How Does a Zener Diode Stabilize Voltage in a Circuit?

[clavin]

well first i saw this solution of a zener diode problem on net
A 5.0v stabilised power supply is required from a 12v d.c. input source. The maximum power rating of the Zener diode is 2W. Load resistance = 1 kiloohm Using the circuit above calculate:

circuit : http://www.electronics-tutorials.ws/diode/diode24.gif

b) The value of the series resistor, RS


12-5/(400mA) = 17.5 ohm

so is the above solution right
cuz i think it should be

12-5/(405mA)


second can somebody explain me a bit about ce transistor circuits
first a little bit about what it does
and secondly how does the circuit works
like why there's grounding
and also i saw that the emitter is connected to the positive terminal of one battery and the negative terminal of other
so in that branch how will the current flow
from postitve terminal of one batter to the negative of the other without passing through emitter? or in some other way

 

[jbsteele]

?Yes, the solution is correct. Common emitter transistor circuits are used to amplify a signal. The circuit works by taking an input signal (typically through the base of the transistor) and amplifying it before it is output from the collector of the transistor. The ground of the circuit is connected to the negative terminal of the power supply in order to establish a reference voltage against which the input and output signals can be measured. The emitter of the transistor is connected to the positive terminal of one battery and the negative terminal of the other in order to provide the necessary bias voltage for the transistor. The current flows from the positive terminal of the one battery, through the collector-emitter junction of the transistor, and then to the negative terminal of the other battery.