What wattage resistor should I use?

Discussion in 'General Electronics Chat' started by nevermind1281, Mar 5, 2008.

  1. nevermind1281

    Thread Starter Member

    Dec 4, 2007
    14
    0
    Hello. I am going to try and make a surface mount version of the project listed below. What wattage resistors should I choose? Thanks
    [​IMG]
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    With SMT components, sometimes handling is a necessary consideration. You may find that anything below 805 is just too hard to handle. That's 1/8 watt. If power is beyond that, use 1206.

    For an exercise, figure current through the resistors. Multiply that by the resistance to get power dissipated.
     
  3. nevermind1281

    Thread Starter Member

    Dec 4, 2007
    14
    0
    Well, I forgot to mention earlier that I am putting this in a wireless Xbox 360 controller so the voltage is going to be 3v max. So Im guessing 1/8 watt would be fine?
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    In your case, I would think 1/10W resistors should do. Whenever SMT resistors are used, they are readily paralleled in order to increase power handling. It is common to see many 1W SMT resistors in parallel on a PCB.

    You cannot say that it is low voltage, so it will dissipate little power. For example, a 3V supply with a 3 Ohm resistor will draw one ampere, and dissipate I^2*R watts, or 3W.

    Steve
     
  5. nevermind1281

    Thread Starter Member

    Dec 4, 2007
    14
    0
    Ok thanks. I am trying to get all of the parts on mouser, but I cant find the 1n914 diode. What diode can I replace it with? I decided to get the 1/8 0805 resistors. Thanks for the help
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Try 1n4148 diode instead
     
  7. nevermind1281

    Thread Starter Member

    Dec 4, 2007
    14
    0
    Now the only thing I cant find in SMT is the transistors. Are there any equivilants of the 2n2905 and 2n3053 for SMT that I can use? And I know it says it pulses between 3.5 and 6.8 times per second, if I wanted to increase that what would I do?
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    To change the pulse frequency, you'll need to change the RC time constant:

    f = 1/(.693 x C1 x (R1 + 2 x R2))
     
  9. nevermind1281

    Thread Starter Member

    Dec 4, 2007
    14
    0
    I did the formula with C1 as .00001, R1 as 10,000, and R2 as 100,000. I got 2.079. The circuit says it pulses between 3.5 and 6.8 timer per second. Did I do someting wrong?
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    According to the list, C1 is .000001 rather than .00001

    I get: 1/(.693 x 0.000001 x 210000) = 6.8Hz.

    I don't know why Jared lists the range. Perhaps one of the resistors is supposed to have been drawn as a potentiometer.
     
  11. nevermind1281

    Thread Starter Member

    Dec 4, 2007
    14
    0
    I found what I need to make it faster. If I change R2 to a 50,000 ohm resistor, it will pulse at 13.1Hz. The last thing I need to make a fully SMT version of this circuit is the transistors. I checked mouser and tried to find an equivilant of the 2n2905heres the link. For the 2n3053 I found this. Could these transistors work for me? Thanks for all the help :)
     
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