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4.4 THREE OPERATIONAL REGION OF BJT

In order for the transistor to operate properly as an amplifier, the two PN junctions
must be correctly biased with external dc voltages.
The proper bias arrangement for both NPN and PNP transistor is the base-
emitter (BE) junction is forward biased and the base-collector (BC) junction
is reverse biased. This is called forward-reverse bias.

(a) : NPN circuit (b) : PNP circuit

Figure 2.4 : Forward-reverse bias of a bipolar transistor

4.4.1 TRANSISTOR CURRENTS

Electrons in the n-type emitter are repelled into the base by the negative terminal
of the emitter supply voltage,VEE. Since the base is very thin and lightly doped,
only a few electrons combine with the holes available in the base.

The small current flowing out of the base lead (which is the base current, IB) is
called recombination current. This is because free electrons injected into the base
must fall into a hole before they can flow out the base lead.

Figure 2.4 (c) : Internal effects of forward-reverse bias.

Most of the emitter-injected electrons pass through the base region and into the
collector region. The reason is two-fold, first, only a few holes are available for
recombination in the base. Second, the positive collector-base voltage attracts
the free electrons in the p-type base over to the collector side before they can
recombine with holes in the base.

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In most transistors the collector current, IC, is nearly identical to the emitter
current, IE, and base current, IB, is very small. The current in a transistor are
related as shown;

IE = IB + IC
IC = IE - IB
IB = IE - IC

4.5 TROUBLESHOOTING COMMON FAULT

(a) (b)
Figure 2.5 (a) Transistor tester (b) Analog Multimeter

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4.5.1 USING METER TO SEPARATE NPN FROM PNP

If you have a transistor but you don't know if it is NPN or PNP, then you can find out
which it is using your Ohm-meter if you know which lead of your meter is positive.

Assuming you know where C, B, and E are on the transistor, do the following. Connect
the positive lead of your Ohm-meter to the base. Touch the other lead of your meter to
the collector. If you get a reading, the transistor is NPN. To verify, move the lead from
the collector to the emitter and you should still get a reading.

If your meter reads open-circuit, then connect the negative lead to the base and touch
the positive lead to the collector. If you get a reading, then the transistor is PNP. Verify
by measuring from base to emitter.

4.5.2 USING DIGITAL MULTIMETERS

Many digital multimeters have a diode test position that provides a convenient way to test
a transistor. Refer to figure xxx, when set to diode test, the meter provides an internal
voltages sufficient to forward-bias and reverse-bias a transistor junction.The meter
provides a voltage reading ti indicate the condition of the transistor junction under test.

Figure 2.5 (c) , the red (positive) lead of the meter is connected to the base of npn
transistor and the black (negative) lead is connected to the emitter to forward-bias the
base-emitter junction. If the junction is good, you will get a reading between 0.5v and 0.9v,
with 0.7v being typical for forward bias.

Figure 2.5 (c) Transistor not defective

Refer figure 2.5 (d), when transistor has failed with an open junction or internal connection,
you get an open circuit voltage reading (2.6v is typical for many DMM) for both the

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forward-bias and the reverse-bias conditions for that junction. If a junction short, the meter
reads 0v in both forward-bias and reverse-bias.

Figure 2.5 (d) Transistor is defective

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SUMMARY

In this unit we have studied:
 The bipolar junction transistor (BJT) is constructed with three regions base,
collector and emitter.
 The BJT has two pn junctions, the base-emitter junction and the base-
collector junction
 Current in a BJT consists of both free electrons and holes, thus the term bipolar
The base region is very thin and lightly doped compared to the collector and
emitter region
 The two types of bipolar junction transistor are the npn and the pnp.
 To operate as an amplifier, the base-emitter junction must be forward-biased and
the base-collector junction must be reverse-biased. This is called forward-
reverse bias.
 The three currents in the transistor are the base current (Is), emitter current (IE),
and collector (IC)
 IB‘a is very small compared to ‘IC and IE.
 βDC is usually referred to as hFE on transistor data sheets.
 The ratio of IC TO IE is called aDc. Values typically range from 0.95 to 0.99.
 When a transistor is forward-reverse biased, the voltage gain depends on the
internal emitter resistance and the external collector resistance.
 A transistor can be operated as an electronic switch in cutoff and saturation.
 In cutoff, both pn junctions are reverse-biased and there is essentially no
collector current. The transistor ideally behaves like an open switch between
collector and emitter.
 In saturation, both pn junctions are forward-biased and the collector current is
maximum. The transistor ideally behaves like a closed switch between collector
and emitter.
 There are many types of transistor packages using plastic, metal, or ceramic.
 It is best to check a transistor in-circuit before removing it.

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EXERCISE UNIT 2
1. The three terminals of a bipolar junction transistor are called

A. Pnp
B. Npn
C. Input, output, ground
D. Base, emitter, collector

2. In a pnp transistor the p regions are
A. Base and emitter
B. Base and collector
C. Emitter and collector

3. The emitter current is always
A. greater than the base current
B. less than the collector current
C. greater than the collector current
D. answers (a) and (c)

4. When operated in cutoff and saturation , the transistor acts like
A. a linear amplifier
B. a switch
C. a variable capacitor
D. a variable resistor

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5. Alpha (α) is the ratio of _________.
A. collector current to emitter current
B. collector currant to base current
C. emitter current to base current
D. base current to collector current

6. A BJT has a value of βDC = 426. Determine the value of αDC for the device.
A. 1
B. 0.998
C. 0.95
D. 200

7. Which statement are TRUE about the basic construction of BJTs?

I. The BJT has two pn junctions, the base-emitter junction and the

base-collector junction.

II. The BJT is constructed with three regions, base, collector and emitter.
III. The base region is very thin and lightly doped compared to the collector

and emitter regions.
IV. One type consists of two n regions separated by a p region (pnp), and the

other consists of two p regions separated by an n region (npn).

A. I, II and III
B. I, III and IV

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C. II, III and IV
D. I and II only
8. Current in a BJTs consists of both free electrons and holes, thus the
term ________.

A. unipolar
B. bipolar
C. trivalent
D. pentavalent

SUBJECTIVE

1. (a) What is the major difference between a bipolar and a unipolar
device?

