Bridge rectifier

Thyristor control of a resistive load 1
THYRISTOR CONTROL OF A RESISTIVE LOAD
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University
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Course
City/State
Date

Abstract
This report is a study and design of single full wave controlled rectifiers using two
single phase half-wave thyristor converters designs with R load. Using an oscilloscope, the
output voltage waveforms were observed. At firing angles within the range of 0° < 𝛼 < 90°,
the converter was operating in the rectifying mode, hence power was flowing from the AC
source into the DC load while at firing angles within the range of 90° < 𝛼 < 180°, there
were small conducting angle for the output voltages. The converter was operating more in the
inversion mode hence little power was flowing into the DC load. It was found that at firing
angle, 𝛼, of 0° the thyristor exhibited the characteristics of a normal diode, while at firing
angle of 180° it did not conduct at all.

Introduction
Earlier, most of the DC power was achieved via AC power or motor generator
conversion into the DC power by using thyratrons or the mercury arc rectifiers. Nowadays,
thyristor are commonly used in conversion of the AC to DC power, this is possible by the
application of phase controlled AC to DC converters that change constant AC voltage into
controlled DC output voltage (Visintini, 2014).
The thyristors are part of the semiconductor control devices that fall under the Silicon
Controlled Rectifiers (SCR). They are applied mostly applied in industries since they more
efficient, reliable, rugged, and less expensive. They are mainly used in generator field
control, solid state relays, DC motor control, lighting systems and Variable Frequency Drive
(VFD) (Visintini, 2014).
Objective
To study a full wave half controlled bridge rectifier in common cathode configuration
with a resistive load.
Background
Single phase half-wave thyristor converters are not widely applied in power
electronics due to the fact that they low DC voltage, as well low DC power; hence full wave
rectifiers are suitable instead (Bird, 2010). These rectifiers, half wave controlled rectifiers,
employ only one SCR in their circuit, that is, between the load and the AC source. The single
thyristor device used in this rectifier gives output controls for each half of an input AC
supply. The characteristics or performance of a controlled rectifier is dependent on the
parameters and type of load or output circuit (Irwin, 2002).

The single full wave controlled rectifiers are a combination of two half wave rectifiers
in a single circuit. This implies that the input’s half cycles are utilized as well as converted
into a unidirectional output current via the load for the production of two pulse output
waveform. The full wave controlled can be Achieved either via a centre tapped transformer
or bridge. A single phase full wave bridge controlled rectifier is further divided into a half
(semi) and fully controlled bridge converter (Krein & Pilawa-Podgurski, 2016).
Phase controlled bridge rectifiers are used in the provision of unregulated DC voltage
that can be processed into either a regulate AC or DC output. A phase controlled rectifier is
made up of a thyristor, load, and AC supply as the main circuit components. These rectifiers
are divided into R load, RL load and RL load with freewheeling diode; their classification is
dependent on the load requirements.
Figure 1. Single phase half wave controlled bridge rectifier with a resistive load
From Figure 1 above, the input AC voltage to the thyristor converter from the
transformer is dependent on the required voltage output. 𝑣 𝑝 and 𝑣𝑠 represent primary and
secondary AC supply of the transformer respectively. At positive half cycle of the input,
usually when the upper end of the secondary part of transformer has a positive potential when
compared to the lower end, the thyristor’s anode becomes positive as compared to its
cathode; hence the thyristor enters a forward biased state (Dollah, 2013).

