Need a bit of help understanding this circuit

[electrophile]

Hi, I got this circuit from the L78S12 voltage regulator datasheet. Can you please help me understand what is Vbe and Isc here? I went through a couple of transistor datasheets and they all have a Vbe value ranging from -2V to 5V. However I also read that the average Vbe value for silicon transistors is 0.7V. I'm confused!!! 8-O.

Also I found the Isc value in the 78S12 datasheet and that is the short-circuit current(?). Again confused!! Any help would be really appreciated. Thanks.

 

[Venkadesh_M]

Q1 is the pass transistor(power).. If circuit is short circuited or over loaded the voltage drop in the Rsc turns the Q2 ON and that turn OFF the Q1...

 

[electrophile]

Q1 is the pass transistor(power).. If circuit is short circuited or over loaded the voltage drop in the Rsc turns the Q2 ON and that turn OFF the Q1...

OK so if I wanted the circuit to shut-off at 15V, then Vbe=15V, Isc=500mA (from the 78S12 datasheet) and hence Rsc=15/0.5=30 ohms. Would that be right?

 

[Venkadesh_M]

Its only for limiting current, If the Q2 is silicon transistor it will turn ON on 0.7 V that means there shd be 0.7 V when the desired cuttoff

Rsc = V / I

Rsc = 0.7 / 0.500;

if turning on below 0.7 means reduce the according Rsc value...

 

[Audioguru]

I went through a couple of transistor datasheets and they all have a Vbe value ranging from -2V to 5V. However I also read that the average Vbe value for silicon transistors is 0.7V. I'm confused!

A transistor is normally turned on. Then its base-emitter is forward-biased at 0.6V to 1.2V.
A transistor's maximum allowed reverse bias voltage for its emitter-base is usually 5V but in this circuit they are never reverse-biased.

The datasheet of nearly every transistor shows its maximum forward-biased base-emitter voltage and a graph of the typical forward-biased voltage. For the 2N6124 transistor the maximum forward-biased voltage is 1.2V when its collector current is 1.5A but is typically 1.0V. Then if RSC is 1 ohm the current is limited to 1V/1 ohm= 1A. If RSC is 0.33 ohms then the current is limited to 1.2V/0.33 ohms= 3.64A. The base-emitter voltage increases a little when the collector current increases.

 

[electrophile]

Thanks Audioguru and Venkadesh_M!! This certainly helps a lot!