Primary Circuit 1 Main Switch Location Between AC

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 Location – Within primary circuit & attached to primary of autotransformer Purpose – Maintains constant voltage in primary circuit 4) Autotransformer Location – Between the AC source and primary of the step-up transformer Purpose – Allows control of k. Vp by varying voltage to primary of step-up transformer Principle of operation – Self-induction

The X-Ray Circuit LOCATION FOR LINE VOLTAGE COMPENSATOR

Primary Circuit 5) Pre-Reading kilovoltmeter Location – Between autotransformer and primary of step-up trans. Purpose – Indirectly measures k. Vp 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) m. As Meter 4) Automatic Exposure Control (AEC) 5) Back-up Timer

Electronic Timer

AEC Exposure Timers

Phototimer Operation

AEC Exposure Timers

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 equipment Setting Manual Back-up Time - Divide m. As/m. A - Time must be at least 1. 5 times expected exposure time or 150% of required m. As value for manual setting - m. As is limited to 600 m. As for exposures over 50 k. Vp Example: An AEC calls for 200 m. A 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 m. As 200 m. A X. 5 S = 100 m. As X 1. 5 = 150 m. As back-up m. As

Primary Circuit 8) Filament Circuit – Supplies heating current to the filament. - Supplies 3 – 5 amps at 6 – 10 volts - This process is controlled by m. A button This circuit also consists of: m. A 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 impedance)

m. A Selector in Filament Circuit m. A selector

8) Filament Circuit (continued) – Also contains: Filament Stabilizer – Corrects for variation in line voltage Space Charge Compensator – Maintains filament current for different k. Vp 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) m. A 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) m. As 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 transformer

The Secondary Circuit

The X-Ray Circuit

The Control Panel Varies by machine, but may include some of the components below.

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 k. Vp generated Advantages: 1) Higher tube rating with short exposures • More m. A 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 k. Vp 1Ǿ 3Ǿ x m. As 2/3 (6 pulse) 4) Higher m. As x m. As 1/2 (12 pulse) x k. Vp - 12%

Conversion Factors When Changing From 1Ǿ to 3Ǿ 1Ǿ x m. As 3Ǿ 2/3 (6 pulse) x m. As 1/2 (12 pulse) x k. Vp - 12% Example: If 30 m. As is required for a single phase exposure, how much m. As will be required for the same density on the image with a 3 phase, 6 pulse generator? 30 X 2/3 = 20 m. As Example: If 100 k. Vp 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 k. Vp

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 k. Vp and m. A can be used with short exposure times • Very little ripple (1% ripple) • Less variation in line voltage • k. Vp can be more easily calibrated and controlled • Real-time monitoring of k. V, m. A & exposure time • Error detection circuitry

Power Rating of X-ray Generators and Circuits Rated in k. W (typically 30 – 80 k. W) 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 = m. A X k. V

Generator Problems A high frequency generator uses 100 m. A at 80 k. V 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 m. A for 100 k. V is 300 m. A. Find the maximum power rating if the maximum exposure factors for a particular x-ray machine are 800 m. A at 70 k. Vp.

Falling Load Generators A generator that automatically starts the exposure at the highest m. A for a selected k. Vp curve and drops it during the exposure based on maximum heat loading capacity of the tube. 1) A microprocessor automatically drops m. A in small steps based on the selected k. Vp curve. 2) Tube operates at near maximum rating to produce optimal m. As at each point on k. Vp curve. Two Types of Technique Selection: 1) One-knob selection - R. T. sets k. Vp (microprocessor sets m. As) 2) Two-knob selection – R. T. sets both k. Vp & m. As (microprocessor controls exposure time by the m. A it selects)

Falling Load Generators Advantages 1) Reduction of exposure time when using high m. A 2) Simplifies technique selection by R. T. 3) Takes advantage of maximum tube loading capacity Disadvantages 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 1 k. Hz AC for transformer use b) Step-up transformer increases voltage c) Rectification system changes AC to DC for tube operation d) Microprocessor control of k. Vp and m. As for improved accuracy • Uses rotating anode • Nominal focal spot size of. 75 m. m. (varies by manufacturer) • Allows selection of k. Vp & m. As (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) Operation: a) k. Vp/m. As 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 tube

Capacitor Discharge Diagram

Grid-Controlled X-ray Tubes • Act as switches to start and stop exposure • Grid is negatively charged focusing cup insulated from filament • 1 – 2 k. Vp is applied to cup to break m. A current in tube Operation • 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.

Grid-controlled X-Ray Tube

Wavetail Cutoff With Capacitor Discharge Units • Process of stopping capacitor discharge at a pre-set point on a discharge curve.