Tactile Sensor circuit

Discussion in 'Homework Help' started by AD633, Jul 6, 2013.

  1. AD633

    Thread Starter Member

    Jun 22, 2013
    96
    1
    T circuit represents a tactile sensor to turn on / off a relay to turn on / off a load of greater power.The tactile sensor consists of two separated metallic contacts about 2mm.When you place your finger on one of these contacts circulates certain current between them.

    hFE (T1) = 300 hFE (T2) = 220 Vgama VCEsat = 0.7 V = 0.5 C

    Considering that the relay needs a current of 70 mA to close the contacts and that in this case both transistors should be saturated.Determine

    1)hFE min for transistor T2

    2)The minimum current that should flow in the tactile sensor when ativated(that is pressed by the finger)


    1)How can i determine the mininum hfe of T2.The hFE of T2 must a have so that we have Ic2 sat =70 mA

    I have tried to do  12 V=Vcesat1+1kOhm*(Ic1sat)+VBE2 sat<br />
<br />
                 12 V=0.5 V+1kOhm*(70 mA)+VBE2sat

    but i am not going nowhere form here.

    Where should i start for determing this?

    Thanks
     
  2. antonv

    Member

    Nov 27, 2012
    149
    27
    I think you are making it too complicated. What is hFE?
     
  3. AD633

    Thread Starter Member

    Jun 22, 2013
    96
    1
    hFE represents the gain of the transistor. I have to determine the minimum gain for the transistor T2 which ensures that the collector current of transistor T2 is 70 mA.Therefore the transistor T2 has to be in saturation.

    I also thought about doing something like  12V-VCE1sat=VE1<br />
<br />
VE1=11,3 V

    Therefore  Ib2=(11,3V)/(1kOhm)=11,3 mA

    In the ative region  Ic=hFE*IB<br />
<br />
70mA=hFE*(11,3mA)<br />
hFE=6,19


    Which i dont think its correct

    Thanks
     
    Last edited: Jul 7, 2013
  4. antonv

    Member

    Nov 27, 2012
    149
    27
    I am a little confused. Where does the following come from in your first post:

    hFE (T1) = 300 hFE (T2) = 220 Vgama VCEsat = 0.7 V = 0.5 C
     
  5. AD633

    Thread Starter Member

    Jun 22, 2013
    96
    1

    hFE (T1)=300 is the typical value for the gain of transistor T1
    hFE(T2)=220 is the typical value for the gain of transistor T1
    VCEsat of T1 is 0.7 V
     
  6. AD633

    Thread Starter Member

    Jun 22, 2013
    96
    1
    What i want to determine is minimum value of the gain for transistor T2 in order to have T2 in saturation.
     
  7. AD633

    Thread Starter Member

    Jun 22, 2013
    96
    1

    Could someone help me with this?I still haven't found a solution..


    Thanks
     
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