Transformers and X-Ray Circuits

Transformer

Electrical device that changes voltage using electromagnetic induction

Primary coil

First coil where AC enters the transformer

Core

Iron center that carries the changing magnetic field between coils

Secondary coil

Second coil where induced voltage appears

Electromagnetic induction

Changing magnetic field creates voltage in a coil

Transformer sequence

AC enters primary coil → changing magnetic field → core carries field → voltage appears in secondary coil

Turns ratio

Number of secondary turns divided by number of primary turns

Turns ratio formula

Ns ÷ Np

Secondary voltage ÷ Primary voltage = Secondary turns ÷ Primary turns

Transformer Voltage Formula

Turns ratio

Secondary over primary; output over input

More secondary turns

Increases secondary voltage

Fewer secondary turns than primary turns: Decreases secondary voltage

step-down transformer

Step-up transformer

Increases voltage and decreases current

Step-up transformer

Increases voltage to create the high kVp that accelerates electrons from cathode to anode

Step-down transformer

Decreases voltage and increases current

Step-down transformer

Supplies low voltage and high current to heat the filament

Step-up turns relationship

Fewer primary turns, more secondary turns

Step-down turns relationship

More primary turns, fewer secondary turns

Autotransformer

One-coil transformer used to select or adjust the voltage before the step-up transformer

Autotransformer job

Selects the starting voltage for kVp

Autotransformer

Auto chooses; step-up boosts

Self-induction

Changing current in one coil creates a magnetic field that induces voltage back into the same coil

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Mutual induction

Changing current in one coil induces voltage in a separate coil

mA selector

Controls filament current, usually by changing resistance in the filament circuit

Exposure timer

Controls how long the high-voltage circuit is active

Incoming power

AC electricity enters the x-ray system from the building

Autotransformer pathway

Incoming AC → autotransformer selects voltage

High-voltage circuit

Autotransformer → timer → step-up transformer → rectifiers → x-ray tube

Filament circuit

mA selector → step-down transformer → filament heats → thermionic emission

Thermionic emission

Release of electrons from the heated filament

Prep stage

Filament heats and anode begins rotating

Step-up, rectifiers

High-voltage circuit sequence: Incoming AC → autotransformer → exposure switch/timer → ______ transformer → ______ → high voltage across x-ray tube → electrons accelerate cathode to anode → x-rays produced

Filament circuit job

Supplies electrons

High-voltage circuit

Provids the high potential difference (kVp) that accelerates electrons from cathode to anode

preps, electrons, moves

Circuit memory phrase: Primary ___, filament makes ____, secondary _____ electrons

mA selector, thermionic emission

Filament circuit sequence: Incoming AC → autotransformer →______ → step down transformer → filament heats → ________ → electrons become available at cathode

Frequency

Number of AC cycles per second

US line frequency

60 Hz

60 Hz

60 AC cycles per second means

Pulse

One rise and fall of voltage sent to the tube

Phase

Number of separate AC voltage waves used by the generator

Single-phase

One AC voltage wave

Three-phase

Three AC voltage waves timed apart to keep voltage more constant

Voltage ripple

Amount the voltage rises and falls during an exposure

High voltage ripple

More fluctuation and less efficient x-ray production

Low voltage ripple

Smoother voltage and more efficient x-ray production

Single-phase voltage ripple

100%

3-phase 6-pulse voltage ripple

14%

3-phase 12-pulse voltage ripple

4%

High-frequency voltage ripple

About 1%

Voltage ripple sequence

100 → 14 → 4 → 1

More pulses

Less voltage ripple

Less voltage ripple

Higher average photon energy and more efficient x-ray production

3-phase 6-pulse factor

About 1.35

3-phase 12-pulse factor

About 1.4

High-frequency factor

About 1.4-1.45

Voltage ripple vs generator factor

Ripple uses 100%, 14%, 4%, 1%; heat-unit generator factors use about 1, 1.35, 1.4, 1.45