When a suitable gate trigger pulse is applied to the thyristor’s gate lead, it implies that
there is a triggered delay angle expressed as 𝜔𝑡 = 𝛼. An ideal thyristor is assumed to behave
like a closed switch when a suitable gate trigger pulse is applied. Therefore, when a thyristor
is triggered to delay at an angle 𝛼, the input supply voltage is equal to the voltage across the
load as the thyristor conducts from 𝛼 to 𝜋 radians. This implies that that the output voltage
𝑣 𝑜 = 𝑣𝑠 for an ideal thyristor conducting from 𝛼 = 𝜔𝑡 to 𝜋 radians (Erney Fabian et al.,
2016).
The load current or output, 𝑖 𝑜 , of purely resistive load flowing when the thyristor 𝑇1
is on is expressed as follows:
𝑖 𝑜 =
𝑣 𝑜
𝑅 𝐿
𝑤ℎ𝑒𝑛 𝛼 ≤ 𝜔𝑡 ≤ 𝜋
At the thyristor’s conduction time 𝛼 to 𝜋, its output load current and output voltage
waveforms are similar; that is, they are in phase. The load current increases in a direct
proportion to the input supply voltage; hence the maximum load current appears at 𝜔 =
𝜋
2
as
the supply voltage is maximum at the input. The thyristor is turned off when the current
flowing through it is zero; when 𝜔𝑡 = 𝜋.
The peal value or maximum value of the current is obtained as:
𝐼 𝑚 = 𝑖 𝑜( 𝑚𝑎𝑥) =
𝑉𝑚
𝑅 𝐿
Where 𝑅 = 𝑅 𝐿 =Load Resistance; and 𝐼 𝑚 =peak value of the load current when 𝜔 =
𝜋
2
When the thyristor is on, the source current is expressed as:
𝑖 𝑠 = 𝑖 𝑇𝑖 = 𝑖 𝑜 =
𝑣 𝑜
𝑅
=
𝑉 𝑚 sin 𝜔𝑡
𝑅
; 𝑤ℎ𝑒𝑛 𝛼 ≤ 𝜔𝑡 ≤ 𝜋
Where 𝑖 𝑇1=Thyristor current; 𝑖 𝑜= Load current via 𝑅 𝐿; 𝑖 𝑠=Source current flowing via the
transformer’s secondary windings.

When the supply voltage reverses, in negative half cycle, and becomes negative at
𝜔𝑡 = 𝜋 to 2𝜋 radians, the thyristor’s anode has a negative potential with respect to the
cathode (Hughes, 2008). This implies that the thyristor is reverse biased hence it is at cut-off
or reverse blocking mode. In this state, a thyristor cannot conduct and therefore an ideal
thyristor in this mode behaves like an open switch. At cut-off, the load current and voltage
are zero, at 𝜔𝑡 = 𝜋 to 2𝜋. The maximum reverse voltage appearing Across the cathode and
anode terminal is given as 𝑉𝑚.
The delay or trigger angle, 𝛼 is obtained at beginning of each half cycle up to an
instant a gate trigger pulse is provided. Therefore, the thyristor’s conduction angle is 𝛿 =
( 𝜋 − 𝛼); in other terms it from 𝜋 to 𝛼. The maximum conduction angle occurs at 180° (𝜋
radians) when trigger angle is 0° (𝛼 = 0°).
𝑣𝑠 = 𝑉𝑚 sin 𝜔𝑡= AC supply voltage across the secondary side of the transformer.
𝑉𝑚=maximum value of the input AC supply voltage across the secondary side of the
transformer.
𝑉𝑠 =
𝑉 𝑚
√2
=RMS value of AC supply voltage across the secondary side of the transformer.
𝑣 𝑜 = 𝑣𝐿 =output voltage at the load
𝑖 𝑜 = 𝑖 𝐿=output current
𝑣 𝑜 = 𝑣𝐿 = 𝑉𝑚 sin 𝜔𝑡 at 𝜔𝑡 = 𝛼 to 𝜋 the thyristor is switched on or Act as a closed switch.
𝑖 𝑜 = 𝑖 𝐿 =
𝑣 𝑜
𝑅
=Load current at 𝜔𝑡 = 𝛼 the thyristor is switched on.
Finding the average output voltage, 𝑉𝑑𝑐;
𝑉𝑜( 𝑑𝑐) = 𝑉𝑑𝑐 =
1
2𝜋
∫ 𝑣 𝑜. 𝑑( 𝜔𝑡)
𝜋
𝛼
𝑉𝑜( 𝑑𝑐) = 𝑉𝑑𝑐 =
1
2𝜋
∫ 𝑉𝑚 sin 𝜔𝑡 . 𝑑( 𝜔𝑡)
𝜋
𝛼

𝑉𝑜( 𝑑𝑐) =
𝑉𝑚
2𝜋
∫ sin 𝜔𝑡 . 𝑑( 𝜔𝑡)
𝜋
𝛼
𝑉𝑚
2𝜋
[−cos 𝜔𝑡] 𝛼
𝜋
𝑉 𝑚
2𝜋
[−cos 𝜋 + cos 𝛼] where −cos 𝜋 = −1
𝑉 𝑚
2𝜋
[1 + cos 𝛼] ; 𝑉𝑚 = √2 𝑉𝑠
√2 𝑉𝑠
2𝜋
[1 + cos 𝛼]
𝑉𝑑𝑐(𝑚𝑎𝑥) = 𝑉𝑑𝑚 =
𝑉𝑚
𝜋
The single phase half-converter circuits consist of two diodes and thyristors in order
to make a full wave half controlled bridge converter. Half controlled bridge converter is
divided into ssymmetrical and asymmetrical configurations (Robertson, 2008). Figure 2 is
symmetrical configuration, which is the commonly used, since a single trigger is used when
firing the two thyristors without any application of electrical isolation.
Figure 2. Single phase full wave converter
The two thyristors in the circuit above are controlled by applying suitable gating
signal or trigger pulse, hence they Act as power switches. On the other hand, the two diodes
are uncontrolled switches that turn-on or conduct when forward biased. During the positive
half cycle with respect to the input AC supply voltage, line ‘A’ is positive when compared

