Transcript Resident Physics Lectures

Resident Physics Lectures
• Christensen, Chapter 3A
X-Ray
Generators:
Transformers &
Rectification
George David
Associate Professor of Radiology
Current Flow and Magnetic
Fields
• Whenever an electric current flows
through a conductor, a magnetic field
surrounds the conductor
• Magnetic field can be concentrated by
coiling conductor
Magnetic Field
N
Magnetic Field
S
Current Flow
Transformer Construction
• Transformers have 2 coils of wire
 coils not in electrical contact with each another
• When electric current passed through
one coil
 magnetic field develops around first coil
 second coil near enough to feel magnetic field
Magnetic Field
Current Flow
Transformer Core
• Coils usually wrapped around core
material such as steel
• Improves magnetic coupling
between coils
Core
Transformer Coil
Designations
Incoming
AC Power
• primary
coil to which power is
applied
• secondary
coil which feels magnetic
field of primary coil
Primary
Coil
Secondary
Coil(s)
Transformer Coil
Designations
Secondary can have >1 set of
secondary coils
Primary coil induces voltage in
both secondary coils
Transformer Coils
• When secondary coil feels changing
(increasing or decreasing) magnetic
field of primary coil
 power is induced in secondary coil
 no physical connection
Incoming
AC Power
Primary
Coil
Secondary
Coil(s)
Turns Ratio Definition
• number of windings of secondary
coil divided by number of windings
of primary coil
850 / 1200 for transformer below
NP = 1200
NS = 850
Transformer Theory
• transformers do not work with direct
(unchanging) current (DC)
• Current induced in secondary coil only when
primary coil current / magnetic field is changing
Input DC
Voltage &
Current
No Output
Voltage
Voltage
Time
Time
Transformer Theory
• Transformers alter both voltage & current of
AC waveforms
• Voltage in secondary can be > or < voltage
in primary
Input AC
Voltage &
Current
Output AC
Voltage &
Current
Transformer Law
• Voltage Ratio = Turns Ratio
# Sec. Coils
Sec. Voltage
Turns Ratio = ---------------- = ------------------# Prim. Coils Prim. Voltage
Transformer Law
# Sec. Coils
Sec. Voltage
Turns Ratio = ---------------- = ----------------# Prim. Coils Prim Voltage
NS VS
----- = ----NP VP
240
VAC
?V
NP = 1200
NS = 850
850
?
------ = ----1200 240
If VPRIM = 240 Volts then VSEC = 170 Volts
Transformer Types
• Step down Transformer
# primary coils > # secondary coils
primary voltage > secondary voltage
Step
up Transformer
• # primary coils < # secondary coils
• primary voltage < secondary voltage
Current Change Opposite
of Voltage Change
• If one goes up, the other goes down
 If Secondary voltage > primary voltage means
» Secondary current < primary current
 If Secondary voltage < primary voltage means
» Secondary current > primary current
Input AC
Voltage &
Current
Output AC
Voltage &
Current
Voltage
Current
Transformer Law (cont.)
• Current ratio is inverse of voltage ratio
# Sec. Coils
Sec. Voltage Prim. Current
----------------- = ------------------ = -----------------# Prim. Coils Prim. Voltage Sec. Current
NS
VS
IP
----- = ----- = ----NP
VP
IS
VPRIM X IPRIM = VSEC X ISEC
Power
Power is rate of energy usage
Power defined as Voltage X Current
Units
 Voltage => Volts
 Current => Amps
 Power => Watts
•Voltage => Kilovolts
•Current => milliamps
•Power => Watts
Power in Transformers
• Transformer primary power =
secondary power
 transformer neither creates nor consumes
power
PowerPRIM = PowerSEC
VPRIM X IPRIM = VSEC X ISEC
Power
• Transformers do not change power
• Transformers transform voltage /
current mix
 High voltage --- Low current
 Low voltage --- High current
PowerPRIM = PowerSEC
VPRIM X IPRIM = VSEC X ISEC
Autotransformer
• Only one winding
• incoming AC voltage
connected across
coils
 primary
Taps
Input
• Output voltage
proportional to # coils
between taps
NP
NS
 secondary
Primary
Secondary
Autotransformer
• Voltage law for
autotransformers
same as for
transformers
• Secondary voltage
adjustable by moving
Input
to a different tap
 changes # secondary coils NS
NS VS
----- = ----NP VP
NP
Primary
NS
Autotransformer
Input
100
80
50
20
Primary
Rectification
• Changes alternating current output of
high voltage transformer to direct
current
• allows current flow in one direction only
• x-ray tube is a rectifier because current
will not flow from anode to cathode
no source of free electrons at anode
Rectifier
• Also called diode
• Diode is 2-component device
cathode
anode
• Note symbol for diode below
Cathode
Anode
Types of Rectifiers
• Vacuum tubes (thermionic diode
tubes)
x-ray tube is one example
• solid state (semiconductor) materials
junction of p & n type semiconductors
» contains impurities with excess or shortage of electrons
materials
» silicon
• most common today
• packaged in cylindrical stack called a “stick”
» selenium
Diodes are One Way Streets
for Electric Current
• Allows current to flow cathode to anode
• Blocks current from flowing anode to
cathode
C
A
C
A
Biasing Semiconductor Rectifiers
• forward biasing
P
N
electron flow
attempting to pass current (electrons) from Ntype to P-type semiconductor across barrier
junction allows current to cross barrier
• reverse biasing
attempting to pass current (electrons) from Ptype to N-type semiconductor across barrier
junction does not allow current to cross barrier
X
P
N
electron flow