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<font color=3D"#0000ff" size=3D"6">Diodes</font><br>
<img src=3D"index_files/line_001.gif" alt=3D"" align=3D"bottom" height=3D"13=
" width=3D"454"><br>
<table bgcolor=3D"#ffffff" cellpadding=3D"10" width=3D"90%">
<tbody><tr>
<td>
<font size=3D"2">A diode is a semiconductor device which allows current
to flow through it in only one direction. Although a transistor is also
a semiconductor device, it does not operate the way a diode does. A
diode is specifically made to allow current to flow through it in only
one direction.</font><br>
<font size=3D"2">Some ways in which the diode can be used are listed here.</=
font><br>
<img src=3D"index_files/bol_s_gr.gif" alt=3D"" align=3D"bottom" height=3D"11=
" width=3D"11"><font size=3D"2"> A diode can be used as a rectifier that con=
verts AC (Alternating Current) to DC (Direct Current) for a power supply dev=
ice.</font><br>
<img src=3D"index_files/bol_s_gr.gif" alt=3D"" align=3D"bottom" height=3D"11=
" width=3D"11"><font size=3D"2"> Diodes can be used to separate the signal f=
rom radio frequencies.</font><br>
<img src=3D"index_files/bol_s_gr.gif" alt=3D"" align=3D"bottom" height=3D"11=
" width=3D"11"><font size=3D"2"> Diodes can be used as an on/off switch that=
controls current.</font><br>
<font size=3D"2">This symbol </font><img src=3D"index_files/diode1.gif" alt=
=3D"" align=3D"top" height=3D"14" width=3D"39"><font size=3D"2"> is used to =
indicate a diode in a circuit diagram.</font><br>
<img src=3D"index_files/diode5.gif" alt=3D"" align=3D"left" height=3D"94" hs=
pace=3D"10" vspace=3D"10" width=3D"165"><br>
<br>
<font size=3D"2">The meaning of the symbol is (Anode)</font><img src=3D"inde=
x_files/diode1.gif" alt=3D"" align=3D"top" height=3D"14" width=3D"39"><font =
size=3D"2">(Cathode).</font><br>
<font size=3D"2">Current flows from the anode side to the cathode side.</fon=
t><br>
<br>
<br clear=3D"left">
<font size=3D"2">Although all diodes operate with the same general
principle, there are different types suited to different applications.
For example, the following devices are best used for the applications
noted.</font><br>
<br>
<table>
<tbody><tr>
<td valign=3D"top"><img src=3D"index_files/bol_s_gy.gif" alt=3D"" align=3D"b=
ottom" height=3D"14" width=3D"14"></td><td>
<a name=3D"diode2"></a><font color=3D"#008000" size=3D"4">Voltage regulation=
diode<strong> </strong></font>(Zener Diode)<br>
<font size=3D"2">The circuit symbol is </font><img src=3D"index_files/diode2=
.gif" alt=3D"" align=3D"top" height=3D"18" width=3D"40"><font size=3D"2">.</=
font><br>
<font size=3D"2">It is used to regulate voltage, by taking advantage of
the fact that Zener diodes tend to stabilize at a certain voltage when
that voltage is applied in the opposite direction.</font></td>
</tr>
<tr>
<td valign=3D"top"><img src=3D"index_files/bol_s_gy.gif" alt=3D"" align=3D"b=
ottom" height=3D"14" width=3D"14"></td><td>
<font color=3D"#008000" size=3D"4">Light emitting diode</font><br>
<font size=3D"2">The circuit symbol is </font><img src=3D"index_files/diode3=
.gif" alt=3D"" align=3D"middle" height=3D"26" width=3D"38"><font size=3D"2">=
.</font><br>
<font size=3D"2">This type of diode emits light when current flows through i=
t in the forward direction. (Forward biased.)</font></td>
</tr>
<tr>
<td valign=3D"top"><img src=3D"index_files/bol_s_gy.gif" alt=3D"" align=3D"b=
ottom" height=3D"14" width=3D"14"></td><td>
<font color=3D"#008000" size=3D"4">Variable capacitance diode</font><br>
<font size=3D"2">The circuit symbol is </font><img src=3D"index_files/diode4=
.gif" alt=3D"" align=3D"top" height=3D"20" width=3D"39"><font size=3D"2">.</=
font><br>
<font size=3D"2">The current does not flow when applying the voltage of
the opposite direction to the diode. In this condition, the diode has a
capacitance like the capacitor. It is a very small capacitance. The
capacitance of the diode changes when changing voltage. With the change
of this capacitance, the frequency of the oscillator can be changed.</font><=
/td>
</tr>
</tbody></table><br>
<a name=3D"graph"></a><img src=3D"index_files/diode_ge.gif" alt=3D"" align=
