1, 78XX series is a forward output three-terminal voltage regulator integrated circuit, not a triode; The last two digits XX represent its stable output voltage.
2, the internal structure of the three-terminal voltage regulator integrated circuit is based on the principle of linear series voltage regulator power supply, this power supply needs a certain pressure difference between the input voltage and its stable output voltage when working, in general, this pressure difference can not be less than 2V, otherwise the voltage regulator effect becomes worse; When the input voltage is lower than the designed stable output voltage, the internal power supply regulator is in a saturated on-state, and the actual output voltage is close to the input voltage (lower a C-E saturation voltage drop) and changes with the input voltage, that is, there is no voltage regulation.
3, the situation in the problem, such as excluding the line fault, most of you are unfamiliar with the application of the integrated circuit, the use of too low input voltage caused, such as the need to convert a lower DC voltage into a higher DC voltage, you need to use a boost type DC-DC conversion module.