(b) What does the arrow on the BJT schematic symbol indicate?
(c) What are the primary differences between pnp and npn transistor circuit?
(d) What are the bias conditions of the base-emitter and base-collector

junctions for a transistor to operate as an amplifier?

2. (a) Define βDC.
(b) Determine the value of βDC if αDC = 0.987.
(c) What is the αDC of a transistor if βDC =120.
(d) Given that βDC = 180 and IC = 2 mA. Find IE and IB.

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REFERENCE

1. ELECTRONIC PRINCIPLES

- BY ALBERT PAUL MALVINO
- SIXTH EDITION.

2. ELECTRONIC FUNDAMENTALS : CIRCUIT DEVICES AND APPLICATIONS.

- BY THOMAS L. FLOYD.
- SIXTH EDITION.

3. ELECTRONICS PRINCIPLES AND APPLICATIONS.

- BY CHARLES A. SCHULER.
- SIXTH EDITION.

4. ELECTRONIC DEVICES AND CIRCUIT THEORY

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY
- EIGHTH EDITION.
- www.prenhall.com/boylestad

5. ELECTRONIC DEVICES AND CIRCUIT THEORY.

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY.
- NINTH EDITION

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MODULE NO 5: PNPN AND OTHER DEVICES

INTRODUCTION

The two-layer semiconductor diode has led to three, four and even five-layer
devices. A family of four-layer pnpn devices will first be considered: the SCR (silicon
controlled rectifier), the SCS (silicon-controlled switch), the GTO (gate turn-off switch), the
LASCR (light-activated SCR) and then an increasingly important device- the UJT (uni-
junction transistor). Those four-layer devices with a control mechanism are commonly
referred to as thyristors, although the term most frequently applied to the SCR.

LEARNING OBJECTIVES

The objectives of this module are to:
1. Define the PNPN and Other devices.
2. Explain the silicon Controlled Rectifier (SCR).
3. Explain the silicon – Controlled Rectifier Operation.
4. Identifies SCR characteristics and Ratings.
5. Explain SCR Applications.
6. Define the Silicon Controlled Switch.

LEARNING OUTCOMES

After completing the module, students should be able to:
1. Define the PNPN and Other devices.
2. Explain the silicon Controlled Rectifier (SCR).
3. Explain the silicon – Controlled Rectifier Operation.
4. Identifies SCR characteristics and Ratings.
5. Explain SCR Applications.
6. Define the Silicon Controlled Switch.

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5.1 PNPN AND OTHER DEVICES

The two-layer semiconductor diode has led to three, four and even five-layer
devices. A family of four-layer pnpn devices will first be considered: the SCR (silicon
controlled rectifier), the SCS (silicon-controlled switch), the GTO (gate turn-off switch), the
LASCR (light-activated SCR) and then an increasingly important device- the UJT (uni-
junction transistor). Those four-layer devices with a control mechanism are commonly
referred to as thyristors, although the term most frequently applied to the SCR.

5.1.1 Silicon Controlled Rectifier (SCR)
Within the family of pnpn devices, the silicon-controlled rectifier is of greatest

interest. It was first introduced in 1956 by Bell Telephone Laboratories. Some of the more
common areas of application for SCRs include relay controls, time-delay circuits,
regulated power suppliers, static switches, motor controls, choppers, inverters,
cycloconverters, battery charges, protective circuits, heater controls and phase controls.

In recent years, SCRs have been designed to control powers as high as 10 MW
with individual ratings as high as 2000 A at 1800 V. Its frequently range of application has
also been extended to about 50 kHz, permitting some high-frequency application such as
induction heating and ultrasonic cleaning.

Figure 1 (a) SCR symbol, (b) basic construction

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5.1.2 Basic Silicon-Controlled Rectifier Operation

As the terminology indicates, the SCR is a rectifier constructed of silicon material
with a third terminal for control purposes. Silicon was chosen because of its high
temperature and power capabilities. The basic operation of the SCR is different from that
of the fundamental two-layer semiconductor diode in that a third terminal called a gate ,
determines when the rectifier switches from the open –circuit to the short-circuit state. It
is not enough to simply forward-bias the anode-to –cathode region of the device. In the
conduction region, the dynamic resistance of the SCR is typically 0.01Ω to 0.1Ω. The
reverse resistance is typically 100kΩ or more.

The graphic symbol for the SCR is shown in Figure 1 with the corresponding
connections to the four-layer semiconductor structure. As indicated in Figure 1(a), if
forward conduction to be established, the anode must be positive with respect to the
cathode. This is not however a sufficient criterion for turning the device on. A pulse of
sufficient magnitude must also be applied to a gate to establish to a turn-on gate current,
represent symbolically by IGT.

A more detailed examination of the basic operation of an SCR is best effected by
splitting the four-layer pnpn in Figure 1(b) into two three-layer transistor as shown in
Figure 2(b) and then considering the resultant circuit of Figure 2(b).

Figure 2 SCR two- transistor equivalent

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Note that one transistor for Figure 2 is an npn device, whereas the other is a pnp

transistor. For discussion purposes, the signal shown in Figure 3(a) will be applied to the

gate of the circuit of Figure 2(b). During the interval 0→t1, Vgate = 0 V, the circuit of

Figure 2(b) will appear as shown in Figure 3(b) (Vgate = 0 is equivalent to the terminal

being grounded as shown in the figure). For VBE2 =Vgate = 0 V, the base current

IB2 = 0 and Ic2 will be approximately ICO. The base current of Q1, IB1 = Ic2 = ICO, is to

small to turn Q1 on. Both transistors are therefore in the “off” state, resulting in a high

impedance between the collector and the emitter of each transistor and the open-circuit

representation for the controlled rectifier as shown in Figure 3(c).

Figure 3 “off” state of the SCR

At t =t1, a pulse of VG
volts will appear at the SCR
gate. The circuit conditions
established with this input
are shown in Figure 4(a). The potential VG was chosen sufficiently
large to turn Q1 on ( IB1 = IC2). As Q1 turns on, IC1 will increase,
resulting in a corresponding increase in IB2 . The increase in base
current for Q2 will result in a further increase in IC2 . The net result
is a regenerative increase in the collector current of each transistor.
The resulting anode-to-cathode resistance (RSCR = V/IA) is the small because IA is large,
resulting in the short-circuit representation for the SCR as indicated in Figure4(b). The
generative action described above results in SCRs having typical turn-on times of 0.1µs
to 1 µs. However, high-power devices in the range 100 A to 400 A may have 10-to 25-µs
turn-on times.

