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let's see if i'm getting this right.

Discussion in 'General Electronics Chat' started by mybuickskill6979, Jan 26, 2007.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The MPSA14 is a darlington with a typical current gain of 60,000.
    Motorola invented it and their datasheet shows a saturation voltage chart with Ic/Ib= 1000.
     

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  2. mybuickskill6979

    mybuickskill6979 New Member

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    okay i see that one. cool i was looking at the fairchild!! Awesome!!
     
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  3. mybuickskill6979

    mybuickskill6979 New Member

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    i just DL'ed and installed cadlite 3.5.9 so i have a schematic drawer now i'll have to mess with it a little i'm gonna have to find a friend with an inkjet printer though :D
     
  4. dave

    Dave New Member

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  5. ljcox

    ljcox Well-Known Member

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    Well done!

    The 1000 comes from the Vce sat conditions.

    I have the Fairchild data sheet and it gives Vce sat for Ic = 100 mA and Ib = 0.1 mA.

    Therefore, hFE = 100/0.1 = 1000.

    Also note that Vce sat = 1.5 Volt max under these conditions.

    The pulsed current gain for Ic between 1 mA and 100 mA is about 100k, ie. 100 000.

    The minimum is specified as 10k at Ic = 10 mA & Vce = 5V and 20k at Ic = 100 mA & Vce = 5V.
     
    Last edited: Feb 7, 2007
  6. mybuickskill6979

    mybuickskill6979 New Member

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    cool. i'll do a couple more today a couple more npn's then for the pnp's till i get comfortable. for the next couple days or until i have the datasheets for all the transistors i have =)

    i found a little compartmentalized box to put all my resistors and stuff in. the kitchen table loks nicer now haha!!
     
  7. fingaz

    fingaz Member

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  8. mybuickskill6979

    mybuickskill6979 New Member

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    i can't seem to get that one i don't know why.
     
  9. mybuickskill6979

    mybuickskill6979 New Member

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    okay i've looked at the fair child and now at the motorola sheet for the 2n5551 fairchild said somewhere between 10 and 12. motorola says the same but in figure1 at 25d C and 20mA it says between 60@1vdc and 150@5vdc then in figure 4 it shows ic/ib=10 so which should i go with? everything points to 10 but i just want to make sure.
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    They show a base current that is 1/10th of the collector current when it is used as a saturated switch. Then all of them will work, even the weak ones that still pass the spec's.
     
  11. mybuickskill6979

    mybuickskill6979 New Member

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    okay. sweet i'll go with the twelve just to make sure!! Thank you!!
     
  12. mybuickskill6979

    mybuickskill6979 New Member

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    sorry for the hugeness. okay i got to change that for next time lol!!

    Post with MASSIVE graphic deleted, scale your pictures sensibly before posting them!

    Moderator.
     
    Last edited by a moderator: Feb 8, 2007
  13. mybuickskill6979

    mybuickskill6979 New Member

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    okay from now on i will go with the higher HFE i find as they all 3 HFE's as audio said work and get the same results(brightness of the LED)

    okay i got the switch for all my npn's working and proper i'm gonna do this same thing for my TIP42 and see if i can get it. then i don't know if'n i should go on to learning about bias and the amplification stuffs or if i should mess around with other types of transistors. like FET's and things like that! what do you guys think would be best?
     
    Last edited: Feb 8, 2007
  14. Sceadwian

    Sceadwian Banned

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    For driving a static load like an LED the HFE isn't going to affect brightness, it's only going to affect the needed drive current at the base.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    This circuit uses the transistor as an emitter-follower. The pot adjusts the voltage to the base of the transistor and the voltage of the emitter follows it with a higher current. The pot adjusts the brightness of the LED.
     

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  16. mybuickskill6979

    mybuickskill6979 New Member

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    awesome it works. i ad to use different resistors or a couple in parallel to get the proper resistance but it works. awesome. i'm gonna do some probing now to see just how it works.

    one note on top-ramen noodles that stuff stay good for a while. but watch out for the bugs that might grow from it. i just learned my lesson on that and thought i'd share!!
     
  17. mybuickskill6979

    mybuickskill6979 New Member

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    okay i was reading about follower circuits. and from what i read HFE comes into play again and so does voltage saturation(sort of) for an HFE of 100 for every amp that does to the base the trans lets 100 amp go through the collector and the emitter. at a smaller voltage that concurs with the voltage saturation ratio/curve of the transistor. is this true?
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A follower has a small base current from the pot and its hFE makes a higher current at the emitter.

    The pot makes an adjustable voltage and the emitter follows it (but it is 0.7V less than the base). Then use Ohm's Law to calculate the current in the LED and in its current-limiting resistor. High current makes high brightness, low current makes low brightness.
     
  19. ljcox

    ljcox Well-Known Member

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    In the case of the Emitter follower, the transistor does not saturate and hFE is largely irrelevant.

    The name "emitter follower" means that the emitter voltage "follows" that of the base.

    So if the base voltage is 4 Volt, the emitter voltage will be about 0.7 Volt lower (for an NPN), ie. 3.3 Volt.

    In the PNP case, it will be about 0.7 Volt higher. Think about it.
     
  20. mybuickskill6979

    mybuickskill6979 New Member

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    will do. i think i moved the breadboard too much yesterday i have to dismantle the circuit and put it back today before i can test anymore lol!!
     

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