Quote Originally Posted by Jumper View Post
Let's say you have a battery (9V) and a circuit that more or less have the same current consumption over time (i.e 100 mA)

In this case you would need to burn away 4V using a resistor.... but after a while you only have 7V in your battery and then you would need a different resistor value .....

Or your circuit sometimes uses 2 mA (then you need one value) and then suddenly you shoot your IR LED and draw 200 mA .... then you would need a new value...

So.. by having a voltage regulator you would always have the correct output voltage no matter what, as long as you stay inside the datasheet for the component.

Look at a LM7805, it will require from 20V to 7V as the battery voltage to provide 5V out at 1A loads.
Right, thank you for your answer.
In fact I am already using a L7805 voltage regulator, a TO-220 with its nice huge heatsink for my PCB but yesterday when I tried to insert all components on the board I noticed this TO-220 is a little bigger than I first estimated when I drawed the PCB, so to fit on my PCB I have to skew the leads of the two neighboring resistors, this is not very professional. I thought of replacing it by the smaller TO-92 (LM75L05) but this component gives 'only' 0.1A while the L7805 gives 1A.
I quickly checked the current needed for the components on the board. Everything is fine except when I turn of the LCD backlight (7*10mA) then 100mA won't be enough. That's why I wanted to use a LM78L05 for everything on the board but for the LCD backlight supply and use a current limiter between 9V battery and the LCD backlight anode.
But when the battery is weakening, I might have a problem. Is it a real problem? The LCD light up very shortly from time to time. You see?