1. Principles of Computer Engineering Lecture 7: Transformers Dr Steve Alty

2. 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

3. 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 IpIp NpNp VpVp NsNs IsIs Ideal transformer – + VsVs – +

4. Measuring AC Voltages  Different methods for measuring AC voltages  Peak voltage V pk  Peak-to-Peak voltage  Root-Mean-Square (RMS)  Average Power P ave

5. Principles of Computer Engineering Lab Exp 7: Transformers & Power Supply Units

6. 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!

7. Measurements and calculations Voltage (RMS) Turns ratioTurns N s AC Input201100 V BA V CA V DA V EA V FA

8. 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

9. 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

10. Add a Capacitor to Smooth DC Output  Use a 1000  F 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

11. Summary  Set up the step-down transformer with 20VAC power supply  Measure V in and V out (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?