Please circuit please!

Hallo Rudra,

here comes a circuit.

Assume you want to supply +5V circuit with about 70mA and you want to use 5 NiCd cells of AA size. Typical cell has about 700mAh capacity. When discharging a fully charged cell by C/10, means 70mA, voltage per cell rapidly drops from 1.4V to 1.3V. Voltage stays at 1.3V up to nearly total discharge. Then voltage drops to less than 1.25V.

So, total voltage of 5 cells is about 7.0V at the very beginning of discharge, 6.5V during almost the whole discharge and finally less than 6.25V.
LM78L05 needs about 1.8V for 70mA current, so, at <=6.5V battery voltage problems can occur.
While LM78L05 requires 6 NiCd cells, circuits shown below will only need 5 cells for working.



The left design does not produce a very precise voltage. It highly depends on tolerance of zener diode and of Ube, which depends on collector current. When zener voltage is 5.6V (at 5mA), output voltage between 4.7V and 5.0V is produced. This is valid for output currents between 1mA and 70mA, where Ube varies from about 0.6V to 0.8V.
If output voltage must be greater than 4.75V, because a reset chip with brown-out detection is used, zener diode should be selected to show about 5.8V (at 5mA).

Right design with TL431 works different: Output voltage is very precise, because highly stable reference voltage of TL431 is used for determing output voltage.

For higher supply currents both 220 Ohm resistors must be decreased: Through zener diode and TL431 at least 1mA should flow. More is better! And BC337-40 needs at least about 1/100 of collector current. More is better!

I want to focuse here, that I do NOT want to recommed the use of these circuits INSTEAD of a common three pin voltage regulator design. It's only for discussing purpose, how such a circuit looks like. It should be kept in mind, that classical three pin voltage regulators, are they NPN-Darlingtons or PNP-LDOs, do offer many advantages over these simple designs, shown above. In any case, circuits using discretes need special care in selecting and/or testing, while three pin regulator chips are ready for use.

Kai