3.01  light emitting diode (LED)
When forward biased it conducts and emits light, when in reverse bias, there is no conduction, and therefore, no light is emitted.
If the LED is in reverse bias and V_{in} is greater than 5v the LED may be damaged.
/\
 
 
  longer leg = anode, positive +
\++/ shorter leg = cathode, negative 
 
  flat edge of the LED denotes the cathode
  or negative pole.

+ 
The symbol for an LED is:
LED’s typically require a series resistor to ensure the current does not
exceed the max input current rating of the LED. The maximum current
rating of an LED is usually ~20mA
To calculate the value of the series resistor:
Where V_{LED} is the maximum LED operating voltage.
3.02  zener diodes
Zener diodes provide a cheap and convenient method of providing a stabilized supply. A zener diode is designed so that at a certain ‘breakdown’ voltage the current increases suddenly, as shown in the graph below:
6 ++ 
 
zener  
breakdown 4 ++ 
voltage, Vz   forward
 .` bias
 2 ++ /
6 v 5 4 2  .'
    +_.'
++++++++
/` 0   
.`  0.5 1 1.5
 + 2
 
 reverse 
 bias + 4
 
 
 + 6
At this ‘breakdown’ voltage the current is limited only by a series
resistor so the voltage across the zener, V_{z(ener)}, remains
reasonably constant over a wide range of reverse currents passing across
the resistor.
Zener diodes are made with ‘breakdown’ voltages between 2.7200v
As well as providing voltage regulation, a zener diode can also be used to:
 prevent the voltage difference from exceeding a certain value
 reduce a voltage by a certain amount
++ +
+Supply+>+
++ ^ 
 +++
Vmax   R
 
+++

*>
Imin }  ^
Pmax }>  
Iz }  
+++ 
 Zener   Regulated
 diode   voltage,
(reverse)  Vz, Iz
+++ 
 
 
 v
0V <+<
V_{max}  =  supply voltage 
I_{max}  =  supply current 
R  =  series resistor value 
I_{min}  =  minimum zener current (required) 
P_{max}  =  zener diode dissipation 
V_{z}  =  regulated voltage supply 
I_{z}  =  regulated current supply 
I_{z} can also be the max amount of current  
that can flow through the zener diode when short  
circuited. 
3.03  type A zener diode calculation
A question will be type A if:
 The power rating of the Zener diode is given
The max current through the zener diode, I_{z} is determined by:
The minimum value of the series resistor, R, is given by:
R=V_{max}/V_{out}/I_{z}
The maximum current that can be used by the load whilst the output voltage is kept constant is:
When short circuited, then the max current that can flow through Iz is:
Under normal operating conditions, the resistor, R, must dissipate power equal to:
When the output (V_{z}) is shortcircuited the resistor, R, must then dissipate power equal to:
3.04 = type B zener diode calculation
How to tell if the question is type B: \
 The maximum output current is given, I_{load}
Voltage across the resistor, R, equals the voltage out:
Current flowing across the resistor, I_{max}, is given by:
Therefore R equals:
For the type B questions, the next largest E24 value is selected.
The maximum current through the zener if the V_{out} circuit is disconnected equals I_{max}
To ensure the zener does not overheat, choose the next largest
Return?