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Figure 4 “on” state of the SCR

In addition to gate triggering, SCRs can also be turned on by significantly raising
the temperature of the device or raising the anode-to-cathode voltage to the break over
value shown on the characteristics of Figure 7.

The next question of concern is: How long is the turn-off time and how is turn-off
accomplished? An SCR cannot be turned off by simply removing the gate signal and only
a special few can be turned off by applying a negative pulse to the gate terminal as shown
in Figure 3(a) at t =t3 .
The two general methods for turning off an SCR are categorized as anode current
interruption and forced commutation.
The two possibilities for current interruption are shown in Figure 5. In Figure 5(a), Ia is
zero when the switch is opened (series interruption), whereas in Figure 5(b), the same
condition is establish when the switch is closed (shunt interruption).

Figure 5 Anode current interruptions

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Forced commutation is the “forcing” of current through the SCR in the direction opposite
to forward conduction. There is a wide variety of circuits for performing this function, a
number of which can be found in the manuals of major manufacturers in this area. One of
the more basic types is shown in Figure 6. As indicated in the figure, the turn-off circuit
consists of an npn transistor, a dc battery VB and a pulse generator. During SCR
conduction, the transistor is in the “off” state, that is, IB = 0, and the collector-to-emitter
impedance is very high (for all practical purposes an open circuit). This high impedance
will isolate the turn-off circuitry from affecting the operation of the SCR. For turn-off
conditions, a positive pulse is applied to the base of the transistor, turning it heavily on,
resulting in a very low impedance from collector to emitter (short-circuit representation).
The battery potential will then appear directly across the SCR as shown in Figure 6(b),
forcing current through it in the reverse direction for turn-off. Turn-off times of SCRs are
typically 5 µs to 30 µs.

Figure 6 Forced commutation technique

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SCR Characteristics And Ratings
The characteristics of an SCR are provided in figure 7 for various values of gate

current. The currents and voltages of usual interest are indicated on the characteristics. A
brief description of each follows.

1. Forward break over voltage V(BR)F* is the voltage above which the SCR enters
the conduction region. The asterisk(*) denotes the letter to be added which is
dependent on the condition of the gate terminal as follows:
O = open circuit from G to K
S = short circuit from G to K
R = resistor from G to K
V = fixed bias (voltage) from G to K

2. Holding current IH is the value of current below which the SCR switches from the
conduction state to the forward blocking region under stated conditions.

3. Forward and reverse blocking regions are the regions corresponding to the open-
circuit condition for the controlled rectifier that block the flow of charge (current)
from anode to cathode.

4. Reverse breakdown voltage is equivalent to the Zener or avalanche region of the
fundamental two-layer semiconductor diode.

Figure 7 SCR characteristics

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It should be immediately obvious that the SCR characteristics of figure 7 are very similar
to those of the basic two-layer semiconductor diode except for the horizontal offshoot
before entering the conduction region. It is this horizontal jutting region that gives the gate
control over the response of the SCR. For the characteristic having the solid blue line in
Figure 7 (IG = 0), VF must reach the largest required break over voltage (V(BR)F*) before
the “collapsing” effects results and the SCR can enter the conduction region
corresponding to the on state. If the gate current is increased to IG1, as shown in the same
figure by applying a bias voltage to the gate terminal, the value of VF required for the
conduction (VF1) is considerably less. Note also that IH drops with increase in IG. If
increased to IG2, the SCR will fire at very low values of voltage (VF3) and the characteristics
will begin to approach those of the basic p-n junction diode. Looking at the characteristics
in a completely different sense for a particular VF voltage, say VF2 (Figure 7) we see that
if the gate current is increased from IG = 0 to IG1 or more, the SCR will fire.

The gate characteristics are provided in Figure 8. The characteristics of
Fig. 17.8b are expanded version of the shaded region of Figure 8(a). In Figure
8(a), the three gate ratings of greatest interest, PGFM, IGFM and VGFM are indicated.
Each is included on the characteristic in the same manner employed for the
transistor. Except for portions of the shaded region, any combination of gate
current and voltage that falls within this region will fire any SCR in the series of
components for which these characteristics are provided. Temperature will
determine which sections of the shaded region must be avoided. At -65°C the
minimum current that will trigger the series of SCRs is 100mA, whereas at +150°C
only 20mA is required. The effect of temperature on the minimum gate voltage is
usually not indicated on curves of this type since gate potentials of 3 V or more are
usually obtained easily. As indicated on Figure 8(b), a minimum of 3 V is indicated
for all units for the temperature range of interest.

Other parameters usually included on the specification sheet of an SCR
are the turn-on time ton, turn-off time toff, junction temperature TJ and case
temperature Tc , all of which by now should be to some extent self-explanatory.

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Figure 8 SCR gate characteristics (GE series C38)

5.2 SCR Construction And Terminal Identification
The basic construction of the four-layer pellet of an SCR is shown in Figure

9(a). The complete construction of a thermal fatigue-free, high-current SCR is
shown in Figure 9(b). Note the position of the gate, cathode and anode terminals.
The pedestal acts as a heat sink by transferring the heat developed to the chassis
on which the SCR is mounted. The case construction and terminals identification
of SCRs vary with the application. Other case-construction techniques and the
terminal identification of each are indicated in Figure 10.
SCR Application

Some of the possible applications for SCR are listed in the introduction to
the SCR. In this section, we consider five: a static switch, a phase-control system,
a battery charger, a temperature controller and a single-source emergency-lighting
system.

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Figure 9 (a) Alloy diffused SCR pellet, (b) thermal fatigue free SCR constructions

Figure 10 SCR constructions and terminal identifications, (a) courtesy general electrical company, (b) and, (c)
courtesy international rectifier corporation

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Series Static Switch
A half-wave series static switch is shown in Figure 11(a). If the switch is

closed as shown in Figure 11(b), a gate current will flow during the positive portion
of the input signal, turning the SCR on. Resistor R1 limit the magnitude of the gate
current. When the SCR turns on, the anode-to cathode voltage (VF) will drop to
the conduction value, resulting in a greatly reduced gate current and very little loss
in the gate circuitry. For the negative region of the input signal, the SCR will turn
off since the anode is negative with respect to the cathode. The diode D1 is
included to prevent a reversal in gate current.

