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Transistor selecting

Discussion in 'General Electronics Discussion' started by Grimm, Mar 10, 2011.

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  1. Grimm


    Mar 10, 2011
    I need help selecting a transistor.

    It will be supplied 9V from a power source and feed two pairs of two series LED's. (4 total)

    The LED's have a If of 70 mA and V of 2.6v.

    The signal comes from a propeller chip at 3v and (specified, not measured) 40 mA.

    I was having trouble designing this circuit, hoping to get some help.


  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You would be looking at a circuit like this:


    R2 is chosen to limit the current to the LEDs (it will have 9 - (2.6 + 2.6 + Vce(sat)) volts across it. For a BC548, the Vce(sat) at 100mA is between 0.2V and 0.6V. Using 0.2, the voltage across R2 will be 3.6V, and at 70mA, it would require a value of just over 51 ohms. The value you choose depends on whether slightly more than 70mA is acceptable. If so, use a 47 ohm resistor. If not, a 56 ohm resistor may be a better choice. (note that if you have easy access to values in other than E12 series, then you may be able to select a value closer to 51 ohms -- indeed 51 ohms is an E24 value.) The resistor will be dissipating almost exactly 1/4 Watt, so get a resistor with a power rating *higher* than 1/4W.

    R1 is chosen to give an appropriate base current. A 1k resistor should provide enough base current to ensure a likely transistor is well saturated. The 1k resistor will be dissipating almost no power at all (well, very little compared to likely power ratings -- 25 thousandths of a watt).

    The transistor could be something like a BC548. Pretty much any small NPN transistor will do. 70mA is not a huge current and with 4mA of base current the circuit is not demanding hfe in excess of about 30.

    A higher gain transistor will allow you to increase the base resistor, but it's not really going to make a lot of difference.

    The circuit could also be changed to use an N channel logic level mosfet, however these are static sensitive, and there is really no compelling reason to use one in this circuit.

    edit: oops 3V, not 5V output from the chip. The generous slack in the design should allow for that. :)

    Attached Files:

  3. solarpanellessons


    Mar 11, 2011
    steve is the man

    great example, I really liked this response. I wouldn't have been able to do the same thing.
  4. Laplace


    Apr 4, 2010
    This would be my solution. It's similar.

    Attached Files:

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yeah, a 2N2222 is more familiar to Americans :)

    I misread it as 2 LEDs or a string of 2 LEDs rather than 2 strings of 2 LEDs, so the solution by Laplace is more correct.

    The 2N2222 has lower gain, but can switch a higher current than the BC548 if I remember correctly. I would expect the voltage on the base to be lower though, the 2N2222 isn't a darlington.

    With a minimum gain of 75, I'd want a base current significantly in excess of 2mA to guarantee saturating the transistor. I'd probably err on the side of caution and use a 560 ohm resistor. The specs show a Vce(sat) of 300mV at 150mA and a base current of 15mA, so there may be some utility in using an even lower base resistor.
  6. Grimm


    Mar 10, 2011
    Thank you so much.

    I will order up some components and let you guys know how it ends up!

    FYI, this is going to be repeated 100 or so times on my tail lights and provide a pattern for car shows!


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