Need a bit of help understanding this circuit

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electrophile

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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.

 

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...
 

Venkadesh_M said:
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...
Click to expand...

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?
 

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...
 
electrophile said:
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!
Click to expand...
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.
 
Thanks Audioguru and Venkadesh_M!! This certainly helps a lot!
 

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