with line ‘B’; this implies that diode 𝐷1 and thyristor 𝑇1 are forward biased. Thyristor 𝑇1
becomes triggered at angle 𝜔𝑡 = 𝛼 by applying a suitable trigger signal to the gate. This
implies that current flows via line ‘A’ via 𝑇1 then via the load in a downward direction and
eventually via 𝐷1 to line ‘B’. Diode 𝐷1 and 𝑇1 conduct together as from 𝜔𝑡 = 𝛼 to 𝜋 while
load is connected to an input AC supply.
Finding the average output voltage, 𝑉𝑑𝑐;
𝑉𝑑𝑐 =
2
2𝜋
∫ 𝑣 𝑜. 𝑑( 𝜔𝑡)
𝜋
𝛼
𝑉𝑑𝑐 =
2
2𝜋
∫ 𝑉𝑚 sin 𝜔𝑡 . 𝑑( 𝜔𝑡)
𝜋
𝛼
𝑉𝑑𝑐 =
2𝑉𝑚
2𝜋
∫ sin 𝜔𝑡 . 𝑑( 𝜔𝑡)
𝜋
𝛼
𝑉𝑑𝑐 =
2𝑉𝑚
2𝜋
[−cos 𝜔𝑡] 𝛼
𝜋
𝑉𝑑𝑐 =
𝑉 𝑚
𝜋
[− cos 𝜋 + cos 𝛼] where − cos 𝜋 = −1
𝑉𝑑𝑐 =
𝑉 𝑚
𝜋
[1 + cos 𝛼]
𝑉𝑑𝑐 varies from
2𝑉 𝑚
𝜋
to 0 with the variation of 𝛼 from 0 to 𝜋.
Maximum average voltage=𝑉𝑑𝑐( 𝑚𝑎𝑥) =
2𝑉 𝑚
𝜋
=𝑉𝑑𝑚

Figure 3. Waveforms of single phase full wave half-converter for R and RL loads
Materials and method
 1 - PE481 power electronics control unit
 1 - PE481B full-wave thyristor circuits module
 1 - PE483 power electronics control unit with integral full-wave thyristor circuits
 1 – Diginess hand-held multimeter
 1 – Tektronix TDS1002 digital double beam oscilloscope
 1 – Slide-wire rheostat
 2 – Resistors, 1 MΩ and 10 kΩ, 0.5 W mounted on a component board
 Inter-connectors.
Experimental procedure
The apparatus were set as shown in Figure 2 using wires. The 240 V, 13 A bench
supply was switched OFF and the PE482A Control “set value” maintained at zero when
connecting the apparatus. The 0 V from the +15-15 V output was connected to the earth. The
rheostat was connected as a fixed resistance using the bottom terminals, one at each end. An

earth from the rheostat’s stud was connected to an earth point. All meters connected were
eventually checked if they were at their zero mark.
The PE481 Base Unit was adjusted to the 30 V/1 A level using the “Max O/P”. The
“Meter 1” was set to “normal” while the 50 V range was adjusted to forward. The Base
Unit’s meter 2 was set to the 60 V range.
The PE483 Unit was set to meter at V/A, the 60 V range and in forward. The PE482A
unit was used in applying the “set value” that controlled the point in the cycle at which the
thyristors fired. The 1 MΩ and 10 kΩ resistors were used to act as potential divider, so as to
enable simultaneous display of the voltage as well as the trigger pulse waveforms. Initially
the thyristor firing circuit on the PE482A unit was set to multi-pulse but later changed to a
single-pulse at some point in order to observe the effect on the pulse wave form.
The Power electronics control unit was switched on. The delay angle range was
maintained for the resistive loads with “set value” requiring minor adjustment in order to
reach the maximum output voltage. The oscilloscope was switched on with the vertical
position controls used in setting the baselines or coupling ground of the Channel 1 and 2. The
observations for the variation of the load voltage with firing angle was achieved by cursor on
the oscilloscope as it can measure or indicate time, therefore, delay angle. Additionally, the
handheld multimeter was used in determining the voltage.
The maximum load voltage was set and the horizontal position control was used for
the arrangement of the voltage waveform into the 180° position; that is the ‘end’ of a half-
cycle. This step facilitated subsequent estimations of the delay angle. The handheld
multimeter was used to check the voltage in PE481.
Results
Input voltage= 42 V

