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Any Electrical Engineers/Gurus?

Discussion in 'Hangout Lounge' started by -Azrael-, Jun 22, 2011.

  1. -Azrael-

    -Azrael- Member

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    Can anyone tell me how much heat a 680 OHM 1 watt resistor will generate?

    It'll be running on a 14V source with one 1.8 V 20mA load...

    Cheers
    Dave
     
    -Azrael-, Jun 22, 2011
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  2. markie

    markie Member

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    If P=Isquared r then 0.02X0.02X680 = 0.272w
     
    markie, Jun 22, 2011
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  3. -Azrael-

    -Azrael- Member

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    The wattage is irrelevant to this case. The wattage of a resistor is just the max load. You are correct in the .272 load so I want a .5 watt but the shop is out so I'll be running a 1 watt. But my question is about the amount of hea being generated as a 680 ohm resistor is quite a large dissipation.

    Cheers
    Dave
     
    -Azrael-, Jun 22, 2011
    #3
  4. darkfibre

    darkfibre Member

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    The larger the resistance the less current flow, the less current flow, the less heat.
     
    darkfibre, Jun 22, 2011
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  5. laptop_geek

    laptop_geek New Member

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    Heat is measured in watts and it is equal to the power dissipated from the resistor. Markie was correct in his calculation. Ask me how I know...
     
    laptop_geek, Jun 22, 2011
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  6. mlew

    mlew Well-Known Member

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    A 1 watt resistor will not generate anymore heat than a .5 watt one. The rating is just the maximum wattage the device can handle. Since you are only using 20ma current thats only .272 watts just like the others have said. Either one will work just fine the 1 watt is just physicall larger.
     
    mlew, Jun 22, 2011
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  7. Gitarded

    Gitarded Member

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    You are better off with a higher wattage resistor. Even if you are dissapating .250 watts or close to it. You would want a 1/2 watt or larger. You should be fine.
     
    Gitarded, Jun 22, 2011
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  8. -Azrael-

    -Azrael- Member

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    ok, I had always thought that the heat was relative to the ohms of resistance versus the wattage but I guess I was wrong. So my 1W will be nothing to worry about melting anything...

    Cheers
    Dave
     
    -Azrael-, Jun 22, 2011
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  9. markie

    markie Member

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    I think you will need to test the circuit and perhaps lower the resistor value as the load will have some resistance. Effectively you are "Dropping" the bulk of the voltage across your resistor.

    This curcuit is called a potential divider and to get 1.8v at the load, you want to be closer to 600ohms.
     
    markie, Jun 23, 2011
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  10. -Azrael-

    -Azrael- Member

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    This is true for a standard operating circuit but under revs the bike will put out up to 14.5V spikes, giving me a need for a 635ohm resistance minimum. So I can wire multiple resistors to get just over that resistance but I had figured that going up to 680 (the next larger single resistor) would be fine seeing as even if it is not as bright as it could be the 3500mcd LED is still damn bright.

    I will try it all out before doing anything final but I wanted to make sure I wouldn't be doing anything potentially dangerous.

    I will post some results as the project comes along.

    Cheers
    Dave
     
    -Azrael-, Jun 23, 2011
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  11. -Azrael-

    -Azrael- Member

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    -Azrael-, Jun 23, 2011
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  12. Polock

    Polock Well-Known Member

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    Polock, Jun 24, 2011
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  13. -Azrael-

    -Azrael- Member

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    -Azrael-, Jun 24, 2011
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  14. KrS14

    KrS14 Active Member

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    Holy crap, I didn't even know they made such a thing lol
     
    KrS14, Jun 24, 2011
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  15. -Azrael-

    -Azrael- Member

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    Well, I have four enroute to me, so I will post in my build how they work

    Cheers
    Dave
     
    -Azrael-, Jun 25, 2011
    #15

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