Production and Control of High VoltageRegulation of Current

Transformer – An electromagnetic device thatchanges an alternating current from low voltage to high voltage or vice versa

Works on the principle of electromagneticmutual induction

The Three Types of Transformers

1) Step-Down

2) Step-Up

3) Autotransformer

The Transformer Law – The emf induced in thesecondary coil is to the emf in the primary coilas the number of turns in the secondary coil is to the number of turns in the primary coil.

Stated as and equation:

Current in a transformer – If the voltageincreases in the secondary, amperage (current)will decrease due to the law of conservationof energy

Stated as an equation:

Types of Transformer Construction

Open Core Transformer

Closed Core Transformer

Air Core Transformer

Shell Transformer

Transformer Power Loss

Transformers:95% efficientAll power loss due to heat loss

Three Forms of Power Loss:

1) Copper Loss- Caused by electrical resistance of wire- Decreased with larger diameter wire

2) Eddy Current Loss- Swirling currents in iron core- Produced by electromagnetic induction of windings- Reduced by laminated silicon plates

3) Hysteresis – Power loss caused by changing magnetic domains in AC core - Also reduced by laminated silicon plates

Autotransformer (Variable Transformer)

•Purpose - A device used to control kVp • Varies input voltage to primary of step-up transformer• Works on principle of self-induction and therefore requires AC voltage

Components:1) Single winding acting as both primary and secondary2) Metal taps to adjust number of turns in secondary of autotransformer3) Iron core (increases back emf)

Autotransformer Law

Works identical to the transformer law

Example: What is the voltage delivered to the primary of the step-up transformer ifthere is 120 volts in the primary of the autotransformer and 2 of 10 turns are tapped?

X = 2120 10

= 240 10x

= 24 V

Control of Filament & Tube Current

X-Ray Tube Circuits:1) Filament Circuit – Carries current to heat the filament (typically 3-5 A, 10 – 12 V)

2) Tube Circuit – Current passing between the electrodes of the x-ray tube (stated in mA)

A small change in filament circuit produces a large change in tube current.

Three Devices Used to Control Filament Current

1) Choke Coil • Works on self-induction• Requires AC• Consists of coil of wire and iron core• Can be used to control current and voltage drop

2) Rheostat• Variable resistor• Increases resistance to decrease current & voltage• Operates on AC or DC

Three Devices Used to Control Filament Current

3) High Frequency Control – The use of high frequency generators (transformers) to control output.- Contains microprocessor feedback circuit for better control of kVp and mA

From high ripple to low ripple means lessvariation in voltage and mA

- More precise control of voltage and mA going to x-ray tube

Operation of High Frequency Generatorand Feedback Circuit

1) Supplied with single or three phase power

2) AC is converted to high frequency DC wave by DC chopper

3) Inverter converts DC to high frequency AC in order to be used by transformer

4) High voltage from secondary of transformer is changed back to DC (rectified) and smoothed.

High Freq Inverter

DC Chopper

Advantages of High Frequency Generators

1) Less ripple (less than 2%)

2) Better control of kVp and mA

3) Increased tube output

4) Shorter exposure times

5) Improved image consistency

6) Less space required

Practice Problems

Answers to Transformer Problems

1) 22 kVp , 100 mA

2) 5.5 kVp, 400 mA

3) 50 volts, 20 A

4) 100 windings, 1 A

5) 7,000 V (7 kVp), 30 A