Principle of voltage regulator triode pin:
Triode here is an emitter output connection, USC=UDW-UBE, because UDW is the voltage at both ends of the regulator tube, is stable, so the output voltage USC is stable.
In addition, the emitter can amplify the change in the base current by (1β) times, that is, the triode extends the operating current of the regulator by β times. For example, triode magnification =20. The operating current of the regulator is 30 mA, and there is an effect of 20*30=600 MA. This can meet the requirements in the case of low power.