Supplementary Reading Materials for Unit 5
Semiconductor Diodes are electronic devices which allow passage of electrical current easily in one direction and
passage of electrical current with great resistance in the opposite direction. Figure 1a shows the circuit symbol of a
diode. The diode consists of a PN junction and has two terminals, as marked in the figure: one terminal is known as
the anode (a) and the other terminal known as the cathode (-). The anode has a positive polarity (‘+’) and the
cathode has a negative polarity (‘-‘).The voltage on the anode must be positive compared to voltage on the cathode
for the current to flow in the diode and so in a diode current can flow from anode to cathode as shown by the arrow
in figure 1a. Picture of diodes is shown in figure 1b that shows how the anode and cathode is represented on a
diode. The cathode is marked by a line painted on the body.
Figure 1. Symbol and picture of diode
a. Symbol of a diode
b. picture of a diode
In very simple terms, according to the characteristics of an ideal diode, the voltage across the diode (between the
anode and the cathode) has to be 0.7 volts for any current to flow through the diode, if it is made from silicon. It has
to be 0.2 volts if it is made from germanium. The interesting point is that the voltage across the diode remains
around the same value (0.7 or 0.2 volts) for any amount of current as shown in figure 2 known as the characteristic
of a diode. This voltage is known as forward voltage drop. The amount of current that flows through the diode is
determined by the values of other components in the circuit. Remember that no current can flow through the diode
as long as the voltage across the anode and cathode is less than the forward voltage drop. The actual minimum
voltage that is required across a diode to allow current flow can be found in catalogues. Diodes are labeled with
their code in small print on the body. There are catalogues that will contain these details for this label on the diode.
Figure 2. Characteristics of a diode shows forward and reverse voltages
Forward Voltage Drop (Vf)
There is a small voltage across a diode when current flows through it. It is about 0.2 for diode made from
germanium and 0.7 for diode made from silicon, It is known as the forward voltage drop and it is almost constant
(very slight variation in reality) whatever the current passing through the diode. So they have a very steep
characteristic (current-voltage graph) as shown in figure 2.
General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to
use in radio circuits as detectors which extract the audio signal from the weak radio signal. For general use, where
the size of the forward voltage drop is less important, silicon diodes are better because they are less easily
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damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage
currents when a reverse voltage is applied.
Reverse Voltage
When a reverse voltage is applied an ideal diode would not conduct, but in reality diodes leak a very small amount
of current in the range of microampere (µA) or less, which can normally be ignored. However, all diodes have a
maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large
current in the reverse direction, this is called breakdown. Diodes are intended to be operated below their
breakdown voltage.
Leakage current
The small amount of current that flows through the diode when the reverse voltage (less than the breakdown
voltage) is applied is known as the leakage current. No leakage current or a negligible possible value is a desired
one.
In an ideal situation diode is like a switch. With a forward bias it is like a closed switch and with reverse bias it is
like an open switch.
Calculation in a diode circuit
Following is an example of a simple diode circuit calculation, using the concepts discussed above and is useful for
most practical applications. However, more accurate answers may be calculated using characteristic diagram of a
diode. The following calculation will serve our purpose. Assume that the diode is made from silicon.
Note that using KVL:
VS = VR + VD So, VR = VS - VD
According to ohm’s law: VR = R*I
0.7V, when
forward
biased
Figure 3. Calculation in diode circuit
So, i = VR / R
If VS = 5 V and R = 1 Kohm = 1000 ohm, Then what will be the
amount of current flowing in the circuit?
Because, the voltage supplied to the circuit is 5V, so the diode
will be forward biased and VD will be .7 volts.
So, VR = VS - VD = 5 - .7 = 4.3 volts
Now from Ohm’s law: i = VR / R
i.e. i = 4.3/1000 = .0043 amperes = 4.3 mA (milliamperes)
If the diode is made from germanium, then
VR = VS - VD = 5 - .2 = 4.8 volts and accordingly the current
i = 4.8/1000 = .0043 amperes = 4.8 mA (milliamperes)
Note that in the above calculation, if you know the current i and VS, you should be able to calculate R. Similarly if
you know i and R you should be able to calculate VS. Try the calculations as they will be asked in the exam.
Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier
diodes which can pass large currents (from1 to 1000 amps or even higher). Many diodes or rectifiers are identified
as 1NXXXX labels on them. ‘XXX..X’ indicates different numbers. In addition there are Light Emitting Diodes
(LEDs) and Zener diodes.
Conversion from AC to DC - Power supply
Alternating current means the current is flowing first in one direction and then in the opposite direction. A diode
placed in a wire with an alternating current will only allow current flow in one direction. Thus the AC signal can be
converted to a DC signal.
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Power Supply
Most power supplies are designed to convert high voltage AC mains electricity from wall outlet to a suitable low
voltage supply for electronics circuits and other devices. For example, 110V AC to 5V DC. It can be achieved
through a number of steps as shown in figure 4.
Figure 4. Steps in a regulated power supply circuit
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In the first step the high voltage AC is stepped down to a desired low voltage DC by using a component
known as Transformer.
In step 2 a rectifier is used to convert AC to DC, but the output remains varying with only positive values.
In step 3 a smoothing circuit smooths the DC from varying to a small ripple.
In the last step a regulator eliminates ripple to make the output to a fixed voltage.
Transformer
Transformers convert the input AC voltage from one value to a different value with. Step-up transformers increase voltage,
step-down transformers reduce voltage. Transformers work only with AC and this is one of the reasons why mains electricity is
AC. Figure 5 shows the circuit symbol and picture of a small transformer.
