The X-Ray Circuit
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The X-Ray Circuit. Primary Circuit. 1) Main Switch Location – Between AC source and primary of step-up transformer Purpose – Completes external circuit to x-ray machine. 2) Fuses – Protects machine from overloaded circuit. 3) Line Voltage Compensator

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Primary Circuit

1) Main Switch

Location – Between AC source and primary of

step-up transformer

Purpose – Completes external circuit to x-ray machine

2) Fuses – Protects machine from overloaded


3) Line Voltage Compensator

Location – Within primary circuit & attached to primary of autotransformer

Purpose – Maintains constant voltage in primary


4) Autotransformer

Location – Between the AC source and primary

of the step-up transformer

Purpose – Allows control of kVp by varying voltage to primary of

step-up transformer

Principle of operation – Self-induction

The X-Ray Circuit


Primary Circuit

5) Pre-Reading kilovoltmeter

Location – Between autotransformer and primary of step-up trans.

Purpose – Indirectly measures kVp selected/adjustment of line v.

Principle of operation – Connected to circuit in parallel & works

on motor principle

6) Exposure Switch

Location – Exposure switch is between autotransformer & primary

of step-up transformer

Purpose – Manually closes circuit between autotransformer &

step-up transformer

Connected in “series”

Special feature – “Deadman” switch

Primary Circuit

7) Exposure Timer

Location – Between autotransformer & primary of step-up trans.

Purpose – Terminates exposure at proper time by opening circuit

between autotransformer & step-up transformer

Types of Exposure Timers:

1) Mechanical Timer

2) Electronic Timer

3) mAs Meter

4) Automatic Exposure Control (AEC)

5) Back-up Timer

Back-up Timer

Purpose – Stops exposure in case AEC fails

- Prevents overexposure to patient and tube overloads

- May be set automatically by machine or manually on some


Setting Manual Back-up Time

- Divide mAs/mA

- Time must be at least 1.5 times expected exposure time or

150% of required mAs value for manual setting

- mAs is limited to 600 mAs for exposures over 50 kVp

Example: An AEC calls for 200 mA at .5 S exposure, what back-up

timer setting should be used?

.5 X 1.5 = .75 S back-up time

If setting back-up timer using mAs

200 mA X .5 S = 100 mAs X 1.5 = 150 mAs back-up mAs

Primary Circuit

8) Filament Circuit – Supplies heating current to the filament.

- Supplies 3 – 5 amps at 6 – 10 volts

- This process is controlled by mA button

This circuit also consists of:

mA Selector

Location – Connected in series between the autotransformer

and step-down transformer

Purpose – Regulates amperage to filament circuit that ultimately

controls tube current.

- May use rheostat (variable resistance), choke coil

(self-inductance) or high frequency circuit or saturable

reactor (application of DC to iron core to primary, creating


8) Filament Circuit (continued) – Also contains:

Filament Stabilizer – Corrects for variation in line voltage

Space Charge Compensator – Maintains filament current

for different kVp selections.

Filament Ammeter – Measures filament current.

9) Primary Windings – Step-up transformer

Secondary Circuit

1) Secondary Coil of Transformer

Principle of operation – Mutual induction

Step-up transformer – Steps up voltage to tube, drives

electrons from cathode to anode

Step-down transformer – Steps voltage down and

steps up amperage to filament of tube

2) mA Meter – Measures average tube current

Principle of operation – Motor principle

Location – Connected in series to the secondary of

step-up transformer (includes connection to ground

to protect operator from being electrocuted)

mAs meter is used for very short exposures

3) Rectifier

Purpose – converts AC to DC to prevent reverse bias

Location – Between secondary of step-up transformer and

x-ray tube

4) Cables to x-ray tube – Conducts high voltage between

rectifier and x-ray tube

The X-Ray Cables

Shock Hazard Minimized in Three Ways:

1) Insulation

2) Wire sheath that is grounded

3) Secondary of high voltage transformer is grounded at its

midpoint to minimize amount of insulation needed

The X-Ray Cables

Consist of 3 Conductors:

Cathode end of cable – All 3 conductors attach to filament

(attach to the 2 filament wires)

Other end of wire connects to secondary of transformer and

filament circuit

Anode end of cable –

One wire attaches to anode

At the other end of the cable, all 3 conductors in the cable attach

to a single conductor that attaches to the secondary of the


The Control Panel

Varies by machine, but may include some of the components


Three Phase Generator Circuits

Consist of 3 single phase currents running 120° out of phase

with each other.

