Resident Physics Lectures (Year 1) Christensen, Chapter 3 X-Ray Generators George David, MS FAAPM, FACR Associate Professor of Radiology

Requirements to Produce X-Rays Filament Voltage High Voltage anode filament filament voltage source + high voltage source

X-Ray Generator + Supplies electrical power to x-ray tube high voltage between anode & cathode filament voltage Controls exposure timing Turns exposure on and off Filament heated before exposure High voltage switched on and off + filament anode voltage source high

Voltage Voltage from US power company Voltage required Home Industrial Most stuff: 120 V AC / Dryer: 240 V Door bell: 15 V Computer 5 V X-Ray Filament: 8-12 V High voltage: 40-150 kV (40,000 – 150,000)

Transformers Magical devices that allow voltage to be changed to any desired value

Current Flow and Magnetic Fields Magnetic field surrounds conductor carrying electric current Magnetic field concentrated by coiling conductor Magnetic Field N S Magnetic Field Current Flow

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 Magnetic Field Current Flow

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

Transformer Coils When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil power induced in secondary coil no physical connection Incoming AC Power Primary Coil Secondary Coil(s)

Turns Ratio Definition TR=NS / NP number of windings of secondary coil divided by number of windings of primary coil 40 / 20 for transformer below NP = 20 NS = 40

Input DC Voltage & Current 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 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

Transformer Law NS VS ----- = ----- NP VP Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage NS VS ----- = ----- NP VP

Transformer Types Step up Transformer Step down Transformer # primary coils > # secondary coils primary voltage > secondary voltage Step up Transformer # primary coils < # secondary coils primary voltage < secondary voltage

How Does a Transformer Magically Increase Voltage without Some Source of Power?

Transformer Law (cont.) Current ratio is inverse of voltage ratio # Sec. Coils Sec. Voltage Prim. Current ----------------- = --------------- = ----------------- # Prim. Coils Prim. Voltage Sec. Current NS VS IP ----- = ----- = ----- NP VP IS VPRIM X IPRIM = VSEC X ISEC

Power = Voltage X Current Transformers Power = Voltage X Current Electrical power not changed Current exchanged for voltage Voltage goes up – current goes down Voltage goes down – current goes up

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

Power PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC Power = Voltage X Current Transformer primary power = secondary power transformer neither creates nor consumes power PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC

Transformer Ratio Ratio = Output voltage / Input voltage Most transformers have fixed ratios X-Ray requires variable ratios Accommodate selection of different kVs

Autotransformer Only one winding Taps Only one winding Incoming AC voltage connected across coils primary Output voltage proportional to # coils between taps secondary Input NS NP Primary Secondary

Autotransformer NS VS ----- = ----- NP VP Voltage law for autotransformers same as for transformers Variable ratio transformer Secondary voltage adjustable by moving to a different tap changes # secondary coils NS Input NS NP NS VS ----- = ----- NP VP Primary

Generator Components control console transformer electronics cabinet kVp adjust mA adjust time adjust transformer high voltage (step up) filament low voltage (step down) electronics cabinet support circuitry or mAs adjust

High Voltage Transformer X-ray Circuit High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA selector Filament Transformer

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA selector Filament Transformer Line Incoming line voltage connected to generator through a circuit breaker. Typ. 220-240 volt AC single phase 240, 480 volt AC three phase

Circuit Breaker Generator connected to power line through a circuit breaker Limits current from power line to generator Allows generator to be disconnected from power line Incoming Power Line Generator Circuit Breaker

High Voltage Transformer Rectifier Circuit Variable ratio Timer Circuit Fixed ratio + Auto- trans-former Line mA regulator Filament Transformer Auto transformer Variable ratio transformer allowing operator to specify kVp High voltage transformer Boosts output of autotransformer by fixed ratio

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA regulator Filament Transformer Timer Starts & stops exposure Turns transformer primary (low voltage) on & off

Exposure Timing Manual Operator sets time Automatic (Phototimed) Equipment measures radiation Terminates exposure when designated radiation is measured

Phototiming Detector in front of receptor Must be essentially invisible Grid Image Receptor Radiation Sensor

Phototiming Fields 1, 2, or 3 fields may be selected individually or in combination proper positioning critical

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA regulator Filament Transformer Timer Develops DC high voltage for x-ray tube

High Voltage Transformer Grounded metal box filled with oil electrical insulator Function Develops proper high voltage for tube Also contains rectifier circuit

Rectification allows current flow in one direction only Transformers only work with alternating current (AC) Rectifier changes alternating current output of high voltage transformer to direct current DC voltage applied to tube

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA regulator Filament Transformer mA regulator Circuitry for mA selection Adjusts mA on the fly during exposure.

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA selector Filament Transformer Filament Transformer Changes AC voltage to smaller voltage required by filament (8-12 volts typical)

Power Storage Generators Use batteries Application Remote locations Inadequate power from power line or power line not accessible Outlet inaccessible

Battery-Powered Generators Batteries used for x-ray transport Independent of power line during exposure Disadvantages Require charging Heavy