About a zener regulator...

[kashmir]

I am referring to this schem:

http://www.mif.pg.gda.pl/homepages/tom/files/ce2.gif
http://www.datasheetarchive.com/search.php?q=rd11eb&sType=part&ExactDS=Starts

The datasheet says the D7 (RD11EB) is a 11V zener diode. Why would one put a 11V zener after a 9V power supply? 

[bioroids]

I think it is not for regulation on normal use. It is there to protect the circuit if you plug it to the wrong adapter.

Luck!

Miguel

[Sir H C]

Yep, as he said, clamps the supply as most of those regulators might give 12 volts if they are not well loaded.

[George Giblet]

As the others said it's largely for protection.

With a battery the zener does nothing - 11V is high enough to ensure there is virtually no zener current wasting battery power in battery mode.

In normal adaptor operation the input supply is about 12V (IIRC actually 13V).  The components D5 and R53 are tuned to give around 9.5V when the correct adaptor is used.  So even through it looks like a regulator the zener isn't active because the voltage across it is too low.

If you plugged in an adaptor with a high enough voltage the zener kicks in and prevents the rails from going too high.  The resistor R53 limits the zener current and prevent the zener from frying.

PS: the actual voltages might be slightly different to the values I've indicated -  I'm only giving you the general idea.

[Sir H C]

Oh and it is a crowbar if the supply is reversed.