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Discussion in 'Electronics Homework Help' started by MWalton, Oct 14, 2013.

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  1. MWalton

    MWalton

    3
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    Oct 14, 2013
    Not actually homework, but dangerousnewbie help so probably best forum to get pointers?

    Wish to prevent smoke coming out of resistors :) Here is the situation:

    I have a lamp which will requires 12 to 18 V, 2.2 watts.

    I am using a pack of 10xAA batteries which produces 14.25v

    Using Maths I decided I needed a 6.5ohm resistor.

    Just to test I was in the correct ballpark, I tried a 10ohm resistor (because I have some) and got a reading of 1.4w OMG SMOKE

    At this point I realised resistors also have a rating for the amount of current they can safely dissipate.I knew this before but sometimes it takes smoke and a binned resistor before I really learn anything.

    THE QUESTIONS:
    1. On a well-known electronics supplier website, some resistors have a power rating of 3/5W, which I read as three fifths, but others specifically have a rating of 0.6 - am I just reading this wrong?

    2. If I connect several resistors in parallel, say I use 4 of 10ohm resistors, they will share the power load and if this reduces each resistors load to their rated level this would be safe, am I correct in this?
     
  2. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    Why are you using a resistor with a lamp? The effective resistance of the lamp when powered at 12V is 65Ω but what math are you doing? You've also misused some terminology in your statements such as "reading wattage" and resistors having "current ratings" that indicate misunderstanding; some study may be needed. I'm being intentionally circumspect in my post because I believe you're a student and need guidance more than a quick answer.

    1. No.

    2. Yes, but the total resistance is reduced to 2.5Ω. Can you think of a way to connect your four 10Ω resistors to still have 10Ω total resistance while quadrupling the wattage?
     
    Last edited: Oct 14, 2013
  3. MWalton

    MWalton

    3
    0
    Oct 14, 2013
    Thanks for your prompt reply,

    my thinking was, if the lamp says it needs 2.2W that this is what I should supply it... perhaps my thinking is backward? Have I, instead of working out the resistance I need to get 2.2W, worked out the resistance of the lamp?
     
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    3/5W
    I doubt if this is 0.6W. It could be either 3W or 6W, perhaps depending on size or how it is connected.
     
  5. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    Check my edited post above.

    Neither. The lamp will consume the rated wattage at the rated voltage. There's no need for a resistor if the voltage supplied is correct.
     
    Last edited: Oct 14, 2013
  6. MWalton

    MWalton

    3
    0
    Oct 14, 2013
    Thanks for your help, as you say I need to study and I am doing so while applying what I *think* I know... hope I haven't been too annoying to anyone in my current knowledge state.
     
  7. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    No problem; we've all been there.
     
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