Principles of Computer EngineeringLecture 7: Transformers

Dr Steve Alty

Introduction 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 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

Ip

NpVpNs

Is

Ideal transformer

–

+

Vs

–

+

dt

dNV ss

dt

dNV pp

p

s

s

p

s

p

I

I

N

N

V

V

Measuring AC Voltages Different methods for

measuring AC voltages Peak voltage Vpk Peak-to-Peak voltage Root-Mean-Square (RMS) Average Power Pave

rmsrmsrmsrms

ave ivRiR

vP 2

2

2).(cos).(

0

221

0

21 pkT

pkT

T

Trms

VdttVdttvv

Principles of Computer EngineeringLab Exp 7: Transformers & Power Supply Units

Test the transformer unit & PSU 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

Voltage (RMS)

Turns ratio

Turns

Ns

AC Input 20 1 100

VBA

VCA

VDA

VEA

VFA

p

s

p

s

V

V

N

N

Select tap to apply rectification 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 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 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 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?