i have been doing a project on battery charger, so happen to saw http://www.aaroncake.net/circuits/charger1.htm , may i know why R2 resistor is 3k? and why must is be 3k? Dear aaron, reply me soon THX
I edited the title of your post to better describe the content. Just posting "help" doesn't work very well since it does not describe what the post is actually about.
R2 sets the output of the voltage regulator. It is 3K to provide the approximately 14V needed to charge a battery.
Both are filters to prevent the circuit from oscillating. I don't know the exact purpose of C3 since I don't have the datasheet for the 741 op-amp handy.