I need to supply ATmega 328 with 5 volt, but have not a good source. So I decided to use a mobile charger which gives 6.4 volts. I will add two diodes in series to decrease voltage till 5 volt. At the same time I want to use Zener diode which will regulate voltage in case of some voltage changes. Can anybody suggest the type of Zener diode to use in the circuit? Or some other idea? Thanks in advance.

 

1N5231, 1N5231B -5.1V 500 mW, but use a resistor in series with the charger instead of two diodes. Select the resistor value for 10 ma + the current your ATMEGA328 is likely to need. If the output of the charger is still 6.4 volts when you connect the shunt regulator (resistors + zener) then for 40 ma for your circuit + 10 ma for your zener you would use 27 ohms 1/8 watt or larger.

The circuit would be charger => 27 ohms => Zener.

Shunt regulators with such a small difference between input and output voltage van be tricky, so you might want to use a low dropout regulator such as the Linear Technology LT3010-5.

http://www.linear.com/product/LT3010

 

1N5231, 1N5231B -5.1V 500 mW, but use a resistor in series with the charger instead of two diodes. Select the resistor value for 10 ma + the current your ATMEGA328 is likely to need. If the output of the charger is still 6.4 volts when you connect the shunt regulator (resistors + zener) then for 40 ma for your circuit + 10 ma for your zener you would use 27 ohms 1/8 watt or larger.

The circuit would be charger => 27 ohms => Zener.

Shunt regulators with such a small difference between input and output voltage van be tricky, so you might want to use a low dropout regulator such as the Linear Technology LT3010-5.

http://www.linear.com/product/LT3010

Can I ask you, why it is not recommended to use diodes?

 

I don't have confidence in the 6.4 volts out of the charger -it might sag when loaded, and with such a setup, particularly when the load is a microcontroller you need to make sure that there is a minimum load (and not the microcontroller because it doesn't draw much current util the oscillator startup is complete) in order to be assured that the voltage after the diode is not higher than is safe. Something like a resistor to ground would work.

To make things simple and to minimize the run-around a low dropout regulator would be the best solution (IMO) and a shunt regulator next best.

I have and used and still occasionally use dropping diodes to adjust power supply voltages but I've also had problems with them working correctly. By the way, I found that small signal diodes can be used as fuses.