The Signal Diode
The semiconductor Signal Diode is a small non-linear semiconductor devices generally used
in electronic circuits, where small currents or high frequencies are involved such as in radio, television and digital
logic circuits or where a low value of capacitance is required between the terminals of the device. Signal diodes which
are also sometimes known by their older name of the Point Contact or Glass Diode, are physically very
small in size compared to their larger Power Diode
cousins and control small currents up to about 100mA. Generally, the PN-junction of a signal diode is encapsulated in
glass to protect it and they generally have a red or black band at one end of their body to help identify which end is
its Cathode terminal. The most widely used of all the glass encapsulated signal diode is the very common 1N4148 signal diode.
The characteristics of a signal point contact diode are different for both germanium and silicon
types and are given as:
- Germanium Signal Diodes - These have a low reverse resistance value giving a lower
forward volt drop across the junction, typically only about 0.2-0.3v, but have a higher forward resistance value
because of their small junction area.
-
- Silicon Signal Diodes - These have a very high value of reverse resistance and give
a forward volt drop of about 0.6-0.7v across the junction. They have fairly low values of forward resistance giving
them high peak values of forward current and reverse voltage.
The electronic symbol given for any type of diode is that of an arrow with a bar or line at its end
and this is illustrated below along with the Steady State V-I Characteristics Curve.
Silicon Diode V-I Characteristic Curve
The arrow points in the direction of conventional current flow through the diode meaning that the
diode will only conduct if a positive supply is connected to the Anode (A)
terminal and a negative supply is connected to the Cathode (K) terminal
thus only allowing current to flow through it in one direction only, acting more like a one way electrical valve,
(Forward Biased Condition). However, we know from the previous tutorial that if we connect the external energy
source in the other direction the diode will block any current flowing through it and instead will act like an open
switch, (Reversed Biased Condition) as shown below.
Forward and Reversed Biased Diode
Then we can say that an ideal small signal diode conducts current in one direction (forward-conducting)
and blocks current in the other direction (reverse-blocking). Signal Diodes are used in a wide variety of applications such
as a switch in rectifiers, limiters, snubbers or in wave-shaping circuits.
Signal Diode Parameters
Signal Diodes are manufactured in a wide range of voltage and current ratings and care
must be taken when choosing a diode for a certain application. There are a bewildering array of static characteristics
associated with the humble signal diode but the more important ones are.
1. Maximum Forward Current
The Maximum Forward Current (IF(max)) is as its name
implies the maximum forward current allowed to flow through the device. When the diode is conducting in the
forward bias condition, it has a very small "ON" resistance across the junction and therefore, power is dissipated
across this junction (Ohm´s Law) in
the form of heat. Then, exceeding its (IF(max)) value will cause more heat to
be generated across the junction and the diode will fail due to thermal overload, usually with destructive consequences.
When operating diodes around their maximum current ratings it is always best to mount them onto suitable aluminium
heatsinks to dissipate the heat produced by the diode.
For example, our small 1N4148 signal diode has a maximum current rating of about 150mA with a power
dissipation of 500mW at 25oC. Then a resistor must be used in series with the diode to limit the forward current,
(IF(max)) through it.
2. Peak Inverse Voltage
The Peak Inverse Voltage (PIV) or Maximum Reverse Voltage
(VR(max)), is the maximum allowable Reverse operating voltage that can be applied
to the diode without reverse breakdown and damage occurring to the device. This rating therefore, is less than the
"Avalanche Breakdown" level on the reverse bias characteristic curve. It is mainly used for rectifying diodes in AC rectifier
circuits with reference to the amplitude of the voltage were the sinusoidal waveform changes from a positive to a negative
value on each and every cycle.
3. Maximum Operating Temperature
The Maximum Operating Temperature actually relates to the Junction Temperature
(TJ) of the device. It is the maximum temperature allowable before the structure of
the device deteriorates and is expressed in units of degrees Centigrade per Watt. This value is linked closely to the
maximum forward current of the device so that at this value the temperature of the junction is not exceeded. However,
the maximum forward current will also depend upon the ambient temperature in which the device is operating so the maximum
forward current is usually quoted for two or more ambient temperature values.
Diode Arrays
When space is limited, or matching pairs of signal diodes are required, diode arrays can be used.
They are generally high speed silicon diodes such as the IN4148 connected together in multiple diode packages called an
array for use in switching and clamping circuits. They are generally single inline packages (SIP) containing 4 or more diodes
connected internally to give a Common Cathode, (CC) or a Common Anode,
(CA) configuration as shown.
Diode Array
Signal diode arrays can also be used to connect together diodes in either series or parallel combinations
to form voltage regulator or voltage reducing type circuits or to produce a known fixed voltage. We know that the forward
volt drop across a silicon diode is about 0.7v and by connecting together a number of diodes in series the total voltage
drop will be the sum of the individual voltage drops of each diode. However, when signal diodes are connected together
in series, the current will be the same for each diode so the maximum forward current must not be exceeded.
Connecting Signal Diodes in Series
As the forward voltage drop across a silicon diode is about 0.7v, these individual voltage drops will be
subtracted from the supply voltage to leave a certain voltage potential across the load resistor, and in our simple example
above is given as 10v - (3 x 0.7v) = 7.9v. By adding more diodes in series a greater voltage reduction
will occur. Also series connected diodes can be placed in parallel with the load resistor to act as a voltage regulating
circuit. Here the voltage applied to the load resistor will be 3 x 0.7v = 2.1v. We can of course
produce the same constant voltage source using a single
Zener Diode. Resistor, RD
is used to prevent excessive current flowing through the diodes if the load is removed.
Flywheel Diodes
Signal diodes can also be used in a variety of clamping, protection and wave shaping circuits with the
most common form of clamping diode circuit being one which uses a diode connected in parallel with a coil or inductive
load to prevent damage to the switching circuit by suppressing the voltage spikes caused when the load is turned "OFF".
This type of diode is generally known as a "Flywheeling Diode" or "Flywheel diode". Flywheel diodes can also be
used to protect solid state switches such as power MOSFETS from damage by reverse battery protection as well as
protection from highly inductive loads such as relay coils or motors, and an example of its connection is shown below.
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Other types of specialized diodes not included here are Photo Diodes, PIN Diodes, Tunnel Diodes
and Schottky Barrier Diodes. By adding more PN-junctions to the basic diode structure other types of semiconductor
devices can be made. For example a 3 layer semiconductor device becomes a
Transistor and a 4 layer semiconductor
device becomes a Thyristor or Silicon Controlled Rectifier. 5 layer devices known as Triacs are also available.
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