Generator KD.pptxROSTER OF 05.08.2025 MORNING Dr Priyadarshini (10 am to 3:30pm)

X-RAY GENERATORS
Presented by: Moderator:
Dr. KANAJ DHAR DR. JAYBRATA RAY
PGT 1, Radiodiagnosis Asso. Professor, radiodiagnosis

Introduction :
• An x-ray generator is the device that supplies electric power to the x-ray tube.
• An x-ray generator begins with a source of electrical energy. Most radiology
departments will have three-phase power available in the range of 208 to 230 V ,
the x-ray generator modifies this energy to meet the needs of the x-ray tube.
• The x-ray tube requires electrical energy for two purposes-
a) To boil electrons from the filament,
b) To accelerate these electrons from the cathode to the anode.

• The x-ray generator has a circuit for each of these functions:
a) Filament circuit
b) High voltage circuit
c) Timer mechanism- for regulating the length of x-ray exposure

• X-ray generator has 2 components:
1.CONTROL PANEL
2.TRANSFORMER ASSEMBLY

Control panel/CONSOLE :
• Allows operator to select kVp, mA and exposure time for a particular radiographic
examination.
• Two exposure buttons:
First button- readies the x ray tube for exposure by heating the filament and rotating
the anode.
Second button- starts the exposure.
The timing mechanism terminates the exposure.

Transformer Assembly :
• IT is grounded metal box filled with oil.
• Contains - low voltage transformer (filament circuit)
- high voltage transformer and a group of rectifiers (high voltage circuit )
• The potential differences in these circuits can be as high as
150,000 V , so immersed in oil, which acts as an insulator.

TRANSFORMER :
• A device that either increases or decreases the voltage in a circuit.
• Incoming Power supply- 230 V, 60 Hz AC.
a. Filament heating requires- 10 V
b. Electron acceleration requires- between 40,000 and 150,000 V.
So transformers are used to change the voltage of incoming power supply to appropriate levels.

Design of a transformer –
Two wire coils (copper) wrapped around a closed core (special iron alloys).

Working of a Transformer :
• Principle- When current flows through the primary coil, it creates a magnetic field
within the core, and this magnetic field induces a current in the secondary coil.
• Current only flows through the secondary circuit when the magnetic field is changing
(increasing or decreasing). No secondary current flows while the magnetic field in the
core is in a steady state.
• So steady direct current in the primary coil can not be used to produce a continuous
current through the secondary coil.
• Hence alternating current is used for a trans former as its voltage changes continuously,
so it produces a continuously changing magnetic field

LAWS OF TRANSFORMERS :
1. The voltage In the two circuits is proportional to the number of turns in the two coils.
Np/Ns = Vp/Vs
2. The product of the voltage and current in the two circuits must be equal.
Vp.Ip = Vs.Is

TRANSFORMER TYPES :
• Step up Transformer
No. primary coils < No. secondary coils
Turns ratio < 1
Primary voltage < Secondary voltage
• Step down Transformer
No primary coils > No secondary coils
Turns ratio > 1
Primary voltage > Secondary voltage

Autotransformer :
• The voltage supplied to the x-ray room connects to the x-ray generator through an
autotransformer.
• Functions:
1. Supplies voltage for x-ray tube filament circuit
2. Provides voltage for primary coil of the high voltage
transformer.
3.Provides a convenient location for kVp meter.
• Consists of SINGLE WINDING wound on a laminated
closed core.
• Works on the principle of self induction.

X-RAY CIRCUITS :
1. FILAMENT CIRCUIT - regulates the current flow through filament of x-ray tube.
-It contains a step-down transformer – FILAMENT TRANSFORMER - provides power to
heat the filament.
- Current flow of 3-5 A and voltage of 10 V- sufficient to heat the filament to necessary
high temperature to cause emission of electrons (thermionic emission).

2. HIGH VOLTAGE (cathode- anode) CIRCUIT –
Contains two transformers:
• Auto-transformer ( kVp selector)
- located in control panel
-adjusts the input voltage to a desired
level before feeding into the high volt
transformer.
• Step-up( high voltage ) transformer
-increases voltage by a factor of approx. 600
-potential difference across secondary coil as high as150,000 V – so immersed in oil.
Two meters are incorporated into the high-voltage circuit, one to measure kVp (voltmeter)
and the other to measure mA (ammeter).

RECTIFICATION :
• Process of changing AC into DC.
WHY NEEDED?
• The high-voltage trans former provides an alternating voltage for the x-ray tube.
• When the cathode is negative with respect to the anode, electrons flow from the
cathode to the anode and x-rays are produced.
• During the next half of the electrical cycle the target (anode) of the x-ray tube is
negative and the filament positive, so electrons, if available, would flow away from
the target toward the filament. Such electrons would not produce useful x rays ,
rather would further heat the filament and reduce its lifetime.

