Diode and capacitor in voltage protection circuit

[fishfude]

Hi Folks,

In the image below, I believe the diode and capacitor are there to protect the circuit from too high a voltage coming from the DC power source (correct?). My question is what is the exact purpose of the diode and the capacitor are, and why were they specifically chosen?

http://imgur.com/j1hm9

Many Thanks,

ff

[frequencycentral]

Diode shunts wrong polarity to ground, will eventually burn out. Cap is there for decoupling, not polarity protection.

[fishfude]

So is it for reverse voltage polarity protection rather than protecting the circuit if say 12 volts was applied instead of 9?

[frequencycentral]

Yup.

12V won't hurt most 9V circuits, assuming the caps are over spec'ed. Whether it would sound the same depends on the circuit.

[fishfude]

Yeah I suspected that. What I'm asking is, I guess, are those two components actually capable of protecting the circuit from a high voltage (24V?), for whatever reason. Or are they just there for reverse voltage polarity protection?
Sorry, the reason I'm confused is because of this schematic and comment:

http://imgur.com/NbEP3

Thanks again,

ff

[slacker]

The diode gives some protection against reverse polarity. If the polarity is correct the diode does nothing, so it offers no protection against high voltages.

Like Rick said the cap is nothing to do with protection.

[R.G.]

It is possible to put a zener diode in that position and get some high voltage protection, but the 1N4000 series is definitely not that. I looked at the pic and to me it only means "wrong polarity of power supply" not wrong power supply voltage.