Transistor Equation help

Discussion in 'General Electronics Chat' started by ibigpapa, Jun 18, 2009.

  1. ibigpapa

    Thread Starter New Member

    May 9, 2009
    7
    0
    So i'm using the site to help determine the calcuations for a transistor i'm setting up. Here is the site.http://www.kpsec.freeuk.com/trancirc.htm

    I'll talk about the circuit first.

    I have a 9v battery.

    The positive terminal connects to an array of leds with a resistor in front of each array. The array configuration is 3 x 8. That is 3 leds in a series and 8 of those in parrallel. The resistor that goes in front of all of those would be a 100 ohm 1/4w. The led has a forward voltage of 2.4v and a forward current of 20ma. That brings the total current of the array to 160ma.

    The positive terminal also connects to a voltage regulator that takes it down to 3.3v to power a controller.

    I have 2n3409 bjt npn transistor here is the data sheet: http://www.sparkfun.com/datasheets/Components/2N3904.pdf

    the led array negative connects to the collector of the transistor
    the controller pin connects to the base of the transistor
    the emitter of the transistor is then sent to a common ground
    There is a figure very similar to the setup i have on the page for the transistor calculations in the section "Connecting a transistor to the output from an IC" http://www.kpsec.freeuk.com/trancirc.htm

    The transistor's maximum collector current Ic(max) must be greater than the load current Ic.
    load current Ic =
    supply voltage Vsload resistance RLThe transistor's minimum current gain hFE(min) must be at least five times the load current Ic divided by the maximum output current from the IC.
    hFE(min) > 5 ×
    load current Ic max. IC currentChoose a transistor which meets these requirements and make a note of its properties: Ic(max) and hFE(min).
    There is a table showing technical data for some popular transistors on the transistors page.
    Calculate an approximate value for the base resistor:
    RB =
    Vc × hFE where Vc = IC supply voltage
    (in a simple circuit with one supply this is Vs)5 × IcFor a simple circuit where the IC and the load share the same power supply (Vc = Vs) you may prefer to use: RB = 0.2 × RL × hFE

    My problem is not in the calculations of the math but taking all my information and inputing it into the proper variable.
    So if anyone could show me where to put the numbers i have and input those into the equations that would be awesome.
     
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