Figure 5. Circuit Symbol and picture of a Transformer
The input coil is known as the primary coil and the output coil is known as the secondary coil. The coils are placed
around a soft-iron core. The two lines in the middle of the circuit symbol represent the core. There is no electrical
connection between the two coils. An alternating magnetic field is created in the soft-iron core by the input AC
voltage applied at the primary coil produces a voltage at the output of the secondary coil. The ratio between the
number of turns of the primary coil and secondary coil (known as the ‘Turns Ratio’) determines the ratio of the
input and output voltages of the transformer. So, ‘Turns Ratio’ may be given by the following equation.
Turns Ratio = Np/Ns,
where Np = number of turns on primary coil
Ns = number of turns on secondary coil
Also, the ratio of the primary and secondary coil is equal to the ratio of the input and output voltages of the
transformer. It can be expressed as shown in the following equation.
Np/Ns = Vp/Vs
where Vp = input voltage at the primary coil
Vs = output voltage at the secondary coil
In the above equation, if any three of the values is known, then the fourth value can be calculated. For example,
the equation to calculate the output voltage Vs, if the other values are known, can be written as follows.
Vs = Vp * Ns/Np
Accordingly, a step-down transformer should have a large number of turns on its primary (input) coil and a small
number of turns on its secondary (output) coil to give a low output voltage.
Input power at the primary (Pp) may be calculated by the equation:
Pp = Vp * Ip
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Output power at the primary (Ps) may be calculated by the equation:
Ps = Vs * Is
Where, Ip and Is are current in the primary (input) coil and current at the secondary (output) coil respectively.
Note that power in an electric circuit is calculated by the product of the voltage (V) and the current (I) flowing
through the circuit and is expressed in watts.
Transformers are designed so that they waste very little power. It means that in ideal situation the input power at
the primary coil is (almost) equal to the output power at the secondary coil.
So, Vp * Ip = Vs * Is
Therefore, Is can be calculated as Is = (Vp * Ip) / Vs
Note from the above equation, that if the secondary voltage is stepped down, then the current at the output is
stepped up. The frequency and shape of the signal at the secondary (output) remains the same, only the
magnitude changes as designed by the turns ratio and is shown in figure 6.
Figure 6. Output frequency and shape of the voltage remains same. Only the magnitude changes.
Rectifier
Rectifier is a circuit component that passes current in one direction and blocks current flow in the other direction. A
rectifier is used to convert alternating current into direct current. There are several ways of connecting diodes to
make a rectifier to convert AC to DC. We will cover the following two approaches in this unit.
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The single diode rectifier
The Bridge rectifier
Single diode rectifier: A single diode is used as a rectifier as shown in figure 7. It uses only the positive (+)
parts of the AC wave to produce half-wave varying DC at the output, but not a smooth constant output over time.
The value of the output voltage is zero when the input AC is negative, because of the reverse bias of the diode. It
is hard to smooth this sufficiently well to supply power to electronic circuits.
Figure 7. Single diode rectifier - input and output
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Bridge rectifier: The bridge rectifier is the most common, which produces full-wave varying DC. It consists of
four individual diodes, which may be build from individual diodes or also available in special packages containing
the four diodes required. Figure 8 shows connection of the diodes to form a bridge rectifier. The figure shows
different ways of drawing the bridge rectifier that is found in different sources, but note that they all have the
same connection and functionality.
a.
b.
c.
Figure 8. Different ways of drawing a bridge rectifier
The bridge rectifier is also known as full-wave rectifier, because it uses both positive and negative sections of the
AC wave as shown in figure 7 and 8. A full-wave rectifier can also be made from just two diodes if a centre-tap
transformer is used. This method is rarely used now as the diodes are cheaper and we will not discuss it in this
unit.
Figure 7. Bridge rectifier - input and output
a. functional diodes and path of current with
positive part of the AC signal
b. functional diodes and path of current with
negative part of the AC signal
Figure 8. Function of bridge rectifier using both positive and negative sections of the AC signal
Note that always there are two diodes on the path of the circuit, which means that 1.4 volts (2*0.7) drops across
the two diodes (for silicon made).
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Smoothing
In the next step smoothing of the full wave varying DC output is performed by a large value electrolytic capacitor
connected across the varying DC supply. The capacitor is charged to the peak of the varying DC when current
flows through it. Later when voltage is dropping it acts as a reservoir, supplying current to the output (discharges).
Figure 10 shows the unsmoothed varying DC with dotted line and the smoothed DC with solid line. Smoothing is
not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage.
Figure 10. Smoothing of varying DC with large value electrolytic capacitor
Note that smoothing increases the average DC voltage to almost the peak value (1.4 × RMS value). For example, if
6V RMS AC is the output from the secondary side of the transformer, then the smooth DC output will be 6.4 volts
calculated as below.
Output from the transformer is: 6V (RMS)
Voltage after the bridge rectifier is: 6 - 1.4 = 4.6 V
Smoothed output will be 4.6 * 1.4 = 6.4 V DC
(1.4 V is voltage drop on the two diodes)
Regulator
Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable
output voltages. They are also rated by the maximum current they can pass.
Negative voltage regulators are available, mainly for use in dual supplies. Most
regulators include some automatic protection from excessive current ('overload
protection') and overheating ('thermal protection').
Many of the fixed voltage regulator ICs have 3 leads, such as the 7805 +5V 1A
regulator shown in the figure. They include a hole for attaching a heatsink if
necessary. The regulated DC output is very smooth with no ripple. It is suitable
for all electronic circuits.
Simple circuit diagram of a power supply
A simple circuit diagram of a power supply with all the units described above is shown in figure below.
Sources for this unit:
http://www.kpsec.freeuk.com/powersup.htm#trsr
http://engnet.anu.edu.au/DEcourses/engn2211/notes/diodenode9.html
http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
http://www.americanmicrosemi.com/tutorials/diode.htm
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