What is the purpose of these transistors in power indicator circuit?

[canadaboy25]

I have a USB-C power supply that is capable of supplying 5V, 9V, 12V, 18V, and 20V depending on what device it is connected to.  The USB-C connector on the end was torn from the cable.

The USB connector is mounted on a little PCB at the end of the cable and there are also two little LEDs that light up whenever the supply is plugged in.  I have some replacement USB connectors with little breakout PCBs that are similar to the original to replace the damaged connector.  The supply has 4 wires, red, white, black, and blue.

I wanted to be sure that I got the sensing connections on the new connector correct to avoid sending incorrect voltage to a connected device, so I worked out the schematic of the original board.

Clearly, I just need to connect the black, white, and blue wires to the new connector and the supply will function normally.  The red wire is obviously just supplying voltage for the LEDs.

However, I am curious as to the purpose of the two PNP transistors in the circuit for the LEDs.  As far as I can tell, Q2 will conduct, lighting the LEDs.  But I do not see the purpose of Q1.  Also, since this is just a constant supply for two LEDs, why have the transistors as all?  A current limiting resistor and two LEDs would be all that is needed for a simple power on indicator.

I don't plan on adding LEDs to the replacement connector, I am just curious about the function of the original circuit.

[Andy Chee]

A current limiting resistor and two LEDs would be all that is needed for a simple power on indicator.

If the voltage at RED is wide ranging 5V-20V, a simple current limiting resistor may give dim to bright, hence the constant current circuit.

A dim/bright power indicator may be ambiguous.

[canadaboy25]

A current limiting resistor and two LEDs would be all that is needed for a simple power on indicator.

If the voltage at RED is wide ranging 5V-20V, a simple current limiting resistor may give dim to bright, hence the constant current circuit.

A dim/bright power indicator may be ambiguous.

I am pretty sure the red wire provides a constant voltage, regardless of the output voltage on the white wire.

I will double check this once I have the new connector in place.

[magic]

I can't explain why they included it, but it's a well known constant current regulator. When Q2 and LED current becomes ~0.7mA, voltage drop across R2 turns on Q1 and Q1 collector steals R1 current away from Q2 base, preventing further increase in Q2 current.

[canadaboy25]

I am pretty sure the red wire provides a constant voltage, regardless of the output voltage on the white wire.

My mistake.  I just completed the repair and the voltage on the red wire does in fact change with the voltage on the white wire.   

This makes a lot more sense and obviously some active regulation is required for constant brightness.

I can't explain why they included it, but it's a well known constant current regulator. When Q2 and LED current becomes ~0.7mA, voltage drop across R2 turns on Q1 and Q1 collector steals R1 current away from Q2 base, preventing further increase in Q2 current.

Thanks for the explanation.  It all makes a lot more sense now.