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Integral regulator MC7805

  1. Apr 9, 2015 #1
    1. The problem statement, all variables and given/known data
    My question concerns MC7805, integral regulator, used as a high current voltage regulator. See figure attached. Now, I am told that Ry=7.5Ω and that Q2 is on the verge of conductance with VBE,on=0.5V. Using the Max. Quiescent Current from the manufacturer's data sheet (see attachment) I am asked for the maximal load resistance which could be attached so that Q2 would still not conduct.

    2. Relevant equations


    3. The attempt at a solution
    Based on the data sheet (unless I misread the info. there), the Max. Quiescent Current is 8.0mA. Should the calculation hence be 5V/8mA=625Ω? Am I given the value of Ry so that I may deduce that most of the current would be passing through it (as its value is quite small)? On the verge of conductance the current flowing through it would be 0.5V/7.5Ω but how does/should that affect my calculation?
     

    Attached Files:

  2. jcsd
  3. Apr 9, 2015 #2

    Hesch

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    Gold Member

    Max current through Ry = 0.5V/7.5Ω = 66.67mA.
    Quiescent current = 8.0mA, so the critical load current = 58.67mA.
    The critical value of the load resistor = 5V/58.67mA.

    ( It must be the minimal load resistance in ohms ).
     
    Last edited: Apr 9, 2015
  4. Apr 9, 2015 #3
    But is that the maximal load resistance so that Q2 is yet inactive?
     
  5. Apr 9, 2015 #4

    Hesch

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    If the value of the load resistor is smaller than 5V/58.67mA = 85.22Ω, the current through Ry will be greater than 8mA+58.67mA = 66.67mA, and Q2 will conduct current as VBE becomes greater than 0.5V. ( 66.67mA*7.5Ω = 0.5V ).
     
  6. Apr 9, 2015 #5
    Question is, what would happen should that 85.22ohm resistor be replaced with a 90ohm resistor, for instance? As, if in that case the BJT would still be inactive, then 85.22ohm could not be the answer.
     
  7. Apr 10, 2015 #6

    Hesch

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    As said in #2: The critical value of the load resistor (85.22Ω) is the minimum value. Replacing it with 90Ω will not activate the BJT, but if you replace it with 80Ω, the BJT will be activated.
     
    Last edited: Apr 10, 2015
  8. Apr 10, 2015 #7
    So now I am under the impression that I am truly not following. I was asked to find the MAXIMAL load for which the BJT would still be inactive. From your answer I understand that it would be possible to connect an even greater load than 85.22ohm and yet the BJT would remain inactive. If so, 85.22ohm could by no means be the answer to my question.
    What is the GREATEST load possible so that the BJT is yet inactive?
     
  9. Apr 12, 2015 #8

    NascentOxygen

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    Staff: Mentor

    This is a point of confusion to many.

    You are most likely asked to find the maximum load for which Q2 will not be conducting. This means determine the lowest load resistance under which Q2 can be guaranteed to not be conducting.

    maximum load is the same concept as maximum current, it's when the heaviest demand is made on a power supply---in the same way that 80W is greater than 20W
     
  10. Apr 12, 2015 #9
    So is 85.22ohm indeed the answer?
     
  11. Apr 12, 2015 #10

    NascentOxygen

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    Staff: Mentor

    It looks like that's it.

    @peripatein WAIT! Where did that 5V come from, that you are using in your maximum load calculations?
     
    Last edited: Apr 12, 2015
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