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Electronics Need help with resistor/wattage

Discussion in 'Modding' started by bassboi72, 14 Feb 2005.

  1. bassboi72

    bassboi72 My dremel is broken.

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    I am building a circuit that supposedly operates at 1.8w, current being 0.20A (which I assume is 20mA) and I have calculated the resistance needs to be around 33 ohms. Since I cannot find a 2w 33ohm resistor, and I don't think it would be much trouble and cost too much to make this, I have come up with this sketch up, and I was wondering if this resistor network is capable of having a resistance value of 33 ohms, and still being able to tolerate 1.8w of power.

    If I calculated things right from my knowledge, total resistance would be 33.6 ohms and the wattage would be 2.5w.

    Here's the image:
    [​IMG]

    Thanks for the help, if any, I appreciate it!
     
    Last edited: 14 Feb 2005
  2. Kerri Ann

    Kerri Ann What's a Dremel?

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    what's the value of I? current....
     
  3. bassboi72

    bassboi72 My dremel is broken.

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    Image updated due to errors in my work. Also, added value of I in the description.
     
    Last edited: 14 Feb 2005
  4. Kerri Ann

    Kerri Ann What's a Dremel?

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    actually, i don't think the current even matters... :)
    anyways, yeah, i think 33.25 is right on.
     
  5. Kerri Ann

    Kerri Ann What's a Dremel?

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    oops.... nm,

    but this time i got 42 for R.
     
  6. Hazer

    Hazer In time,you too will be relixalated

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    0.20 Amps = 200 mAmps.
     
  7. bassboi72

    bassboi72 My dremel is broken.

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    isn't the formula for resistance in parallel somethin like..

    1 / ( 1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5 )

    if I use that formula in MS excel, it gives me 33.6...

    because I'd add all the resistors that would be in a series together, thus giving me 168 ohms x5...

    and then the formula in excell would be:

    =1/(1/A35+1/B35+1/C35+1/D35+1/E35)

    Where you'd enter that line of code in cell F, and then input all your R values in cells A35, B35, C35...etc..

    A little trick I've learned is that if you have all the same value resistors in parallel, you just take 1 value and divide it by how many resistors are in the array.

    Like in my case, 5x "168 ohm resistors" (adding 100 and 68), then dividing that by 5, would give me 33.6...

    But besides all that, I'm just worried about the wattage.

    Wouldn't the wattage be like, adding all the wattages of the resistors? Since they're all going to be 1/2w, and 1/2 x 5 = 2.5, that would mean that little circuit has the power to handle 2.5w, wouldn't it?
     
  8. SteveyG

    SteveyG Electromodder

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    33.6 Ohms.

    The power rating won't be 2.5W, since the power isn't evenly distributed between the 100R and 68R resistors.

    If you give us the rest of the details - i.e. how many volts this network is supposed to drop, then the power rating can be calculated.
     
  9. Kerri Ann

    Kerri Ann What's a Dremel?

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    oops, youre right, i was doing 168 x 4.... :duh: didn't notice at first you had 5 rows... :D
     
  10. bassboi72

    bassboi72 My dremel is broken.

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    I had changed the pic... because the first one wasn't correct...

    Basically, I wanted to take this fan, and have a voltage drop of 4v to reduce the amount of exhaust airflow, and prevent my case from being infested with dust.

    But, I have figured things out.

    Two of these for intake, in the bottom front of the case

    This one as the rear exhaust (this is the one I was going to add those resistors to, since I thought it was going to be pushing 32cfm like the first one, instead of 22cfm

    And I'd have this on the top for exhaust, since I'll be watercooling and this fan will cool the water in the radiator

    I personally think that this machine may be rather loud, but I doubt it being that the total dbA for all 4 fans is 27.1
     
    Last edited: 14 Feb 2005
  11. cpemma

    cpemma Ecky thump

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    If you pass 200mA through a 33R resistor, the heat produced is I2R, 1.32W. Voltage dropped is IR, 6.6V.

    Three 100R 0.5W in parallel will just do, but they'll get quite hot.
     
  12. Dngrsone

    Dngrsone What's a Dremel?

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    Weighing in kinda late here, but three 100 ohm 1w resistors should do the job well.
     

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