X-Ray Generators

A transformer is a device that either increases or decreases the voltage in a circuit. A rectifier changes alternating current into direct current.

                230V, 60 Hz

                Adjust  mA, kVp and Exposure time

                Low voltage transformer for the filament circuit and

High Voltage transformer and Group of Rectifiers for the high-voltage circuit.

Immersed in oil that acts as an insulator and prevents sparking of various components.  

v  TRANSFORMERS

Filament heating requires potential difference of 10V. Electron acceleration requires potential difference of between 40 000 to 150 000 V.

Potential and Potential Difference are measured in Volts. Voltage meter measures the potential difference.

§  Primary circuit and Secondary circuit

The core of a transformer is laminated  - to prevent eddy currents – which waste power and appear as heat in the transformer core.

§  When current flows through a primary coil, it creates a magnetic field within the core, and this magnetic field induces a current in the secondary coil.  

A current only flows in the secondary circuit when the current is increasing or decreasing.  No current flows while the magnetic field is in steady state.

Ø  Laws of transformers

§  The voltage in the two circuits is proportional to the number of turns in the two coils

Step up transformers and step down transformers. Step up transformers increase the voltage and decrease the current. Used to supply the high voltage to the x-ray tube.

 Step-down transformers decrease the voltage and increase the current. (remember that no transformer is 100% efficient). Supplies the power that heats up the filament.

§  The second law of transformers is simply a restatement of the law of conservation of energy. A transformer cannot create energy. An increase in voltage must be accompanied by a corresponding decrease in current. The product of the voltage and current in the two circuits must be equal.

§   The product of voltage and current is power

Ø  The autotransformer

Provides voltage for the x-ray tube filament circuit

Provides voltage for the primary of the high-voltage transformer

Provides a convenient location for the kVp meter that indicates the voltage to be applied across the x-ray meter

§  Single winding – wound around a laminated closed core

Ø  Filament circuit

§  The filament circuit regulates current flow through the filament of the x-ray tube.

The filament is a coiled tungsten wire that emits electrons when it is heated by current flow – thermoionic emission. (3-5 A with 10 V heats the filament)

§  The power to heat the x-ray tube filament is provided by a small step-down transformer called the “filament transformer”

§  Precise control over of filament heating is critical, because a small variation in filament current results in a large variation in x-ray tube current.

Using resistors to reduce voltage – Ohm’s Law :

Ø  High-Voltage Circuits

Ø  Rectification

§  Rectification is the process of changing alternating current into direct current – rectifier

§  Half-wave rectification

§  Rectifiers: a rectifier is a device that allows an electrical current to flow in one direction but does not allow current to flow in other directions

§  Semiconductor: The heart of a solid state rectifier is a semiconductor, which usually a piece of crystalline silicon.

·         N-type semiconductors. Donor

·         P-type semiconductors. acceptors

·         P-N Junctions. Diode. Solid state rectifiers are diodes.

¨       Forward bias and reverse bias

¨        

§  Silicon rectifiers

§  Half wave rectification

§  Full wave rectification

v  TYPES OF GENERATORS

Ø  Three phase generators

Ø  Three phase transformers – delta and wye

§  Six-pulse, six-rectifier

·         Ripple factor

·         Load ripple-factor, and is always greater than theoretical ripple

§  Six-pulse Twelve rectifier

§  Twelve pulse recitifier

Ø  Power storage generators

Ø  Capacity Discharge Generators

Ø  Battery-Powered Generators

Ø  Medium-frequency generators. In a transformer, the voltage induced in the secondary coil is proportional to the rate of change in the current of the primary coil.

V = output voltage, F= frequency, n=number of windings, A=core cross-sectional area

v  TRANSFORMER RATING

Ø  Kilowatts

 = 0.707 peak

§  Kilowatt ratings of x-ray generators are determined when the generator is under load, 100 kVp

v  EXPOSURE SWITCHING

Ø  Primary switching and

§  Silicon-controlled rectifiers (SCRs) or thyristers

Ø  secondary switching

Ø  Primary versus Secondary Switching

v  FALLING LOAD GENERATORS

Ø  The purpose of a falling load generator is to produce an x-ray exposure in the shortest possible exposure time by operating the x-ray tube at its maximum kilowatt rating for the entire exposure

v  EXPOSURE TIMERS

Ø  Electronic Timers

Ø  Automatic Exposure Control (Phototimer)

§  Entrance or exit types

§  Photomultiplier Phototimers

§  Ionization Chamber Autotimers

§  Solid-State Autotimers

§  Miscellaneous Autotimer Topics

§  Pulse-Counter Timers