Quenstion about resistor around transistor(HELP)

Discussion in 'General Electronics Chat' started by rightx2, Nov 6, 2013.

  1. rightx2

    Thread Starter New Member

    Nov 6, 2013
    HI. I want to know how to choose suitable resistors(value) around transistor.
    this contents is from Electronics all in one for Dummies.
    Please refer the attached file, and answer the question.

    Contents in Book
    - R1: 330ohm resistor limits the current through the LED to prevent the LED burning out. You can use Ohm's law to calculate the amount of current that the resistor will allow to flow. Because the supply vol is 6v, and the LED drops 1.8v, the voltage across R1 will be 4.2v (6-1.8). Dividing the voltage by the resistance gives you the current in amperes, 0.127A

    - R2 : You can use Ohm's law to calculate the current at the base. Because the base-emitter junction drops about 0.7v, the voltage across R2 is 5.3v

    Question 1 : I don't know how to use Ohm's law when there is transister in schmatic. In the Contents, "Because the supply vol is 6v, and the LED drops 1.8v, the voltage across R1 will be 4.2v (6-1.8)." this means it will ignore transistor's inner resistors when using Ohm's law? or it uses Ohm's law from voltage source(+6v) to point "a"??

    Question 2 : "Because the base-emitter junction drops about 0.7v, the voltage across R2 is 5.3v" -> how could i know base-emitter junction drops is about 0.7v?? i experiment with various resistors, and this junction drops seems to change every time. Or could i know this in transistor's data sheet?(if it is, could you let me know where can i find it?? i attach the datasheet. please refer sentence(or paragraph) indicating this.)

    Question 3 The calculating(Ohm's law) will be different if i put R3 under the point "c"?? i mean R3 between point "c" and ground. How it will change?? please let me know ...

    Question 4 : How can i choose certain resistor's value like 330 Ohm, 1K in this picture?? is it just intuition??? or simulate with Circuit Simulate Software before?? I don't know which resistor i have to use when using transistors.

    I stuck in depreesion because of this. I can not go further cuz i can't understand how to use and apply transistor T-T... Please Help me !
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    Your right. Not the best advice for a book. Most of your questions are answered on page 4 of the data sheet, but the current can not be predicted perfectly.
    Question 1. You can include the transistors loss by using VCEsat. This will be the drop across the transistor in saturation.
    Question 2. VBE sat is the spec for the drop between B and C when the transistor is used as a saturated switch. Usually for a transistor to be fully turned on (saturated) people assume a gain of 10. That is also what is most often used in the data sheet. So you should calculate the worst case base current to be 1/10 of the expected collector current.
    Question 3. If the resistor is below the emitter (C) the transistor will become an emitter follower so the emitter will be ~.6 volts lower than the base (B) when it is turned on. This will lower the voltage at the collector(A) by about .6 volts.
    Hope this helps.
  3. wayneh


    Sep 9, 2010
    ronv gave correct answers but I'll add mine.

    1. In most cases and certainly for simple lighting of an LED, yes, ignore the internal drop across the transistor. If it is being used as a switch and is fully turned on, that drop will likely be well under 0.5V and this is usually not worth the trouble to include in the calculations. The error in estimating Vf of the LED may be a larger unknown.

    2. The base-emitter junction is essentially a diode and introduces a diode drop. Unlike #1, this is sometimes important for calculating R2 in low voltage circuits.

    3. ronv said it well. I'll just add that a problem with the emitter follower configuration is that the required base voltage increases with the voltage on that resistor, or on the load if that were also placed between emitter and ground. The base must be at least about 0.6V above the emitter in order to turn on. If you wanted a 1V signal on the base to turn on the LED, it wouldn't work in an emitter-follower arrangement.

    4. A great place to start is to realize you are building a switch, and in that application it's typical to aim for a base current of 10% of the collector current. Not intuition, just a rule of thumb based on experience. If current consumption is a big concern in your circuit, you can then reduce the base current by experimentation until the load current begins to fall unacceptably.