The waveforms for the resulting load current and voltage are shown in
Figure 11(b). The result is a half-wave-rectified signal through the load. If less than
180° conduction is desired, the switch can be closed at any phase displacement
during the positive portion of the input signal. The switch can be electronic,
electromagnetic or mechanical depending on the application.

Figure 11 Half wave series static switch

Variable-Resistance Phase Control
A circuit capable of establishing a conduction angle between 90° and 180°

is shown in Figure 12(a). The circuit is similar to that of Figure 11(a) except for
the addition of a variable resistor and the elimination of the switch. The
combination of the resistors R and R1 will limit the gate current during the positive
portion of the input signal. If R1 is set to its maximum value, the gate current may
never reach turn-on magnitude. As R1 is decreased from the maximum, the gate
current will increase from the same input voltage. In this way, the required turn-on

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gate current can be established in any point between 0° and 90° as shown in
Figure 12(b). If R1 is low, The SCR will fire almost immediately, resulting in the
same action as that obtained from the circuit of Figure 11(a) (180° conduction).
However, as indicated above, if R1 is increased, a larger input voltage (positive)
will be required to fire the SCR. As shown in Figure 12(b), the control cannot be
extended past a 90° phase displacement since the input is at its maximum at this
point. If it fails to fire at this and lesser values of input voltage on the positive slope
of the input, the same response must be expected from the negatively sloped
portion of the signal waveform. The operation here is normally referred to technical
terms as half-wave variable-resistance phase control. It is an effective method of
controlling the rms current and therefore power to the load.

Figure 12 Half wave variable resistances phase control

Battery-Charging Regulator
A third popular application of the SCR is in a battery-charging regulator.

The fundamental components of the circuit are shown in Figure 13. The control
circuit has been blocked off for discussion purposes.

As indicated in the figure, D1 and D2 establish a full-wave-rectified signal
across SCR1 and the 12-V battery to be charged. At low battery voltage, SCR2 is
in the “off” state reasons to be explained shortly. With SCR2 open, the SCR1
controlling circuit is exactly the same as the series static switch control discussed
earlier in this section. When the full-wave-rectified input is sufficiently large to
produce the required turn-on gate current (controlled by R1), SCR1 will turn on and
charging of the battery will commerce. At the start of charging, the low battery
voltage will result in a low voltage VR as determined by the simple voltage-divider
circuit. Voltage VR is turn too small to cause 11.0-V Zener conduction. In the “off”

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state, the Zener is effectively an open circuit, maintaining SCR2 in the “off” state
since the gate current is zero. The capacitor C1 is included to prevent any voltage
transients in the circuit from accidentally turning on SCR2 . Recall from your
fundamental study of circuit analysis that the voltage cannot change
instantaneously across a capacitor. In this way, C1 prevents transient effects from
affecting the SCR.

As charging continues, the battery voltage rises to a point where VR is
sufficiently high to both turn on the 11.0-V Zener and fire SCR2. Once SCR2 has
fired, the short-circuit representation for SCR2 will result in a voltage-divider circuit
determined by R1 and R2 that will maintain V2 at the level too small to turn SCR1
on. When this occurs, the battery is fully charged and the open-circuit state of
SCR1 will cut off the charging current. Thus the regulator recharges the battery
whenever the voltage drops and prevents overcharging when it is fully charged.

Figure 13 Battery charging regulator

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Temperature Controller
The schematic diagrams of a 100-W heater control using an SCR appear

in Figure 14. It is designed such that the 100-W heater will turn on and off as
determined by thermostats. Mercury-in-glass thermostats are very sensitive to
temperature change. In fact, they can sense change as small as 0.1°C. They are
limited in application, however in that they can handle only very low levels of
current-below 1mA. In this application, the SCR serves as a current amplifier in a
load-switching element. It is not an amplifier in the sense that it magnifies the
current level of thermostat. Rather, it is a device whose higher current level is
controlled by the behavior of the thermostat.

It should be clear that the bridge network is connected to the ac supply
through the 100-W heater. This will result in a full-wave-rectified voltage across the
SCR. When the thermostat is open, the voltage across the capacitor will charge to
a gate-firing potential through each pulse of the rectified signal. The charging time
constant is determined by the RC product. This will trigger the SCR during each
half-cycle of the input signal, permitting a flow of charge (current) to the heater. As
the temperature rises, the conductive thermostat will short-circuit the capacitor,
eliminating the possibility of the capacitor charging to the firing potential and
triggering the SCR. The 510-kΩ resistor will then contribute to maintaining a very
low current (less than 250µA) through the thermostat.

Figure 14 Temperature controller ( courtesy general electrical semiconductor

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Emergency-Lighting System
The last application for the SCR to be described is shown in Figure 15. It

is a single-source emergency-lighting system that will maintain the charge on a 6-
V battery to ensure its availability and also provide dc energy to a bulb if there is a
power shortage. A full-wave rectified signal will appear across 6-V lamp due to
diodes D2 and D1. The capacitor C1 will charge to a voltage slightly less than a
difference between the peak value of the full-wave-rectified signal and the dc
voltage across R2 established by the 6-V battery. In any event, the cathode of
SCR1 is higher than the anode and the gate-to-cathode voltage is negative,
ensuring that the SCR is non-conducting. The battery is charged through R1 and
D1 at a rate determined by R1. Charging will only take place when the anode of D1
is more positive than its cathode. The dc level of the full-wave-rectified signal will
ensure that the bulb is lit when the power is on. If the power should fail, the
capacitor C1 will discharge through D1, R1 and R3 until the cathode of SCR1 is less
positive than the anode. At the same time, the junction of R2 and R3 will become
positive and establish sufficient gate-to-cathode voltage to trigger the SCR. Once
fired, the 6-V battery discharges through the SCR1 and energizes the lamp and
maintains its illumination. Once power is restored, the capacitor C1 recharges and
reestablishes the non-conducting state of SCR1 as described above.

