led from receiving too much current and burning out. For an led that is 3.4 Vf @ 1000mA with a 6 volt supply you need at least 2.6 ohms of resistance. 2.7 is the closest most common resistor when rounding up.

    Study this http://forums.thecustomsabershop.com...e-to-Ohm-s-Law

    Quote Originally Posted by jbkuma View Post
    Rapid unplanned disassembly.
    He's saying you need to fix the issue before you are a very unhappy camper with a burnt out led.
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  7. #7

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    Quote Originally Posted by jbkuma View Post
    Rapid unplanned disassembly.
    that sounds like a disaster in outer space, like Apollo 13 or something. Got my laugh for the night at that one. Hey, ever had a "rapid unplanned disassembly?" Yep, this one time..........

    Tom

    "Mistakes are our greatest teacher."

  8. #8

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    Is putting a 4.7ohm 5 watt resistor on the battery assembly a viable solution? I'm trying to not mess up the color mix I have made, but I also do not want for my LED to burn out.

    Also, if the LED was going to burn out, how long would it take before it would? I've been using this current setup (with the .47ohm resistor on the blue) since this past Thursday.

  9. #9
    Sith Warrior darth_chasm's Avatar
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    I’m surprised it hasn’t happened already. I’ve accidentally popped them just testing with bench power supply. I don’t believe there is a specific answer as to when. It’s more like, yes it will definitely shorten the life of that diode.

    As for your question on a resistor on the battery assembly. If you mean one resistor for both red and blue, we tend to highly discourage that. 1 resistor per diode.

    You can leave it as is and tempt fate, but you have been informed
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  10. #10

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    The life of the LED is not really the issue for the power rating, as it gets hotter the resistance will increase only exacerbating the real problem. The wattage refers to the heat dissipation of the component. If you use a 1/2 watt resistor when you need a 5 watt resistor there is every possibility something could burn, and possibly even catch fire depending on if there is enough air, the flammability of the neighboring materials, and the whether the component fails first or not. In any case at the minimum you can expect is that something will burn out and any jacketting and shrink tubing near the resistor will likely melt. Even if you aren't using lithium batteries, your batteries won't be happy if they are short circuited. The longer the underrated component is in use the hotter it will get and the more likely something disastrous will happen.

    With the LEDs you have a different problem, the more voltage available, the more current they will draw. If you used a resistor with a lower resistance rating than required the LED will have a higher voltage available and draw more current dissipating more and more heat until it pops. You can drive an LED a little hotter than the spec and get a bit of brightness at the cost of life, but if you go to low the current will spike and it will pop. You can really get your LED cooking to the point of burning hot if you use a resistor in between the proper value and the instantly popping limit. Most cheap LED lightbulbs are over driven like this and that is why they get very hot and die quickly. (also because their circuit designs don't allow for attrition)

    "Rapid unplanned disassembly" did indeed originate in the aerospace industry (or at least made popular by stories from the legendary days of the "right stuff" etc) but it is often used in both the engineering and prop building communities today.

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