Rectifiers :
• Device that allows an electrical current to flow in one direction but does not allow
current to flow in the other direction.
• Rectifiers can be : vacuum-tube type or solid-state type.
• Solid-state rectifiers are smaller, more reliable, and have a longer life. Today most
xray generators use silicon rectifiers.

SEMICONDUCTORS :
• The heart of a solid state rectifier is a semiconductor, which is usually a piece
of crystalline silicon.
• There are basically 2 types of semiconductors:
1. N-type Semiconductors – Formed when a material with 5
valence electrons (e.g arsenic) is added as an impurity
to the silicon lattice having 4 valance electrons per atom.
The unbound electron can move about freely in the
crystal lattice.

2. P-type Semiconductors-
-Impurity with only three valence electrons is added to
silicon.
-The absence of 4th electron of the impurity creates a
hole(positive particle).
-A hole can migrate through the lattice structure by an
electron from a neighboring bond filling the hole, thus leaving a hole at a new site.

P-N JUNCTION :
• When N-type and P-type crystals are joined,a P-N junction is created.
• When electrons leave the N-type material ,the junction area is left with
a net positive charge.
• Similarly ,the p-type material acquires a negative charge.
• This creates what is called a “depletion layer”, which has a junction
potential

SOLID STATE RECTIFIER (diode) :
• The device formed by a P-N junction is called a diode.
• Solid-state rectifiers are diodes.
• Symbol for a solid-state rectifier-

RECTIFICATION
1.HALF-WAVE RECTIFICATION-
The voltage reverses during the inverse half of the
alternating cycle, the rectifier stops current flow.

2.FULL-WAVE RECTIFICATION-
Both halves of the alternating voltage are used to
produce x rays.

SELF RECTIFICATION –
• When x-ray tube itself acts as rectifier, a form of half-wave rectification.
• Two disadvantages of self rectification-
1. Half of available electrical cycle not utilized to produce x-rays- so exposure time is
doubled.
2. Repeated / prolonged exposures heat the anode, become hot enough to emit electrons
and bombard the filament and destroy it .

Types of generators :
1. SINGLE PHASE GENERATORS:
• Input power with a single phase AC line.
• The low voltage AC waveform is first transformed into
high-voltage AC - rectified - then applied to the x-ray
tube as a DC.

• Disadvantage of single phase generators-
-Single phase input power results in pulsed radiation.
-Considerable portion of exposure time is lost while the voltage is in the valley between two
pulses.
-Intensity of x-rays produced is significant only when voltage is near peak.
-Low voltage heats target and produces low-energy x-rays - absorbed in patient - raise
patient dose.

2. THREE PHASE GENERATORS:
• Has three waves of power flowing at evenly spaced intervals from each other, each phase
120 apart.
⁰
• The overall waveform never reaches zero.
• Three basic types:
Six pulse, six-rectifier
Six pulse, twelve-rectifier
Twelve-pulse

THREE PHASE TRANSFORMERS :
• Three sets of primary and secondary copper windings.
• Windings arranged in two configurations- DELTA or WYE.
• Output voltage has same max. value but 30º shift in phase
between the two.

Six-pulse,Six rectifier :
• one primary delta
• one secondary wye
• six rectifiers
• When rectified, there will be 6 positive maximum voltages
per cycle.

Six-pulse,Twelve rectifier :
• 1 delta primary
• 2 wye secondaries
• 12 rectifiers
• Has a fixed potential to ground- an advantage over six
rectifier circuit

Twelve-pulse :
• Similar to 6 pulse 12 rectifier transformer but, secondary
has a delta and wye connection.
• Output of delta lags behind wye by 30 ⁰- output of
one winding will fill in the ripple of other- resulting in
12 pulse output.

Advantages of three phase generators over single phase generators-
• Produce a nearly constant potential.
• Allows generators to produce x rays efficiently throughout the exposure.
• Much higher tube rating for extremely short x-ray exposures.
• Produce radiographs with extremely short exposure times and high repetition rates,
so they are excellent for angiography.

