I found this design on internet:
It is said to be a regulated power supply with variable DC output 5-15 V, 400 mA. My first question: Is this design around 7805 correct? I am asking because it doesn't correspond to the the 7805 datasheet:
What I miss there is R1. I expect there actually is some resistor inside 7805, so R1 is not required to let it work. But we don't know its value so we are unable to say what will be the exact output voltage when using the design on the first image. Am I right?
Then I am confused about voltages. The transformer's output on the above picture is 12V, then 2 diodes take 0.7V each, then 7805 takes another 2V. So we've got only 8.6V left for the output. My second question goes here: How can this give 15 Volt output? (15V 400mA is said to be maximum output of this design by the author.) Is it normally legal and working, does it mean that we can have any output voltage from any legal input voltage using these IC regulators?
And my third question: Isn't LM317 generally more suitable for adjustable voltage supplies of this kind than 7805? The 7805 seems to me like a LM317 with those R1 and R2 added inside. Also, LM317 has got a lower dropout voltage than 7805 and can give a higher current.
I found the following information in National Semiconductor's LM340/LM78XX datasheet:
RAISING THE OUTPUT VOLTAGE ABOVE THE INPUT VOLTAGE Since the output of the device does not sink current, forcing the output high can cause damage to internal low current paths in a manner similar to that just described in the “Shorting the Regulator Input” section.