Table 1. Input and output values of the rectifier
𝜶(°) 𝑽 𝒐 (Measured) V 𝑽 𝒐 (Calc) 𝑽 𝒐 =
𝑽 𝒎
𝝅
[ 𝟏 + 𝐜𝐨𝐬 𝜶] V
0 36.2 37.8
36 32.8 34.2
54 28.5 30.0
72 23.3 24.7
90 18.3 18.9
108 12.1 13.1
126 7.3 7.8
144 3.3 3.6
162 0.9 0.9
180 0.0 0.0
Figure 4. Vo (DC) versus the trigger angle ( 𝜶)
From Figure 4 and Table 1, the voltage out is inversely proportional to the trigger
angle.
Table 2. Error analysis
𝜶(°) 𝑽 𝒐 (Measured) V 𝑽 𝒐 (Calc) V Absolute Error, V Relative Error (%)
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0 20 40 60 80 100 120 140 160 180 200
VO(dc)
Trigger angle α in degrees
VO(dc) Vs. TriggerAngle α in Degrees

0 36.2 37.8 1.6 4.2
36 32.8 34.2 1.4 4
54 28.5 30.0 1.5 5
72 23.3 24.7 1.5 6
90 18.3 18.9 0.6 3
108 12.1 13.1 0.9 7
126 7.3 7.8 0.5 6.5
144 3.3 3.6 0.3 8
162 0.9 0.9 0.1 6
180 0.0 0.0 0.0 0
Average =4.97%
𝜂=95.03%
From Table 2, 𝜂=95.03% which is Acceptable, since typical rectification ration, 𝜂, is about
81%. Additionally, the measured and calculated values have small deviations as seen in
Figure 5.
Figure 5. Error analysis Vo (DC) versus the trigger angle ( 𝜶)
The discrepancies in measured and calculated values are due to the sudden changes in
voltages at the thyristor’s firing point hence large current spikes are produced.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0 20 40 60 80 100 120 140 160 180 200
VO(dc)
Trigger angle α in degrees
ErrorAnalysis of the VO(dc) Vs. TriggerAngle α in
Degrees
Measured Calculated

Figure 6. The output sketch on the oscilloscope

From Figure 6, the wave forms are less regular. The problem emanates from the
process of commutation. In the half controlled case, the thyristor and diode pair does not turn
off and on the same time. In this case, the diodes conduct up to the occurrence of
commutation at the firing point of the next diode or thyristor.
Discussions
The single-phase mains supply was stepped down prior to being fed to the control
circuit; the control circuit has components that work at low voltage and current. The PE481
power electronics control unit played a major role in the experiment because of its duty that
triggers the firing angle to the two thyristors. Therefore, the performance of this circuit
component is vital in determining the outcome or errors. The diodes were used in order to
avert the load voltage from becoming negative, moreover extend the conduction period at
reduced AC ripples (Mubeen, 2014).
The working of single phase full wave half-converter for R is studied along with the
waveforms. The full wave rectifier converted both polarities of AC into DC. The rectifier
exhibited less ripples when compared to the half wave rectifier. Additionally, the circuit
components shared the same main current supply for a synchronized signal that was used for
gating the SCR. If there were no synchronization, possibly the SCR firing signal could be
random in nature hence leading to an erratic system response. From the calculations, its
efficiency was over 81%, hence twice that of the half wave rectifier.
The SCR conducted after meeting the following conditions, the SCR was forward
biased and current had been applied to the gate. It was observed that, the SCR conducted after
being triggered despite the gate current going to zero. At firing angles within the range of
0° < 𝛼 < 90°, there were large conduction angle of the average output voltage. This implies
that the converter was operating in the rectifying mode, hence more power was flowing from
the AC source into the DC load. Moreover, at firing angles within the range of 90° < 𝛼 <