RIPPLE FACTOR :
• Variation in the voltage across the x ray tube expressed as a percentage of maximum
value.
• Values-
single-phase circuit- 100%
six pulse- 13.5%
twelve-pulse circuit-3.5%

POWER STORAGE GENERATORS :
• Used in mobile x-ray machines.
• Provide a means of supplying power for the x-ray tube
independent of an external power supply.
• Two types-
1. Capacitor discharge generators
2. Battery-powered generators

1. Capacitor discharge generators-
• 230V power fed to step up transformer- output rectified- used to charge large capacitor/
bank of capacitors.
• Once the capacitor is charged, it can be discharged through the x-ray tube.
• Advantage-
Small and easy to move
Each exposure starts at the same kVp
• Disadvantage-
Limitation in mAs output (30 to 50 mAs)
The kV will fall during the exposure

2. Battery powered generators-
• Uses large capacity Nickel-cadmium batteries.
• The output from the batteries is fed into a DC chopper, which interrupts the current
about 500 times each second which is supplied to the primary windins of the high
voltage transformer.
• Advantage-
Store considerable energy to generate x rays
Make exposures independent of a power supply
Supplies a constant output of kV and rnA throughout the exposure.
• Disadvantage-
Heavy and requires regular battery maintenance

MEDIUM FREQUENCY GENERATORS :
• Principle- In a transformer, the voltage induced in the secondary coil is proportional to
the rate of change of current in the primary coil.
• Generates high-frequency current to produce an almost constant potential voltage to
the x-ray tube.

Advantage-
• Supply a constant, nearly ripple-free voltage to x- ray tube.
• More efficient and small size.

TRANSFORMER RATING :
• It is the maximum safe output of its secondary winding. If the rating is exceeded, the
transformer may overheat and burn out its insulation and windings.
• Expressed in kilowatts (kW).
• For three-phase generators - kW=( kV x mA)/1000
• For single phase generators - kW= (kV x mA x 0.7)/1000

EXPOSURE SWITCHING:
• Switch is the device that turns the high voltage applied to the x-ray tube on and off.
• 2 types –
1. Primary switching :
Switching occurs in the primary circuit .
Used in most general purpose three-phase units.
High currents and low voltage.
Three types –
Electromechanical contractors
Thyratrons
Solid-state silicon controlled rectifiers

Silicon-Controlled Rectifiers/Thyristers :
• The primary switching found in most modern generators
• Consists of –
cathode (negative end),
an anode(positive end),
a gate,
three junctions.
• If the cathode is made negative and the anode positive
Forward bias N-P junction
Reverse bias P-N junction
• If a small positive voltage (1 V) is applied to the gate, the reverse bias at the PN
junction will be overcome and electrons will flow through the thyrister.

• Primary switching can produce short exposure times, but it cannot produce these
exposures at a high repetitive rate .
2. Secondary switching :
• It takes place on the high voltage side of transformer or at the X ray tube.
• They are of two types.
1.Triode vacuum tubes (angiography)
2.Grid controlled X ray tubes (cinefluorography)
• Used in units designed for rapid, repetitive exposures or where extremely short
exposure times are needed.

GRID-CONTROLLED X RAY TUBES :
• The focusing cup is used as a third electrode is used to control the flow of
electrons from fillament to target.
• The focusing cup can be electrically negative relative to the fillament.
• The voltage across the two produces an electric field along the path of the
electron beam.
• If large enough,the tube current may be completely pinched off - no electrons
go to the target.
• The voltage applied between the focusing cup and filament, acts like a switch
to turn the tube on and off.

EXPOSURE TIMERS
• Used to control the length of an x-ray exposure.
• 4 basic types-
1. Mechanical timers (rarely used today)
2. Electronic timers
3. Automatic exposure control (phototimers)
4. Pulse-counting timer

Automatic Exposure Control (AEC)
/Phototimer :
• The essential element in phototimers is a device that can detect radiation and in
response produce a small electric current.
• There are three such devices:
1. Photomultiplier detectors
2. Ionization chambers
3. Solid-state detectors

Photomultiplier Phototimers :
• Most common type of automatic exposure control.
• The detector is made of lucite, coated with areas of a phosphor that will emit light
when irradiated with x rays.
• The lucite paddle transmits this light to an output region called a
"lightgate“, which directs the light to a photomultiplier tube,
where the light is converted to an electric current that is
amplified to produce an electrical signal.
• The signal is used to charge a capacitor. When the capacitor
reaches a predetermined charge it and cause the exposure
to terminate.

Electronic Timers:
• The length of the x-ray exposure is determined by the time required to charge a
capacitor through a selected resistance.
• The exposure is terminated when the capacitor is charged to a predetermind value.

IONISATION CHAMBERS :
• Consists of two thin parallel sheets of aluminum
or lead foil.
• Prior to exposure a charge is placed on the parallel
plates by applying a voltage across them.
• When the gas is between the plates is ionized by
radiation, the electron moves toward the positive plate and the positive ion (ionized
gas atom) moves toward the negative plate and neutralize part of the charge that
has been placed on the plates.
• This will reduse the voltage in the plates to a previously determind value that will
terminate the exposure.

Solid-State Autotimers :
• Operate on the basis of radiation-producing ionization in or near a PN structure.
• Advantage of small size,almost no x-ray beam absorption, and a consistent, rapid
response.

Generator KD.pptxROSTER OF 05.08.2025 MORNING Dr Priyadarshini (10 am to 3:30pm)

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