Figure 15 single source emergency lighting system

SILICON CONTROLLED SWITCH

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The silicon-controlled switch (SCS), like the silicon-controlled rectifier, is a
four-layer pnpn device. All four semiconductor layers of the SCS are available due
to the addition of an anode gate, as shown in Figure 16(a). The graphic symbol
and transistor equivalent circuit are shown in the same figure. The characteristics
of the device are essentially the same as those for the SCR. The effect of an anode
gate current is very similar to that demonstrated by the gate current in Figure 7.
The higher the anode gate current, the lower is the required
anode-to-cathode voltage to turn the device on.

Figure 16 Silicon controlled switch (SCR), (a) basic construction, (b) graphic symbol, (c) equivalent transistor
circuit

The anode gate connection can be used to turn the device either on or off.
To turn on the device, a negative pulse must be applied to the anode gate terminal,
whereas a positive pulse indicated above can be demonstrated using the circuit of
Figure 16(c). A negative pulse at the anode gate will forward-bias the base-to-
emitter junction of Q1, turning it on. The resulting heavy collector current Ic1 will
turn on Q2, resulting in a regenerative action and the “on” state for the SCS device.
A positive pulse at the anode gate will reserve-bias the base-to-emitter junction of
Q1, turning it off, resulting it off, resulting in the open-circuit “off” state of the device.
In general, the triggering (turn-on) anode gate current is larger in magnitude than
the required cathode gate current. For one representative SCS device, the

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triggering anode gate current is 1.5mA, whereas the required cathode gate current
is 1µA. The required turn-on gate current at either terminal is affected by many
factors, including the operating temperature, the anode-to-cathode voltage, the
load placement and the type of cathode, gate-to-cathode and anode gate-to-anode
connection (short-circuit, open circuit, bias, load etc.). Tables, graphs and curves
are normally available for each device to provide the type of information indicated
above.

Figure 17 SCR turn off technique

Three of the more fundamental types of turn-off circuits for the SCS are
shown in Figure 17. When a pulse is applied to the circuit of Figure 17(a), the
transistor conducts heavily, resulting in a low-impedance (~short-circuit)
characteristic between collector and emitter. This low-impedance branch diverts
anode current away from the SCS, dropping it below the holding value and
consequently turning it off. Similarly, the positive pulse at the anode gate of Figure
17(b) will turn the SCS off by the mechanism described earlier in this section. The
circuit of Figure 17(c) can be turned either off or on by a pulse of the proper
magnitude at the cathode gate. The turn-off characteristic is possible only if the
correct value of RA is employed. It will control the amount of regenerative feedback,
the magnitude of which is critical for this type of operation. Note the variety of
positions in which the load resistor RL can be placed. There are a number of other
possibilities, which can be found in any comprehensive semiconductor handbook
or manual.

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An advantage of the SCS over a corresponding SCR is the reduced turn-
off time, typically within the range 1µs to 10µs for the SCS and 5µs to 30µs for the
SCR. Some of the remaining advantages of the SCS over an SCR include
increased control and triggering sensitivity and a more predictable firing situation.
At present, however, the SCS is limited to low power, current and voltage ratings.
Typically maximum anode currents range from 100mA to 300mA with dissipation
(power) ratings of 100mW to 500mW.

Voltage Sensor
Temperature-, light- or radiation-sensitive resistors whose resistance

increase due to the application of any of the three energy sources described above
can be accommodated by simply interchanging the location of Rs and the variable
resistor. The terminal identification of an SCS is shown in Figure 18 with a
packaged SCS.

Figure 18 SCR, (a) devices, (b) terminal identification.

Some of the more common areas of application include a wide variety of
computer circuits (counters, registers and timing circuits), pulse generators,
voltage sensors and oscillators. One simple application for an SCS as a voltage-
sensing device is shown in Figure 19. It is an alarm system with n inputs from
various stations. Any single input will turn that particular SCS on, resulting in an
energized alarm relay and light in the anode gate circuit to indicate the location of
the input (disturbance).

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Figure 19 SCS alarm circuit.

Alarm Circuit
One additional application of the SCS is in the alarm circuit of Figure 20.

Rs represent a temperature-, light- or radiation-sensitive resistor that is an element
whose resistance will decrease with the application of any of the three energy
sources listed above. The cathode gate potential is determined by the divider
relationship established by Rs and the variable resistor. Note that the gate potential
is at approximately 0 V if Rs equals the value set by the variable resistor since both
resistors will have 12 V across them. However, if Rs decreases, the potential of the
junction will increase until the SCS is forward-biased, causing the SCS to turn on
and energize the alarm relay.

The 100-kΩ resistor is included to reduce the possibility of an accidental
triggering of the device through a phenomenon known as the rate effect. It is
caused by the stray capacitance levels between gates. A high-frequency transient
can establish sufficient base current to turn the SCS on accidentally. The device
is reset by pressing the reset button, which opens the conduction path of the SCS
and reduces the anode current to zero.

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Figure 20 alarm circuit.

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SUMMARY

 The silicon-controlled rectifier (SCR) is a rectifier whose state is controlled by the
magnitude of the gate current.The forward-bias voltage across the device will
determine the level of the gate current recuired to “fire” (turn on)the device.The
higher the level of biasing voltage,the less is the required gate current.

 In addition to gate triggering,an SCR can be turned on with zero gate current
simply by applying sufficient voltage across the device.The higher the gate
current,however,the less is the required biasing the voltage to turn SCR on.

 .The silicon-controlled switch has both an anode gate a cathode gate for
controlling the state of the device,although the anode gate is now connected to an
n-type layer and the cathode gate to a p-type layer.The result is that a negative
pulse at the anode gate will turn the device on,whereas a positive pulse will
turn it off.The reverse is true for the cathode gate.

 A gate turn-off switch (GTO) look similar in construction to the SCR with only
one gate connection,but the GTO has the added advantage of being able to turn
the device off and on at the gate terminal.However,this added option of being able
to turn the device off at the gate results in a much higher gate current to turn the
device on.

 The LASCR is a light-activated SCR whose state can be controlled by light falling
on a semiconductor layer of the device or by triggering the gate terminal in a
manner described for SCRs.The higher the junction temperature of the device,the
less is the required incident light to turn the device on.

 The Shockey diode has essentially the same characteristics as an SCR with
zero gate current.It is turned on by simply increasing the forward-bias voltage
across the device beyond the breakover level.

 The diac is essentially a Shockey diode that can fire in either direction.The
application of sufficient voltage of either polarity will turn the device on.

