I have a circuit board with the following set up with the voltage regulator KA7824
This picture is taken from the datasheet of the voltage regulator as one of its uses.
my circuit board converts AC to DC to power motors using a bridge rectifier. and this regulator set up is connected after the bridge .The motors are connected after this set up.
My question is , what exactly is its purpose? and how does it work.
EDIT:
Thanks to @peter smith, apparently this is called an outboard boost transistor.
Here is an article explaining in detail all about it.
current boost circuit
Best Answer
Q1 is an outboard boost transistor. When the regulator input current gets to about 300mA, it forward biases Q1 to permit current flow to the output in excess of the safe amount for the regulator.
If I set an arbitrary current limit of 2A, then Rsc = 0.3 ohm (0.6 / 2).
At outboard currents below 2A, Q2 is off and is therefore an open circuit.
When the output current gets to 2A, Q2 is forward biased and places a short circuit between Vin and the regulator input, shutting down Q1 and disabling outboard boost.
The output is still short circuited, but the KA78XX series has internal protection.