3 Ǿ may be rectified to provide with 6 pulses using 6

rectifiers, 6 pulses with 12 rectifiers or 12 pulses with 12 rectifiers

(3 Ǿ, 6 p = 13% ripple, 3 Ǿ 12 p = 3% ripple

Three Phase Generator Circuits

  • To work properly must have 3 primary & secondary windings in

  • transformer (one for each current)

  • Must have 3 autotransformers (one for each current)

  • Primary windings must be in delta configuration

  • Secondary may be arranged in delta or star (wye) configuration

Advantages and Disadvantages of 3Ǿ Vs 1Ǿ Power Generation

  • Disadvantages:

  • 1) Possible power surges –

  • Current never reaches 0 potential

  • Circuit cannot be opened or closed at zero potential

  • 2) Less image contrast

  • Due to higher effective kVp generated

  • Advantages:

  • 1) Higher tube rating with short exposures

  • More mA can be applied during short exposure time

  • 2) Nearly constant potential (less ripple)

  • - 13% for 3 Ǿ, 6 pulse

  • - 4% for 3 Ǿ, 12 pulse

  • 3) Higher effective kVp 1Ǿ 3Ǿ

  • x mAs 2/3 (6 pulse)

  • 4) Higher mAs x mAs 1/2 (12 pulse)

  • x kVp - 12%

Conversion Factors When Changing From 1Ǿ to 3Ǿ

1Ǿ 3Ǿ

x mAs 2/3 (6 pulse)

x mAs 1/2 (12 pulse)

x kVp - 12%

Example: If 30 mAs is required for a single phase exposure,

how much mAs will be required for the same density on the

image with a 3 phase, 6 pulse generator?

30 X 2/3 = 20 mAs

Example: If 100 kVp were used on an x-ray machine with single

phase generation, how much should be used on a three phase

machine for the same density? 100 X .12 = 12

100 – 12 = 88 kVp

High Frequency Generation

Changes 60 Hz to high frequency current for even less ripple!

Operational Steps of the High Frequency Generator

1) 1Ǿ or 3Ǿ AC current is supplied to machine

2) A DC chopper converts the AC wave to a high frequency

DC wave that is less subject to line voltage fluctuations

3) An inverter converts the DC waveform to a high frequency

AC wave that can be used by the transformer

4) Voltage from the secondary side of the transformer is then

changed to DC for application to the tube, rectified and smoothed

Operation of a High Frequency Generator

High Freq Inverter

DC Chopper

Advantages of High Frequency Generators

  • Smaller size

  • Shorter exposure times available

  • High kVp and mA can be used with short exposure times

  • Very little ripple (1% ripple)

  • Less variation in line voltage

  • kVp can be more easily calibrated and controlled

  • Real-time monitoring of kV, mA & exposure time

  • Error detection circuitry

Power Rating of X-ray Generators and Circuits

Rated in kW (typically 30 – 80 kW) for x-ray machines

1 Watt = Energy expenditure of 1 joule

For DC P = IV

P (Watts) = Power

I = Current intensity

V = Voltage

Since high frequency generators produce a nearly constant

electrical waveform the same formula for DC can be applied:

P = mA X kV

Generator Problems

A high frequency generator uses 100 mA at 80 kV for an exposure.

How much energy was consumed to produce this exposure?

What is the maximum power rating for an x-ray machine

when the maximum mA for 100 kV is 300 mA.

Find the maximum power rating if the maximum exposure

factors for a particular x-ray machine are 800 mA at 70 kVp.

Falling Load Generators

A generator that automatically starts the exposure at the

highest mA for a selected kVp curve and drops it during the

exposure based on maximum heat loading capacity of the tube.

1) A microprocessor automatically drops mA in small steps

based on the selected kVp curve.

2) Tube operates at near maximum rating to produce optimal

mAs at each point on kVp curve.

Two Types of Technique Selection:

1) One-knob selection - R.T. sets kVp (microprocessor sets mAs)

2) Two-knob selection – R.T. sets both kVp & mAs

(microprocessor controls exposure time by the mA it selects)

Falling Load Generators


1) Reduction of exposure time when using high mA

2) Simplifies technique selection by R.T.

3) Takes advantage of maximum tube loading capacity


1) Takes control away from R.T. in choosing technical factors

Mobile X-Ray Units

  • 1) Battery Powered Mobile Units

  • Uses nine, 12 Volt DC batteries connected in series – Powers

  • mobile unit and x-ray tube (recharged by 110 V AC)

  • Circuit Operation

  • a. Inverter changes DC from battery to 1kHz AC

  • for transformer use

  • b) Step-up transformer increases voltage

  • c) Rectification system changes AC to DC for tube

  • operation

  • d) Microprocessor control of kVp and mAs for improved

  • accuracy

  • Uses rotating anode

  • Nominal focal spot size of .75 m.m. (varies by manufacturer)

  • Allows selection of kVp & mAs (no exp. time selection)

Mobile X-Ray Units

  • 2) Capacitor Discharge Unit

  • Older type of mobile unit

  • Operates by charging a capacitor immediately prior to exposure to operate x-ray tube (does not drive unit)


a) kVp/mAs values are chosen

b) A charger button is pressed immediately prior to exposure

to charge the capacitor

c) Exposure switch is depressed to start exposure

d) Exposure is terminated via a grid-controlled (triode) x-ray


Grid-Controlled X-ray Tubes

  • Act as switches to start and stop exposure

  • Grid is negatively charged focusing cup insulated from filament

  • 1 – 2 kVp is applied to cup to break mA current in tube


  • Exposure is started by removing the negative charge from the

  • grid

  • Exposure is terminated by restoring the negative charge

Advantage – Allows precise control of short exposures.

Wavetail Cutoff With Capacitor Discharge Units

  • Process of stopping capacitor discharge at a pre-set point on a discharge curve.