resistor calculation

Discussion in 'The Projects Forum' started by sangpo, Aug 17, 2013.

  1. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Sir/madm . Using 7805 , i want to make my own varaible voltage .ie. 3vdc,6vdc and 12 vdc. so in this case , how do I calculate value of resistor for 3vdc, value of resistor for 6vdc and value of reistor for 12vdc. Please help me here
    thank you
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You really can't make a proper 3 volt regulator using the 7805 and resistors.

    An LM317 would be better and go even lower than 3 volts.
     
  3. #12

    Expert

    Nov 30, 2010
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    I have too much time that I need to waste today.
     
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  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Can I buy some extra time from u ??

    EDIT : Yard Work :rolleyes:
     
    Last edited: Aug 17, 2013
  5. Metalmann

    Active Member

    Dec 8, 2012
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    Is that for real?:eek: That's all there is to it?:confused:



    :cool:
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Maybe :D

    The thing is, learning what the available components can do.

    Years ago It wasn't (quite) as tough.

    Now, it's a bit mind-boggling (In my opinion)

    But, I still appreciate stuff that glows ;)
     
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  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,019
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    Wish I could get to that point! I seem to have got hooked on the smoke.:p
     
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  8. #12

    Expert

    Nov 30, 2010
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    Sorry. Already stuck with yard work. Finished a plumbing job at 5 pm. Have to wait until 7:30 pm when the heat index goes down to 93 degrees so I can mow the lawn. (Mowing sucks in August in Florida!)
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    A little smoke, once in a while is a good thing ...
     
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  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How in the heck can you STAND plumbing !%$#@$#^%# ???

    I have boxes of plumbing pieces that don't fit what I need at the moment. :cool:

    EDIT: I HAVE plumbing stories .......
     
  11. #12

    Expert

    Nov 30, 2010
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    It comes with the territory, but among things I don't like to do, from worst to less worst, the list goes:

    Painting
    Plumbing
    and gets better from there.
    There's nothing quite like laying in a puddle of poop water, trying to fix ruined metal, while the mosquitoes try to eat you alive...and yet, I hate painting even worse.
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
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    #12,

    Does it still rain every afternoon in Florida ??
     
  13. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I hate painting more than anything............

    But, what about the ghekos ?? (small lizards how do you spell that ??)
     
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  14. #12

    Expert

    Nov 30, 2010
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    It has this week.
     
  15. #12

    Expert

    Nov 30, 2010
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    The native lizard is the anole. Geckos (insurance sales lizards) are very rare here.

    http://en.wikipedia.org/wiki/Brown_Anole
     
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  16. wayneh

    Expert

    Sep 9, 2010
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    Gosh, I enjoy the actual painting. Getting ready to paint - scraping, sanding, chiseling, repairing, cleaning, and on and on - would be considered cruel and unusual punishment if you forced prisoners to do it. Painting goes so quickly and makes such a big difference.
     
  17. #12

    Expert

    Nov 30, 2010
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    I quite agree. Painting is like soldering: 90% of the work is preparation.
    The difference is that if painting takes 4 to 6 hours, the preparation time was 40 to 60 hours. Soldering a pipe takes about 50 seconds, so the preparation takes about 8 minutes.
     
  18. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Thanks for valuable suggestion made by tubeguy. I have gone through the circuit diagrame for LM317. But here, i wan to know how the value of resistor is calulated. And also, from where did we get 1.25 volt.Please kindly explain for me.

    Thank you all
     
  19. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    thanks for the diagrame of LM317. Please kindly explain me how the value of resistors are calculated.
    Thanks you somuch
     
  20. #12

    Expert

    Nov 30, 2010
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    1.25 volts comes from the data sheet for the LM317 chip.
    The value of the resistors is calculated by using the formula I wrote on the drawing or by using the formula on the data sheet for the LM317 chip.
     
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