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Homework Help: Power consumption for a specific time period

  1. Dec 2, 2009 #1
    1. The problem statement, all variables and given/known data

    Given circuit consists of two resistors ( R1 = 330 Ohm , R2 = 22K Ohm ) , an LED ( Forward voltage = 1.6V, forward current = 20 mA) , a BC 337 NPN transistor and a 9V battery supply. I need to find the total power consumed by the circuit for a run time of continous 1 hour?

    2. Relevant equations
    V = IR
    P = VI

    3. The attempt at a solution

    taking the right side of the circuit and appyling the V = IR
    9 = (I *330) +(1.6)+voltage across the NPN transistor

    I am not sure about the voltage across the NPN transistor.

    And on the left side applying V= IR
    9= I *22000
    giving I = 4.09*10^-4=400 uA.

    Is total power consumed by the circuit = sum of power consumed by all individual elements??

    Attached Files:

  2. jcsd
  3. Dec 2, 2009 #2
    It's easier to calculat the total current of the circuit, wich is the sum of the currents through the 2 resistances, or the Base-emitter current and the Collector-emittor current.
    The power is (current drawn) * (supply voltage)

    This is a common emitter ampiflier, without an emitter resistance. Compute the base-emitter current first. You have the 22k resistance in series with the base-emitter voltage (look up on dataheet). Now the collector - emittor current is the current gain (at least 100 for this transistor) * the base-emittor current.
    The current gain can't be larger than the current you get if the transistor is saturated. This maximum current is what you get if you put the 330ohm resistance, the led and the saturation voltage of the transistor in series.
  4. Dec 2, 2009 #3
    Is it a common base transistor or common emitter, ?

    In the data sheet the Base-emitter current is given as 1.2V ( max) [under the given conditons Ic = 500 mA, Vce = 1V]
    So Emitter base current = 1.2/22 *1000 =0.0545 mA
    Collector meitter current = 100 *0.0545 = 5.45mA

    So total current = 5.5045mA
    Therfore P = 5.5045 *9/1000 = 49.5mW..

    This is power consumed for how much time? If the circuit runs for 5 mins continously then power = ???

    Is that so?
    Last edited by a moderator: Apr 24, 2017
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