 The triac is fundamentally a diac with a gate terminal to control the action of
the device in either direction.

 The unijunction transistor is a three terminal device with a p-n junction formed
between an aluminium rod and an n-type silicon slab.Once the emitter firing
potential is reached,the emitter voltage will drop with an increase in emitter

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current,establishing a negative-resistance region excellent for oscillator
applications.Once the valley point is reached,the characteristics of the device take
on those of a semiconductor diode.The higher the applied voltage across the
device,the higher is the emitter firing potential.
 The phototransistor is the three-terminal device having characteristics very
similar to those of a BJT with a base and collector current sensitive to the incident
light intensity.The base current that results is essentially linearly related to the
applied light with a level almost independent of the voltage across the device until
breakdown results.
 Opto-isolators contain an infrared LED and a photodetector to provide a linkage
between systems that does not require a direct connection.The output detector
current is less than but linearly related to the the applied input LED
current.Furthermore,the collector current is essentially independent og the
collector-to-emitter voltage.
 The PUT (progarammable unijunction transistor)is,as the name implies,a device
with the characteristics of a UJT but with the added capability of being able to
control the firing potential.In general,the peak,valley,and minimum operating
voltages of PUTs are less than those of UJTs.

5.3 DIAC

The DIAC is a full-wave or bi-directional semiconductor switch that can be turned on in both
forward and reverse polarities.
The DIAC gains its name from the contraction of the words DIode Alternating Current.
The DIAC is widely used to assist even triggering of a TRIAC when used in AC switches.
DIACs are mainly used in dimmer applications and also in starter circuits for florescent lamps.

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Circuit symbol

The DIAC circuit symbol is generated from the two triangles held between two lines as shown
below. In some way this demonstrates the structure of the device which can be considered
also as two junctions.

Circuit symbol for the DIAC
The two terminals of the device are normally designated either Anode 1 and Anode 2 or Main
Terminals 1 and 2, i.e. MT1 and MT2.

Operation

The DIAC is essentially a diode that conducts after a 'break-over' voltage, designated VBO, is
exceeded.
When the device exceeds this break-over voltage, it enters the region of negative dynamic
resistance. This results in a decrease in the voltage drop across the diode with increasing
voltage. Accordingly there is a sharp increase in the level of current that is conducted by the
device.
The diode remains in its conduction state until the current through it drops below what is termed
the holding current, which is normally designated by the letters IH.
Below the holding current, the DIAC reverts to its high-resistance (non-conducting) state.
Its behaviour is bi-directional and therefore its operation occurs on both halves of an alternating
cycle.

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DIAC applications

Typically the DIAC is placed in series with the gate of a TRIAC. DIACs are often used in
conjunction with TRIACs because these devices do not fire symmetrically as a result of slight
differences between the two halves of the device. This results in harmonics being generated,
and the less symmetrical the device fires, the greater the level of harmonics produced. It is
generally undesirable to have high levels of harmonics in a power system.

Typical DIAC / TRIAC circuit configuration
To help in overcoming this problem, a DIAC is often placed in series with the gate. This device
helps make the switching more even for both halves of the cycle. This results from the fact that
its switching characteristic is far more even than that of the TRIAC. Since the DIAC prevents
any gate current flowing until the trigger voltage has reached a certain voltage in either
direction, this makes the firing point of the TRIAC more even in both directions.

Structure

The DIAC can be fabricated as either a two layer or a five layer structure. In the three layer
structure the switching occurs when the junction that is reverse biased experiences reverse
breakdown. The three layer version of the device is the more common and can have a break-
over voltage of around 30 V. Operation is almost symmetrical owing to the symmetry of the
device.
A five layer DIAC structure is also available. This does not act in quite the same manner,
although it produces an I-V curve that is very similar to the three layer version. It can be
considered as two break-over diodes connected back to back.

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The structure of a DIAC
For most applications a three layer version of the DIAC is used. It provides sufficient
improvement in switching characteristics. For some applications the five layer device may be
used.

5.4 TRIAC
5.4.1 Characteristics And Parameters Of The TRIAC
The TRIAC is a THYRISTOR with the ability to pass current bi-directionally
and is therefore an ac power control device. Although it is one device, its
performance is equivalent to two SCRs connected in parallel in opposite
directions but with a common gate terminal.
The basic characteristic curves for a TRIAC are illustrated in Figure 2.
Because a TRIAC is like two back-to-back SCRs, there is no reverse
characteristic.

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Figure 2: TRIAC characteristic curves
As in the case of the SCR, gate triggering is the usual method for turning on
a triac. Application of current to the triac gate initiates the latching mechanism
discussed in the previous section. Once conduction has been initiated, the
triac will conduct on with either polarity, hence it is useful as an ac controller.
A triac can be triggered such that ac power is supplied to the load for a portion
of the ac cycle.
This enables the triac to provide more or less power to the load depending on
the trigger point. This basic operation is illustrated with the circuit in Figure 3.

Figure 3: Basic TRIAC phase control. The timing of the gate trigger
determines the portion of the ac cycle passed to the load.

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5.4.2 Construct And Test The Operation Of Basic TRIAC Switch
Circuits

A simple lamp dimmer circuit is shown here, complete with the phase-shifting
resistor-capacitor network necessary for after-peak firing.

TRIACs are notorious for not firing symmetrically. This means they usually
won't trigger at the exact same gate voltage level for one polarity as for the
other. Generally speaking, this is undesirable, because unsymmetrical firing
results in a current waveform with a greater variety of harmonic frequencies.
Waveforms that are symmetrical above and below their average centerlines
are comprised of only odd-numbered harmonics. Unsymmetrical waveforms,
on the other hand, contain even-numbered harmonics (which may or may not
be accompanied by odd-numbered harmonics as well).

In the interest of reducing total harmonic content in power systems, the fewer
and less diverse the harmonics, the better -- one more reason why individual
SCRs are favored over TRIACs for complex, high-power control circuits.

One way to make the TRIAC's current waveform more symmetrical is to use
a device external to the TRIAC to time the triggering pulse. A DIAC placed in
series with the gate does a fair job of this:

DIAC break over voltages tend to be much more symmetrical (the same in
one polarity as the other) than TRIAC triggering voltage thresholds. Since the
DIAC prevents any gate current until the triggering voltage has reached a
certain, repeatable level in either direction, the firing point of the TRIAC from
one half-cycle to the next tends to be more consistent, and the waveform
more symmetrical above and below its centerline.

