# 3 - Diodes

##### 2016-04-19 12:17:00 +0000, 2 years and 4 months ago

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 Vin 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 VLED 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, Vz(ener), remains reasonably constant over a wide range of reverse currents passing across the resistor.
Zener diodes are made with ‘breakdown’ voltages between 2.7-200v

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 --------<---+--------<
`
 Vmax = supply voltage Imax = supply current R = series resistor value Imin = minimum zener current (required) Pmax = zener diode dissipation Vz = regulated voltage supply Iz = regulated current supply Iz 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, Iz is determined by:

The minimum value of the series resistor, R, is given by:

R=Vmax/Vout/Iz

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 (Vz) is short-circuited 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, Iload

Voltage across the resistor, R, equals the voltage out:

Current flowing across the resistor, Imax, 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 Vout circuit is disconnected equals Imax

To ensure the zener does not overheat, choose the next largest

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