Calculating Maximum Load Resistance for MC7805 Integral Regulator

[peripatein]

Homework Statement

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.

Homework Equations

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?

 

[Hesch]

Max current through R_y = 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 ).

 

[peripatein]

But is that the maximal load resistance so that Q2 is yet inactive?

 

[Hesch]

But is that the maximal load resistance so that Q2 is yet inactive?

If the value of the load resistor is smaller than 5V/58.67mA = 85.22Ω, the current through R_y will be greater than 8mA+58.67mA = 66.67mA, and Q2 will conduct current as V_BE becomes greater than 0.5V. ( 66.67mA*7.5Ω = 0.5V ).

 

[peripatein]

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.

 

[Hesch]

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.

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.

 

[peripatein]

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?

 

[NascentOxygen]

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?

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

 

[peripatein]

So is 85.22ohm indeed the answer?

 

[NascentOxygen]

So is 85.22ohm indeed the answer?

It looks like that's it.

@peripatein WAIT! Where did that 5V come from, that you are using in your maximum load calculations?