Practically all the characteristics and ratings of SCRs apply equally to
TRIACs, except that TRIACs of course are bidirectional (can handle current

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in both directions). Not much more needs to be said about this device except
for an important caveat concerning its terminal designations.

From the equivalent circuit diagram shown earlier, one might think that main
terminals 1 and 2 were interchangeable. They are not! Although it is helpful
to imagine the TRIAC as being composed of two SCRs joined together, it in
fact is constructed from a single piece of semiconducting material,
appropriately doped and layered. The actual operating characteristics may
differ slightly from that of the equivalent model.

This is made most evident by contrasting two simple circuit designs, one that
works and one that doesn't. The following two circuits are a variation of the
lamp dimmer circuit shown earlier, the phase-shifting capacitor and DIAC
removed for simplicity's sake. Although the resulting circuit lacks the fine
control ability of the more complex version (with capacitor and DIAC), it does
function:

Suppose we were to swap the two main terminals of the TRIAC around.
According to the equivalent circuit diagram shown earlier in this section, the
swap should make no difference. The circuit ought to work:

However, if this circuit is built, it will be found that it does not work! The load
will receive no power, the TRIAC refusing to fire at all, no matter how low or
high a resistance value the control resistor is set to.

The key to successfully triggering a TRIAC is to make sure the gate receives
its triggering current from the main terminal 2 side of the circuit (the main
terminal on the opposite side of the TRIAC symbol from the gate terminal).
Identification of the MT1 and MT2 terminals must be done via the TRIAC's part
number with reference to a data sheet or book.

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4.4 Application THYRISTOR In Solid State Relays
Solid-state relays are much smaller than corresponding single-pole relays.
They are also faster, dissipate less power, and withstand a larger number of
operations.

And unlike electromechanical relays, solid-state switches exhibit no bounce
on closing. Solid-state devices are generally preferred where switch life must
be independent of the number of switching cycles, where switching times
must be less than 2 msec, and for bounce-free or zero-current switching.
They are also generally chosen for applications subject to severe shock or
vibration.

Solid-state relays (SSRs) control load currents through solid-state switches
such as triacs, SCRs, or power transistors. These elements are controlled by
input signals coupled to the switched devices through isolation mechanisms
such as transformers, reed relays, or optoisolators. Some solid-state relays
also incorporate snubber circuits or zero-crossing detectors to reduce spikes
and transients generated by interrupting load current. Since semiconductor
switches can dissipate significant amounts of power, solid-state relays must
generally be heat sinked to minimize operating temperature.

Applications are where rapid on/off cycling would quickly wear out
conventional electromechanical relays. General-purpose SSRs have on/off
cycle lifetimes as high as 100,000 actuations. SSRs that can be actuated with
conventional CMOS and TTL logic-level voltages are available.

SSR failure modes are primarily determined by the triac or SSR switching
characteristics. Most failures take the form of SSR false turn on with no turn-
on signal. For example, turn on may occur if operating temperatures exceed
the thyristor rating. Also, transients from the switched load or from an ac line
can momentarily exceed the thyristor breakover voltage, or steeply rising load

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voltages can couple into the thyristor input through stray capacitances in the
thyristor and cause turn on. This latter effect, called dv/dt turn on, occurs in
highly inductive circuits immediately after the circuit attempts to turn off. To
combat dv/dt turn on, some SSRs use back-to-back SSRs with reverse bias
in place of triacs.

The chief failure mechanism of an SSR is mechanical fatigue in the power
semiconductor structure, caused by thermal cycling.

However, thermal-cycling effects can be controlled by matching the required
load-cycling qualities to relay characteristics. SSRs generate heat because of
the voltage drop present in all semiconductor devices. A 40-A relay, for
example, typically drops 1.2 V during conduction and, thus, dissipates 50 W
of heat. However, SSR heat generation generally does not require special
system design. These devices usually mount on circuit boards or control
panels containing other semiconductor devices.

Some SSRs designed for controlling ac loads incorporate a zero-voltage turn-on
circuit that switches the load on or off only when the power-line sine wave passes
through zero. Highly capacitive loads such as lamps and heaters which produce high
inrush currents at turn on generate little electromagnetic interference if actuated
when line voltage is zero. However, inductive loads such as motors and
transformers can saturate during the first half cycle after turn on and produce
maximum interference when switched on as line voltage passes through zero. Zero-
voltage switches should not be used.
Many SSRs have built-in transient suppressors connected in parallel with the
semiconductor switch output. Common suppressors include RC networks
(sometimes called snubbers), Zener or clipper diodes, varistors (voltage-
dependent resistors), and RC/diode dump circuits.

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REFERENCE

1. ELECTRONIC PRINCIPLES

- BY ALBERT PAUL MALVINO
- SIXTH EDITION.

2. ELECTRONIC FUNDAMENTALS : CIRCUIT DEVICES AND APPLICATIONS.

- BY THOMAS L. FLOYD.
- SIXTH EDITION.

3. ELECTRONICS PRINCIPLES AND APPLICATIONS.

- BY CHARLES A. SCHULER.
- SIXTH EDITION.

4. ELECTRONIC DEVICES AND CIRCUIT THEORY
- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY
- EIGHTH EDITION.
- www.prenhall.com/boylestad

5. ELECTRONIC DEVICES AND CIRCUIT THEORY.

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY.
- NINTH EDITION.

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MODULE NO 6: NUMBER SYSTEM_

INTRODUCTION

The purpose of the binary system is the simply another way to represent
quantities. The binary system is less complicated than decimal system because it
has only two digits (bits) 1 and 0. The binary system with its two digits is a base-
two system.The position of 1 0r 0 in a binary number indicates its weight or value
within the number.

LEARNING OBJECTIVES

The objectives of this module are to:
1. Describe the structure of Binary Numbers.
2. Describe the conversion of number system.
3. Describe the conversion of fractional number.

LEARNING OUTCOMES

After completing the module, students should be able to:
1. Describe the structure of Binary Numbers.
2. Describe the conversion of number system.
3. Describe the conversion of fractional number.