=3D"right" height=3D"289" hspace=3D"10" width=3D"384"><br>
<font size=3D"2">The graph on the right shows the electrical characteristics=
of a typical diode.</font><br>
<br>
<font size=3D"2">When a small voltage is applied to the diode in the forward=
direction, current flows easily.</font><br>
<font size=3D"2">Because the diode has a certain amount of resistance,
the voltage will drop slightly as current flows through the diode. A
typical diode causes a voltage drop of about 0.6 - 1V (V<sub>F</sub>) (In th=
e case of silicon diode, almost 0.6V) </font><br>
<font size=3D"2">This voltage drop needs to be taken into consideration
in a circuit which uses many diodes in series. Also, the amount of
current passing through the diodes must be considered.</font><br>
<br>
<font size=3D"2">When voltage is applied in the reverse direction through a =
diode, the diode will have a great resistance to current flow.</font><br>
<font size=3D"2">Different diodes have different characteristics when
reverse-biased. A given diode should be selected depending on how it
will be used in the circuit.</font><br>
<font size=3D"2">The current that will flow through a diode biased in the
reverse direction will vary from several mA to just =B5A, which is very
small.</font><br>
<br>
<font color=3D"#ff0000" size=3D"2">The limiting voltages and currents
permissible must be considered on a case by case basis. For example,
when using diodes for rectification, part of the time they will be
required to withstand a reverse voltage. If the diodes are not chosen
carefully, they will break down.</font><br>
<br clear=3D"right">
<br>
<br>
<br>
<hr><br>
<br>
<br>
<br>
<a name=3D"diode1"></a><img src=3D"index_files/bol_s_gy.gif" alt=3D"" align=
=3D"bottom" height=3D"14" width=3D"14"> <font size=3D"4">Rectification / Swi=
tching / Regulation Diode</font><br>
<br>
<img src=3D"index_files/diode.gif" alt=3D"" align=3D"bottom" height=3D"52" w=
idth=3D"288"><br>
<img src=3D"index_files/rect.jpg" alt=3D"" align=3D"left" height=3D"253" hsp=
ace=3D"10" vspace=3D"10" width=3D"258"><br>
<font size=3D"2">The stripe stamped on one end of the diode shows indicates =
the polarity of the diode.</font><br>
<font size=3D"2">The stripe shows the cathode side.</font><br>
<font size=3D"2">The top two devices shown in the picture are diodes used
for rectification. They are made to handle relatively high currents.
The device on top can handle as high as 6A, and the one below it can
safely handle up to 1A.</font><br>
<font size=3D"2">However, it is best used at about 70% of its rating because=
this current value is a maximum rating.</font><br>
<font size=3D"2">The third device from the top (red color) has a part
number of 1S1588. This diode is used for switching, because it can
switch on and off at very high speed. However, the maximum current it
can handle is 120 mA. This makes it well suited to use within digital
circuits. The maximum reverse voltage (reverse bias) this diode can
handle is 30V.</font><br>
<font size=3D"2">The device at the bottom of the picture is a voltage
regulation diode with a rating of 6V. When this type of diode is
reverse biased, it will resist changes in voltage. If the input voltage
is increased, the output voltage will not change. (Or any change will
be an insignificant amount.) While the output voltage does not increase
with an increase in input voltage, the output current will. </font><br>
<font size=3D"2"> This requires some thought for a protection circuit so tha=
t too much current does not flow.</font><br>
<font size=3D"2">The rated current limit for the device is 30 mA.</font><br>
<font size=3D"2">Generally, a 3-terminal voltage regulator is used for
the stabilization of a power supply. Therefore, this diode is typically
used to protect the circuit from momentary voltage spikes. 3 terminal
regulators use voltage regulation diodes inside.</font><br clear=3D"left">
<br>
<br>
<br>
<hr><br>
<br>
<br>
<a name=3D"diode3"></a><img src=3D"index_files/bol_s_gy.gif" alt=3D"" align=
=3D"bottom" height=3D"14" width=3D"14"> <font size=3D"4">Diode bridge</font>=
<br>
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