Resident Physics LecturesResident Physics Lectures

• Christensen, Chapter 3A

X-Ray Generators: Transformers & Rectification

George DavidAssociate Professor of Radiology

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 FlowMagnetic Field

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

Transformer Core

• Coils usually wrapped around core material such as steel

• Improves magnetic coupling between coils

Core

Transformer Coil Designations

• primary coil to which power is

applied

• secondary coil which feels magnetic

field of primary coil

PrimaryCoil

SecondaryCoil(s)

IncomingAC Power

Transformer Coil Designations

Secondary can have >1 set of secondary coils

Primary coil induces voltage in both secondary coils

Transformer Coils

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

PrimaryCoil

SecondaryCoil(s)

IncomingAC Power

• number of windings of secondary coil divided by number of windings of primary coil 850 / 1200 for transformer below

Turns RatioTurns Ratio Definition

NP = 1200 NS = 850

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 &

CurrentNo Output

Voltage Voltage

Time Time

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

• Voltage Ratio = Turns Ratio

# Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage

Transformer Law

# Sec. Coils Sec. Voltage

Turns Ratio = ---------------- = -----------------

# Prim. Coils Prim Voltage

Transformer Law

NP = 1200 NS = 850

If VPRIM = 240 Volts then VSEC = 170 Volts

850 ?------ = -----1200 240

NS VS ----- = ----- NP VP

240VAC ? V

Transformer Types

• Step down Transformer # primary coils > # secondary coils primary voltage > secondary voltage

Step up Transformer• # primary coils < # secondary coils

• primary voltage < secondary voltage

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

• Current ratio is inverse of voltage ratio

# Sec. Coils Sec. Voltage Prim. Current ----------------- = ------------------ = ------------------

# Prim. Coils Prim. Voltage Sec. Current

Transformer Law (cont.)

VPRIM X IPRIM = VSEC X ISEC

NS VS IP

----- = ----- = ----- NP VP IS

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

• Transformer primary power = secondary power transformer neither creates nor consumes

power

Power in Transformers

VPRIM X IPRIM = VSEC X ISEC

PowerPRIM = PowerSEC

• Transformers do not change power• Transformers transform voltage /

current mix High voltage --- Low current Low voltage --- High current

Power

VPRIM X IPRIM = VSEC X ISEC

PowerPRIM = PowerSEC

Autotransformer

• Only one winding• incoming AC voltage

connected across coils primary

• Output voltage proportional to # coils between taps secondary

Primary

NP

Input NS

Secondary

Taps

Autotransformer

• Voltage law for autotransformers same as for transformers

• Secondary voltage adjustable by moving to a different tap changes # secondary coils NS

Primary

NP

Input NS

NS VS ----- = ----- NP VP

Autotransformer

Primary

100Input 50

Primary

100Input 50

20

80

RectificationRectification

• 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

Rectifier

• Also called diodediode

• Diode is 2-component device cathode anode

• Note symbol for diode below

Cathode Anode

Types of RectifiersTypes 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

Diodes are One Way Streets for Electric Current

• Allows current to flow cathode to anode

• Blocks current from flowing anode to cathode

C A C A

Biasing Semiconductor RectifiersBiasing 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 Pelectron flow

N Pelectron flowX