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Principles of Computer Engineering
Lecture 7: Transformers
Dr Steve Alty
Introduction
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Introduction to magnetic field theory
Electromagnetic Transformers
Experiment to test different windings on a transformer
Select a winding and rectify the output using diodes
Use a capacitor to smooth the DC supply
Measure the “ripple” voltage
Ideal Transformers
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AC current, change Voltage
Do not consider inductance
Assume perfectly coupled
No power loss
AC current through a coil
cause Magnetic Flux
Flux induces a voltage in the
secondary  number of turns
in the secondary coil
d
Vp  N p
dt
d
Vs  N s
dt
Ip
Is
+
+
Vp
Np
Ns
Vs
–
–
Ideal
transformer
Vp
Np
Is


Vs
Ns
Ip
Measuring AC Voltages
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Different methods for
measuring AC voltages
Peak voltage Vpk
Peak-to-Peak voltage
Root-Mean-Square (RMS)
2
Average Power Pave
vrms
2
Pave 
 irms
R  vrmsirms
R
vrms 
1
T
T

0
v (t ).dt 
2
1
T
T

0
V cos (t ).dt 
2
pk
2
V pk
2
Principles of Computer Engineering
Lab Exp 7: Transformers & Power Supply Units
Test the transformer unit & PSU
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Use prebuilt transformer box
Transformer has one primary winding which
is connected to the AC power supply unit
provided
Transformer has a “multi-tap” secondary
winding, hence each has a different voltage
Use the digital voltmeter (set to AC voltage!)
to measure the primary voltage
Then, use the digital voltmeter to measure
each secondary “tap” voltage relative to ‘A’
Note, that they are not necessarily in order!
Measurements and calculations
AC Input: Vp =
Vs
VBA
VCA
VDA
VEA
VFA
;
NP = 100
Voltage (RMS)
Vs (measured)
Turns
Ns (calculated)
N s Vs

N p Vp
Select tap to apply rectification
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Use a 1N4001 silicon diode to rectify the 12V output tap
The diode allows to current to flow only one-way
The diode also requires about 0.6 volts to function causing a
small voltage drop to occur
Connect the lamp to the diode to load the circuit
Use the oscilloscope to measure the input and output
waveforms
Full-Wave Rectification
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It is possible to achieve a more effective rectification process
by using 4 diodes in a so-called “Bridge Rectifier”
A bridge rectifier will give twice as many positive going
waveforms as before improving the resultant DC voltage
Further smoothing is required however
Add a Capacitor to Smooth DC Output
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Use a 1000mF capacitor to smooth supply
Add the capacitor in parallel with the lamp
Taking great care with the polarity of capacitor (if it’s
connected the wrong way it may explode!)
Ask us to check before turning the power on!
Again use the oscilloscope to measure the input and output
waveforms and estimate the “ripple” voltage
Summary
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Set up the step-down transformer with 20VAC power supply
Measure Vin and Vout (RMS) for the primary and each secondary
tap winding
Estimate the turns ratio for each tap
Calculate the actual number of turns for each tap
Add the diode to rectify the 12VAC tap and measure output
Build a bridge rectifier and compare brightness of lamp
Add a capacitor to smooth the DC and measure the ripple
Any questions?