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6.1 NUMBER SYSTEM

6.1.1 Binary Number System

The purpose of the binary system is the simply another way to represent quantities.
The binary system is less complicated than decimal system because it has only two digits
(bits) 1 and 0. The binary system with its two digits is a base-two system. The position of
1 0r 0 in a binary number indicates its weight or value within the number. The
understanding of binary number is important because it is fundamental to digital electronic.

In decimal number system each of ten digits, 0 through 9 represents a certain
quantity. The simplest number that use positional notation is the binary number system. A
binary number system contains only elements or states. In a binary number system, this
is expressed as a base of 2, using the digits 0 and 1 called bit. These two digits have same
basic value as 0 and 1 in the decimal number system.
Counting in Binary

DECIMAL NUMBER BINARY NUMBER 0
0 000 1
1 000 0
2 001 1
3 001 0
4 010 1
5 010 0
6 011 0
7 011 0
8 100 1
9 100 0
10 101 1
11 101 0
12 110 1
13 110 0
14 111 1
15 111

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Table 1
As have seen in table 1, four bits are required to count from zero to 15. In general,

with n bits we can count up to a number equal to 2n – 1.

Largest decimal number = 2n – 1

Example 1.1 :

With five bits (n=5) you can count form zero to thirty-one.

25 – 1 = 32 – 1 = 31

6.1.2 The weighting Structure of Binary Numbers

A binary number is a weighted number. The right-most bit is the LSB (least
significant bit) in a binary number and was a weight of 20 = 1. The weignt increase from
right to left by apower of two for each bit. The left-most bit is the MSB (most significant bit)
, its weight depend on binary number.

Fraction number can also represented in a binary by placing bits to the right of the
binary point, just as fractional decimal digits are placed to the right of the decimal point.The
left-most bit is the MSB in a binary fractional number and has a weight of 2-1 = 0.5. The
fractional weights decrease from left to right by a negative power of two of each bit. The
structure of a binary number is

2n-1 ……23 22 21 20. 2-1 2-2…..2-n

Binary point

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Binary weights

POSITIVE POWERS OF TWO (WHOLE NEGATIVE POWERS OF TWO

NUMBERS) (FRACTIONAL NUMBERS)

28 27 26 25 24 23 22 21 20 2-1 2-2 2-3 2-4 2-5 2-6

256 128 64 32 16 8 4 2 1 ½ ¼ 1/8 1/16 1/32 1/64

0.5 0.25 0.125 0.0625 0.0315 0.015625

6.1.3 BINARY TO DECIMAL CONVERSION

The decimal value of any binary number can be found by adding the weights of all
bits that are 1 and discarding the weight of all bits that are 0. The following two examples
will illustrate this.

Example 1.2 :
Convert the binary number 1101101 to decimal.

Weight : 26 25 24 23 22 21 20

Binary Number : 1 1 0 1 1 0 1

6.1.4 DECIMAL TO BINARY CONVERSION.

A systematic method of converting whole numbers from decimal to binary is
repeated division by-2. For example to convert the decimal number 12 to binary begin by
dividing 12 by 2. Then divide each resulting quotient by 2 until there is a 0 whole-number
quotient. The reminders generated by each division from the binary number. The first
reminder to be produced is the LSB and the last reminder is the MSB.

Example 1.3 :
Convert the following decimal number 12 to binary.

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2 12

2 6 =0

Stop when the whole number 2 3 =0
quotient is 0
2 1 =1
Binary number =11002

0 =1

1 10 0

MSB LSB

Example 1.4 :
Convert the following decimal number to binary.

a) 19.
b) 45.

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a.

2 9 =1 (LSB)
(MSB)
2 4 =0

2 2 =0

2 1 =0

0 =1

Binary number = 11002 2 45 (LSB)
b. 2 22 = 1 (MSB)
2 11 = 0
. 2 5 =1
2 2 =1
Binary number = 1011012 2 1 =0

0 =1

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6.1.5 CONVERTING DECIMAL FRACTION TO BINARY

Decimal whole number can be converted to binary by repeated division by 2.
Decimal fractions can be converted by repeated multiplication by 2. For example, to
convert the decimal fraction 0.3125 to binary, begin by multiplying 0.3125 by 2 and the
multiplying each resulting fractional part of the product by 2 until the fractional product is
zero or until the desired number of decimal places is reached. The carried digits or carries,
generated by the multiplications produce the binary number. The first carry produced is
the MSB and the last carry is the LSB.

Example 1.5 :
Convert fractional number 0.3125 to binary.

Carry MSB LSB
. 0 1` 0 1

0.3125 x 2 = 0.625 0

0.625 x 2 = 1.25 1

0.25 x 2 = 0.50 0

0.50 x 2 = 1.00 1

Continue to the desired number Binary number = 0.1012
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Example 1.6
Convert decimal number 45.25 to binary.

Carry

0.25 x 2 = 0.5 0

2 45 (LSB) 0.5 x 2 = 1.0 1
2 22 = 1 (MSB)
2 11 = 0 Binary number = 0.01
2 5=1
2 2=1
2 1=0

0=1

Answer: 101101.102

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6.1.6 OCTAL NUMBER SYSTEM
It is important . It has a base of eight,meaning that it has eight possible digits :

0,1,2,3,4,5,6 and 7 Thus each digit of an octal number can have any value from
0-7. The digit positions in an octal number have weight as follows :

84 83 82 81 80 8-1 8-2 8-3 8-4 8-5
Example 1.7
3728 = 3 × (82 ) +3 × ( 81 ) + 3 × ( 80 )

= 3 × 64 + 7 × 8 + 2 × 1
= 25010
24.68 = 2 × (81 ) +4 × ( 80 ) + 6 × ( 8-1 )
= 20.7510

1.1.7 BINARY TO OCTAL NUMBER SYSTEM.
It is the simply the reverse of the foregoing process.
Example 1.8,
Converting 11010.10112 to octal
011 010 . 101 100

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3 2.5 4

Note that 0s were added on each end to complete the group of three.

1.1.8 OCTAL TO BINARY CONVERSION

The conversion from octal to binary is performed by cinverting each octal digit to
its 3-bit binary equivalent.

Octal digit 0 12 3 4 5 6 7
Binary 000 001 010 011 100 101 110 111
Equivalent
Table 5.1

Example 1.9 ,
Convert 4728 to binary :

47 2

100 111 010
Hence octal 472 is equivalent to binary 1001110102

100