Transformers and X-Ray Circuits

Transformer

Electrical device that changes voltage using electromagnetic induction.

Primary coil

Coil that receives the incoming AC voltage.

Secondary coil

Coil where the new induced voltage is produced.

Transformer core

Iron core that carries the changing magnetic field between the primary and secondary coils.

Electromagnetic induction

A changing magnetic field produces an electrical voltage.

Mutual induction

A changing current in one coil induces voltage in a separate coil.

Self-induction

A changing current in a coil induces voltage back into that same coil.

Mutual induction.

Which type of induction is used by step-up and step-down transformers?

Self-induction.

Which type of induction is used by the autotransformer?

Transformer sequence

AC enters primary coil → magnetic field changes → core carries the magnetic field → voltage is induced in the secondary coil.

Turns ratio

Ratio of secondary coil turns to primary coil turns.

Ns ÷ Np.

Turns ratio formula

Vs ÷ Vp = Ns ÷ Np.

Transformer voltage formula

Secondary over primary; output over input.

How can the transformer ratio be remembered?

Step-up transformer

Increases voltage and decreases current.

Produces the high voltage needed to accelerate electrons from cathode to anode.

What is the purpose of the step-up transformer in an x-ray circuit?

Step-down transformer

Decreases voltage and increases current.

Provides low voltage and high current to heat the filament.

What is the purpose of the step-down transformer in the filament circuit?

Autotransformer

One-coil transformer used to select the voltage supplied to the primary side of the high-voltage transformer.

Selects the voltage used to establish the desired kVp.

What is the main job of the autotransformer?

mA selector

Controls filament current and therefore the number of electrons available for tube current.

Exposure timer

Controls the duration of the x-ray exposure.

Thermionic emission

Release of electrons from the heated filament.

Prep stage

Filament heats and the rotating anode begins spinning before the exposure.

Filament circuit

Low-voltage, high-current circuit that heats the filament and supplies electrons.

Filament circuit sequence

mA selector → step-down transformer → filament heats → thermionic emission → electrons become available.

High-voltage circuit

Provides the high potential difference that accelerates electrons from cathode to anode.

High-voltage circuit sequence

Autotransformer → exposure timer/switch → step-up transformer → rectifiers → x-ray tube.

Rectifier

Allows current to flow in only one direction through the x-ray tube.

Three major x-ray circuit jobs

Primary circuit selects and controls; filament circuit supplies electrons; secondary/high-voltage circuit accelerates electrons.

Frequency

Number of AC cycles occurring each second.

Hertz (Hz)

Unit used to measure frequency; 1 Hz equals 1 cycle per second.

60 Hz.

US line frequency

60 Hz

60 AC cycles occur every second.

Single-phase generator

Uses one alternating voltage waveform.

Three-phase generator

Uses three AC voltage waveforms timed apart to provide more constant voltage.

High-frequency generator

Produces very rapidly changing voltage with very little voltage ripple.

Voltage ripple

Percentage that tube voltage rises and falls during an exposure.

High voltage ripple

Greater fluctuation in tube voltage and less efficient x-ray production.

Low voltage ripple

More constant tube voltage and more efficient x-ray production.

100%.

Single-phase voltage ripple

About 14%.

Three-phase 6-pulse voltage ripple

About 4%.

Three-phase 12-pulse voltage ripple

About 1%.

High-frequency voltage ripple

100 → 14 → 4 → 1.

Voltage ripple memory sequence

Voltage ripple decreases.

What happens to voltage ripple as the number of pulses increases?

Average photon energy and x-ray production efficiency increase.

What happens when voltage ripple decreases?

1.0.

Single-phase heat-unit generator factor

About 1.35.

Three-phase 6-pulse heat-unit generator factor

About 1.4.

Three-phase 12-pulse heat-unit generator factor

About 1.4.

High-frequency heat-unit generator factor

HU = kVp × mA × time × generator factor.

Heat unit formula

Voltage ripple vs generator factor

Voltage ripple describes waveform fluctuation; generator factor is used when calculating heat units.