The high tension source

COMPONENTS AND CONTROL IN X RAY CIRCUIT

15-Nov-17

The x-ray tube circuit comprises the following components and controls:-

• The high tension source
• The kilo voltage control
• Filament circuit and milliamperage control
• Timer
• The mains supply, and

The x-ray tube as shown in the diagram below

The high tension source:

The source of high voltage which is used to drive electrons fast across an x-ray tube is a high tension transformer.

The high tension transformer transforms the voltage of the main supply up to the range of voltage from about 20,000 to 100,000 volts (20kV to 100kV).

The function of high tension transformer is to transform the voltage from the main supply up to thousands of volts required to operate the x-ray tube. (step up transformer)

The high tension source:

• Grounded metal box
• filled with oil
• electrical insulator
• Also contains rectifier circuit

changes alternating current into direct current

CONTRUCTION OF A TRANSFORMER

Materials used are soft ion and copper wires. The soft ion core is wound with copper wire on it as indicated on the diagrams below:-

The high tension source

The step-up transformer has many numbers of turns in the secondary winding compared to the primary winding, while the step-down transformer has less number of turns on the secondary winding.

For an ideal transformer the induced voltage in each turn is the same.

Therefore the total primary voltage is the voltage in each turn multiplied by the number of turns.

• Similarly for the secondary:

Vs = V x Ns or Vs/Ns = V

On combining the two equations we get:-

Vp /Np = Vs/Ns

• By cross multiplication we get:

Vs/Vp = Ns/Np

WINDINGS

The primary winding receives electricity from the mains supply. It has few number of turn compared to the secondary winding.

• It is shorter and thicker because carries very high current (it may be 200 Amps).

The secondary winding supplies kilovoltage and mill amperes for the x-ray tube.

Supplies power in form of low current at very high voltage. This is the reason to why it is made of thin wire. It is long compared to the primary winding.

Step-down transformer

This is a type of transformer steps down high volts to low volts e.g. the filament transformer. It steps down the mains voltage which used by the filament.

This type of transformer has few number of turn on the secondary winding.

AUTO TRANSFORMER

Is the type of transformer which has one winding wound around a soft iron core.

There is only one winding

The primary and secondary circuits are in metallic connection with each other. This fact makes an auto transformer unsuitable for transforming high voltages from one value to another or for stepping up voltages to high values.

Filament Circuit

The circuit which supplies power to the filament of the x-ray tube

X RAY TUBE FILAMENT CIRCUIT

The components in the filament circuit

• An autotransformer
• mA Selector
• Filament transformer
• Voltage stabilizer

Frequency compositor

The Autotransformer; Contains an iron core and a single winding or wire, supplies power to the filament transformer.

The step-down transformer; Decreases voltage from primary to the secondary coil and increases current in the same proportion. Has more turns in the primary than in the secondary coil. Used in the filament portion to increase current flow to the cathode.

Filament Transformer

• Intended to lower voltage and increase current
• Allows for thermionic emission to occur at the filament
• Ultimately, provides the electrons necessary for x-ray production.

Tube current measured in mA.

The high tension source

The mA selector: Used for selecting the amount of current flowing in the filament circuit by varying resistors. The amount of filament current determines how hot the filament wire gets.(More current more heat more electrons boiled off the filament wire Higher mA).

mA selector

• Allows for adjusting of the filament current

Precision resistors

The voltage stabilizer: Absorbs or cancels out any voltage fluctuation which may occur in the filament circuit.

It receives voltage from the autotransformer and supplies to the mill ampere selector.

The space charge compensator: Compensates any selected low kilo voltage which will be less able to attract electrons out of the space charge The mA meter: Indicates the tube current

Control

• Factor
• Electrical Device and Location in Circuit
• kVp Selection
• kVp Level
• Autotransformer (between incoming line and exposure switch)
• mA Selection

Filament Current

• Variable resistor (in filament circuit between incoming line and step-down transformer)

Time Selection

• Length of exposure
• Timer circuit (between exposure switch and step-up transformer)

Rotor Switch

• Speed of rotating anode
• Stator (separate circuit from stator of anode motor)

Exposure Switch

• Moment of exposure
• Switch (between autotransformer and timer circuit)

Filament Circuit