I just got a voltage regulator and I was playing with it in a simple DC circuit. My Source is 3.25V, the OUT pin on the voltage regulator (L7809CV) shows 2.25. My confusion starts with the 1V drop in the energy. I'm not sure I understand how this works very well. If my voltage regulator is a 9V isn't it suppose to take in a lower voltage and increase it up to 9V ? I know the L7809CV is a linear regulator. Do all linear regulators drop voltage? Would I need a booster to increase the voltage instead, or this very one can be set up to work as a booster? Another question, what happens when the voltage in my source drops to (say) 200mV, would the voltage regulator be able to maintain the same 2.5V output until the battery is almost dead? I am not sure how to make sense of the information presented in the data sheet. Thanks much
78xx series are linear regulators, the input should be about 2v - 2.5v higher than the output voltage (please read below). For 7809 the input should be about 11v - 11.5v (again read below)
The dropout is given in the datasheet of 78xx, it shows a typical voltage drop of 2v and a max drop of 2.5v (note that this is from a 7805)
The specs for 7809A are the following:
And for 7809C the following
As you can see in the red rectangle there is only the typical value of 2v shown (no max voltage drop), but I think we can assume the same 2.5v as for 7805. Furthermore the voltage range (green rectangular) and yellow rectangle for which the datasheet shows the output voltage spec (blue rectangular) uses a range with 10.6 or 11.5v minimum input depending on the "A" or "C" type.
I think we should design circuits based on the worse case scenario so it is better to use the max drop of 2.5v rather than the typical of 2v, so we should feed the regulator with 11.5v minimum just to stay cleat of troubles.
To boost the voltage you'll need a switching regulator used in a boost configuration. The minimum input voltage specs depend on the specific implementations, as long as the circuit works withing proper input range the 200mv drop will not affect the output