What is the purpose of parallel resistor and reverse diode at supply?

What is the purpose of parallel resistor and reverse parallel diode at supply?

-Most of the output of low voltage power supply (like 5V cell phone charger OR output of 7805 regulator) has a resistor connected between positive to ground, for why? What is the best resistor value for 5V supply? Equation?

- I have found also a Big diode in few circuit connected reversly paralleled at the voltage (battery) input, hope it is protection diode to protect the circuit from if there is mistake in connection of battery polarity, isn't it? Won't if affect the voltage just leaving connected?

The purpose of these diodes is well explained in the 78xx/79xx datasheets and application notes provided by National, ON and many other manufacturers of these regulators. E

- Many regulators require a minimum load.

- Reverse polarity protection. If the diode is reversed biased it doesn't conduct. When the polarity is reversed, the voltage will be limited to essentially -0.6V if a silicon diode is used. Lower if a Schotkey diode is used.

KeepItSimpleStupid said:

- Many regulators require a minimum load.

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7805 (TO-220 case) requires too? But when I am connecting this 7805 permanetly to any load, even should I have to connect this minimum load resistor?

yep, it's in the details in the datasheet. Note 5 mA. https://www.electro-tech-online.com/custompdfs/2012/09/7805-datasheet-fairchild.pdf

It doesn't explicitly say "Minimum load 5 mA" but it does when it's specified from 5 mA to 1A.

No, the minimum load resistor isn't required UNLESS you are drawing less than 5 mA.