Resident Physics Lectures

1. Resident Physics Lectures Christensen, Chapter 3A X-Ray Generators: Transformers & Rectification

2. Current Flow and Magnetic Fields Whenever an electric current flows through a conductor, a magnetic field surrounds the conductor Magnetic field can be concentrated by coiling conductor

3. Transformer Construction Transformers have 2 coils of wire coils not in electrical contact with each another When electric current passed through one coil magnetic field develops around first coil second coil near enough to feel magnetic field

4. Transformer Core Coils usually wrapped around core material such as steel Improves magnetic coupling between coils

5. Transformer Coil Designations primary coil to which power is applied secondary coil which feels magnetic field of primary coil

6. Transformer Coil Designations Secondary can have >1 set of secondary coils Primary coil induces voltage in both secondary coils

7. Transformer Coils When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil power is induced in secondary coil no physical connection

8. number of windings of secondary coil divided by number of windings of primary coil 850 / 1200 for transformer below Turns Ratio Definition

9. Transformer Theory transformers do not work with direct (unchanging) current (DC) Current induced in secondary coil only when primary coil current / magnetic field is changing

10. Transformer Theory Transformers alter both voltage & current of AC waveforms Voltage in secondary can be > or < voltage in primary

11. Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage Transformer Law

12. # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ----------------- # Prim. Coils Prim Voltage Transformer Law

13. Transformer Types Step down Transformer # primary coils > # secondary coils primary voltage > secondary voltage

14. Current Change Opposite of Voltage Change If one goes up, the other goes down If Secondary voltage > primary voltage means Secondary current < primary current If Secondary voltage < primary voltage means Secondary current > primary current

15. Current ratio is inverse of voltage ratio  # Sec. Coils Sec. Voltage Prim. Current ----------------- = ------------------ = ------------------ # Prim. Coils Prim. Voltage Sec. Current Transformer Law (cont.)

16. Power is rate of energy usage Power defined as Voltage X Current Units Voltage => Volts Current => Amps Power => Watts Power

17. Transformer primary power = secondary power transformer neither creates nor consumes power Power in Transformers

18. Transformers do not change power Transformers transform voltage / current mix High voltage --- Low current Low voltage --- High current Power

19. Autotransformer Only one winding incoming AC voltage connected across coils primary Output voltage proportional to # coils between taps secondary

20. Autotransformer Voltage law for autotransformers same as for transformers Secondary voltage adjustable by moving to a different tap changes # secondary coils NS

21. Autotransformer

22. Rectification Changes alternating current output of high voltage transformer to direct current allows current flow in one direction only x-ray tube is a rectifier because current will not flow from anode to cathode no source of free electrons at anode

23. Rectifier Also called diode Diode is 2-component device cathode anode Note symbol for diode below

24. Types of Rectifiers Vacuum tubes (thermionic diode tubes) x-ray tube is one example solid state (semiconductor) materials junction of p & n type semiconductors contains impurities with excess or shortage of electrons materials silicon most common today packaged in cylindrical stack called a �stick� selenium

25. Diodes are One Way Streets for Electric Current Allows current to flow cathode to anode Blocks current from flowing anode to cathode

26. Biasing Semiconductor Rectifiers forward biasing attempting to pass current (electrons) from N-type to P-type semiconductor across barrier junction allows current to cross barrier reverse biasing attempting to pass current (electrons) from P-type to N-type semiconductor across barrier junction does not allow current to cross barrier