Rectification – The process of changing alternating to direct current. Necessary in order to prevent reverse bias in the x-ray tube. - Current must be allowed to pass in only one direction in the tube (cathode to anode) to prevent damage to the tube filament. Can be accomplished by: 1) Half Wave Rectification – Suppressing the negative half of the AC (1 pulse) 2) Full Wave Rectification – Inverting the negative part of AC to a positive pulse and using both pulses. (2 pulse) Half Wave Rectification Full Wave Rectification All rectification systems are located between the secondary side of the step-up transformer and x-ray tube. The Two Methods of Rectification: 1) Self-rectification 2) Valve tube or solid state diode rectification 1) Self-rectification – Performed by the x-ray tube. Results in half-wave rectification Voltage and current only flow when current flows from cathode (-) to anode (+). One half of the wave is suppressed (half-wave rectification) 2) Diode Rectification – Using a device (either valve tube or solid state) that only allows current to flow in one direction. - diodes only allow current to flow from cathode to anode. Cathode Anode One rectifier diode results in half-wave rectification. Two rectifier diodes also result in half-wave rectification, but have better efficiency resulting in less chance of reverse bias Full-Wave (4 Diode) Rectification Positive Cycle high potential Lower Potential Negative Cycle Results in 2 pulses Four Diode Tube Rectification Provides full-wave rectification (2 pulses/cycle) Advantages 1) Allows us to use all of the current, which doubles the amount of radiation produced for each cycle of AC 2) Increases the heat loading capacity of tube Disadvantage 1) Increased percentage of production of low energy x-rays Reduced by: - Tube Filtration - Three Phase or High Frequency Generation Solid State Diode Rectification Do you recall the difference between: Conductors? Insulators (dielectrics)? Semiconductors? Conductivity in Matter Depends on: 1) Energy State – Energy level of a particular electron in an atom - Electrons further from the nucleus of an atom have higher energy levels 2) Energy Bands – Groups of atoms with the same energy states grouped as a crystal lattice. The Three Energy Bands 1) Valence Band – Contains loosely bound (outer shell) electrons 2) Forbidden Band – Opposes the movement of electrons (no electrons can remain here) 3) Conduction Band – Place where electrons can move freely Large Band Overlap Large Forbidden Band Small Energy Gap (electrons flow easily from valence to conduction band) (some electrons flow & we have more control over current) (electrons do not flow due large energy gap) Solid State Diode Rectifiers Stacked silicon diodes used to make current unidirectional. - Conductivity is related to coated impurities - The Octet Rule is basic to understanding why impurities are important Octet Rule – Atoms tend to share electrons so the outer shell of each contains 8 electrons. Silicon (4 outer shell electrons) fulfills the rule through covalent bonding with other silicon atoms Solid State Diode Rectifiers Adding arsenic (As) with 5 valence electrons to silicon leaves one extra electron not needed for bonding (valence +1) - It is used to make n-type silicon N-Type Silicon Adding gallium (Ga) with 3 valence electrons to silicon results in a deficit of one electron for bonding (valence -1) - It is used to make p-type silicon (accepts electrons) P-Type Silicon Solid State Diode Rectifiers By combining n-type and p-type silicon, we can selectively control and rectify a circuit. 1) n-type silicon behaves as negative charges 2) p-type silicon behaves as positive charges or holes to be filled 3) During forward bias, electrons enter the n-type silicon from the source & p-type silicon drift toward source at the n-p junction, completing the circuit 4) During reverse bias, electrons from the n-type pass towards the positive side of the circuit & p-type silicon towards the negative side. This breaks the circuit & current can no longer flow. The Stick Rectifier Consists of stacked diode modules • Each can withstand 1,000 V. • 200 kVp generator requires 200 stacked diodes Advantages of Silicon Diode Rectifiers (Stick Rectifier): 1) Compact 2) No filament burnout 3) Low voltage drop 4) Low reverse bias 5) long Life Rectifier Failure • mA meter on x-ray machine will indicate ½ of selected value • Radiographs appear underexposed Checked with spinning top for single phase machine Shows both rectifier and timer failure Checked with synchronized (motorized) spinning top for 3 phase or high frequency Checks rectifier only Critical Thinking Questions 1) Why is rectification necessary in an x-ray tube? 2) What are the disadvantages to half-wave rectification? 3) On a single phase, fully rectified machine, a .25 sec. exposure produces yields 12 dots. Is the machine functioning properly or is there a problem? If there is a problem, what is it? 4) Explain what prevents reverse bias during the negative half of the AC cycle when using silicon diode solid state rectification? 5) Explain how doping semiconductors helps control the flow of current in solid state electronics. Study Questions 1) What is rectification? 2) Why is rectification needed in an x-ray tube? 3) List the two primary mean of rectifying an x-ray tube. 4) Draw a diagram of a 4 diode rectification system without referring to the book (remember to put labels on all parts of the diagram, including the x-ray tube) 5)Where is the rectification system located on an x-ray tube? 6) What is meant by energy bands and how do they relate to conductivity in semiconductors? 7) Define the term “solid state” diode rectification and what are it’s advantages over valve tube rectification? 8) What are the two signs of valve tube failure? 9) What is the expected number of dots with a single phase, full wave rectified unit using a .15 sec. exposure? How would this be different for a half wave rectified unit? How many degrees of arc would be expected if it were a 3 phase unit? 10) What is meant by 3 phase x-ray equipment?