Resistor Info

Discussion in 'General Electronics Chat' started by kbxc20, Jun 10, 2015.

  1. kbxc20

    Thread Starter New Member

    May 27, 2015
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    Any one seen a resistor like this.
    0.47 ohm 2 watt I assume. However it powers a circuit that is 12 volt DC with a 1.21 amp load flowing thru this resistor.
    I can't see how this size resistor can carry this load unless someone knows otherwise.
    It blew along with a transistor. I replaced the transistor and this resistor with a wire wound 5 watt which blew from overloading.
    Other wise circuit seems to work ok.
    from the above values after a short while. IMG_0616.JPG
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1.21 amps thru a A .47 ohm resistor is just 1.21*1.21*.44=0.7 watts.

    So a 2 watt resistor should be quite fine here.
     
  4. kbxc20

    Thread Starter New Member

    May 27, 2015
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    I get 14.52 watts 12Vx1.21A=14.52W
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I don't. Neither should you.

    If you put 12V across .47 ohms you get 25.5 amps.

    I assumed all of the current was going thru this resistor so the power is I*I*R
     
  6. kbxc20

    Thread Starter New Member

    May 27, 2015
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    This is the amount of amperage flowing THRU the resistor, not as if you if you connected each end of the resistor to the power source.
     
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
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    1.21A*.47Ω = .5687V across the resistor. Not 12V.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    You are making the common mistake of throwing the nearest voltage and the nearest current at an equation just because it needs a voltage and a current.

    P = V·I requires the voltage across THAT device and the current through THAT device in order to determine the power dissipated by THAT device.
     
  9. kbxc20

    Thread Starter New Member

    May 27, 2015
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    Thanks to all replies.
    But back to the original post. The values stated are actual operational measurements. The resistor is just in a series circuit with other components.
    All I know is that it now blows 5 watt wire wound ceramic case resistors after about 2 minutes
    whereas the one in the picture lasted for 4,745 hours. Again is this one made differently or is the current and voltage stated too
    high for the resistor in question. (see pic of disassembled resistor)
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    I would say that something has gone wrong with the circuit such that the "other components" that the resistor is in series with is now presenting a much lower effective resistance to the supply so that you are getting more voltage across (and hence current through) that resistor.

    What is the voltage measured directly across the resistor when it is in operation?
     
  11. kbxc20

    Thread Starter New Member

    May 27, 2015
    11
    0
    Ill have to get one and then measure
     
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