Electronic – Voltage Regulation and Capacitors

If you look at the data sheet you will see that the minimum requirement is a 330 nF capacitor on the input if the regulator is situated some way from the rectifier reservoir capacitor and a 100 nF capacitor on the output to improve stability and transient response. Those are the minimum requirements for the regulator itself. Additional capacitance is often provided for the circuit that is being powered; it depends on the characteristics of the load, and 100 uF is a popular value for most applications. I wouldn't worry much about the value, and use whatever you have.

1. R30 limits the charging peak current to the capacitor somewhat, but at 1 ohm it still allows for 12 A, so of little use there. Also would limit the current through the zener if there are peaks above 20 V.

2. The larger capacitors work less well at higher frequencies, and that's where the smaller ones take over.

3. 470 µF in a non-polarized version would be expensive, but there would be nothing against it. All large capacitors are polarized.

4. On the output it would give an extra load for the 7805 to charge it. On the input too for the battery, but that can deliver more than enough current.

5. No, except that they're larger.

6. Keep in mind that the input-output difference for the 7805 is about 5 V (12 V - 1 V for the diode and 1 V for R30 - 5 V out) and that at 1 A out the regulator will dissipate 5 W, so it will need considerable cooling (sizable heatsink) if you want to draw that much current.