180°, there were less conduction angles for the average output voltages (SCILLC, 2016). The
converter was operating more like in the inversion mode hence less power was flowing to the
DC load from the AC source as seen in Figure 6; it was not possible for the circuit to work in
the inversion because of the presence of the diodes. The diodes averted the average output
voltage from becoming negative. At firing angle 𝛼 = 0°, the circuit behaved liked the one
with diodes only; it had the characteristics of the uncontrolled rectifier. Additionally, the
source current for the circuit with resistive load was sinusoidal; hence it was in phase with the
average voltage at trigger angle, 𝛼 = 0° which further implies that the power factor was
approximately 1, p.f=1 (Hughes, 2008).
Since conduction is via diode, the output voltage wave in R load cannot extend in a
negative direction but in the case of an RL load, the circuit’s output voltage wave will extend
towards the negative portion. If there was an inductive load, there would be a tendency of
slowed change in the anode current with time, hence leading to an increased charge with the
value of inductance. According to ON Semiconductor (2016), long pulse width or DC current
triggering, there are little effects of inductive loads but the effects increase significantly at
short pulse widths. Increase in charge is as a result of short pulse widths hence the trigger
signals are decreased to negligible values prior to the anode current reaching a level sufficient
of turn−on sustenance.
Conclusion
At the firing angle, 𝛼, of 0° the thyristor has the characteristics of a normal diode,
while at firing angle of 180° it does not conduct at all. The circuit exhibited an experimental
error of 4.97%, which is negligible. These are random errors that affected the precision of
measurement due to the sum of component errors as they are not ideal; hence non-traceable.
Therefore, the study objectives of a full wave half controlled bridge rectifier in common

cathode configuration with a resistive load was successively carried out via design,
construction and implementation.

References
Bird, J., 2010. Electrical principles and technology for engineering. Oxford: Newnes.
Bishop, O., 2011. Electronics circuit and systems. 4th ed. Oxford: Elsevier.
Dobkin, B., 2011. Analog circuit design. Oxford: Elsevier.
Dollah, Z., 2013. Single-phase controlled rectifiers. Kelantan: Kota Bharu Polytechnic.
DronACharya, 2015. power electronics lab manual. [Online] Noida: DronACharya Available
at: http://gnindia.dronACharya.info/EEEDept/Downloads/Labmanuals/Power-Electronics-
Lab-Manual.pdf [ACcessed 1 March 2017].
educypedia.info, 2016. Power Electronics Lab: Rectifier Circuit. [Online] Available at:
http://educypedia.karadimov.info/library/Labsheet2y.pdf [ACcessed 2 March 2017].
Erney Fabian, C.B., Diego Alejandro, P.P. & Franklin Meer, A., 2016. Design and
implementation of single phase fully controlled bridge rectifier using PIC microcontroller.
International Journal of Scientific and Research Publications, VI(1), pp.239-50.
Hughes, E., 2008. Electrical and electronic technology. 10th ed. Harlow: Pearson.
Irwin, J.D., 2002. The power electronics handbook. New York: CRC Press LLC.
Jalakas, T. & Vodovozov, V., 2015. Laboratory works in power electronics. Tallinn: Tallinn
University of Technology Department of Electrical Drives and Power Electronics.
Krein, P.T. & Pilawa-Podgurski, R.C.N., 2016. ECE 469: Power electronics laboratory.
Urbana: Department of Electrical and Computer Engineering, University of Illinois.
Kybett, H. & Boysen, E., 2008. All new electronics self-teAChing guide. 3rd ed. Indianapolis:
Wiley Publishing, Inc.
Mehar, H., 2013. The case study of simulation of power converter circuits using Psim
software in teAChing. American Journal of Educational Research, I(4), pp.137-42.
Monaaf, F.A., 2015. Speed control of a separately- excited DC motor. International Journal
of Research in Aeronautical and Mechanical Engineering , III(1), pp.64-77.
Mubeen, M., 2014. Power electronics. Hyderabad: Muffakham Jah College of Engineering
&Technology.
Patel, M.S., 2014. Power electronics lab reference manual. Kattankulathur: SRM University.
Robertson, C.R., 2008. Fundamental electrical and electronic principles. 3rd ed. Burlington:
Elsevier Ltd.
Scientech, 2015. Single phase controlled rectifier with cosine firing scheme. Pardesipura:
Scientech Technologies Pvt. Ltd.

SCILLC, 2016. Thyristor theory and design considerations. [Online] Available at:
http://onsemi.com [ACcessed 1 March 2017]. Excerpt from the DL137/D.
Visintini, R., 2014. Rectifiers. Trieste: Elettra Synchrotron Light Laboratory.

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