1. Resident Physics Lectures Christensen, Chapter 3A X-Ray Generators: Transformers & Rectification George David Associate Professor of Radiology

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 Magnetic Field N S Current Flow Magnetic Field

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 Current Flow Magnetic Field

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

5. Transformer Coil Designations primary  coil to which power is applied secondary  coil which feels magnetic field of primary coil Primary Coil Secondary Coil(s) Incoming AC Power

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 Primary Coil Secondary Coil(s) Incoming AC Power

8. number of windings of secondary coil divided by number of windings of primary coil  850 / 1200 for transformer below Turns Ratio Turns Ratio Definition N P = 1200 N S = 850

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 Input DC Voltage & Current No Output Voltage Time

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

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 N P = 1200 N S = 850 If V PRIM = 240 Volts then V SEC = 170 Volts 850 ? ------ = ----- 1200 240 N S V S ----- = ----- N P V P 240 VAC ? V

13. Transformer Types Step down Transformer  # primary coils > # secondary coils  primary voltage > secondary voltage u Step up 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 Input AC Voltage & Current Output AC Voltage & Current Voltage Current

15. Current ratio is inverse of voltage ratio # Sec. Coils Sec. Voltage Prim. Current ----------------- = ------------------ = ------------------ # Prim. Coils Prim. Voltage Sec. Current Transformer Law (cont.) V PRIM X I PRIM = V SEC X I SEC N S V S I P ----- = ----- = ----- N P V P I S

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

17. Transformer primary power = secondary power  transformer neither creates nor consumes power Power in Transformers V PRIM X I PRIM = V SEC X I SEC Power PRIM = Power SEC

18. Transformers do not change power Transformers transform voltage / current mix  High voltage --- Low current  Low voltage --- High current Power V PRIM X I PRIM = V SEC X I SEC Power PRIM = Power SEC

19. Autotransformer Only one winding incoming AC voltage connected across coils  primary Output voltage proportional to # coils between taps  secondary Primary NPNP Input NSNS Secondary Taps

20. Autotransformer Voltage law for autotransformers same as for transformers Secondary voltage adjustable by moving to a different tap  changes # secondary coils N S Primary NPNP Input NSNS N S V S ----- = ----- N P V P

21. Autotransformer Primary 100 Input 50 Primary 100 Input 50 20 80

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 diodeAlso called diode Diode is 2-component device  cathode  anode Note symbol for diode below Cathode Anode

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 CA CA

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 N P electron flow N P X