Basic Circuitry
and
X-ray Production
Lynn C. Sadler, MSRS, R.T.(R)(QM)
President, WCEC, Inc.
X-Ray Production
• What are X-Rays?
• Where do they come from?
• What are some characteristics of x-radiation?
• How are x-rays produced?
From the Beginning…….
• Why learn this “stuff”?
• Understanding circuits and electricity makes
x-ray production more understandable.
• Understanding x-ray production and the
characteristics of x-radiation will make you a
better technologist.
– How?
1
Circuit
• The path over which an electric current
flows.
• Consists of a source of energy, a conductor,
and a load or resistor(s) - (something that
utilizes the electricity).
Circuit
• Source of energy?
– Battery
– Generator
– Outlet in wall????
• Conductor?
– Gold
– Silver
– Copper
Types of Circuits
• Series
– Type of circuit where electron flow has only
one path from beginning to end.
2
Types of Circuits
• Parallel
– Circuit has branches…. Electron flow has more
than one path to take.
Factors in an Electric Circuit
• Potential Difference or Electromotive Force
– Volt
• Current
– Ampere
• Resistance
– Ohm
Potential Difference
• Potential Difference is the force that “drives”
electron movement in an electric circuit
– Sometimes shortened to PD
– Also referred to as “emf” or electromotive force
• The unit of potential difference is the “volt”
– Voltage is used to measure potential difference
– The higher the voltage, the greater the potential
difference or emf, the greater “speed” at which
electrons “move” through the circuit.
3
Current
• Current is the number of electrons passing a
given point per second in an electric circuit.
– Is referred to as the “strength” or intensity of the
circuit. (it’s a numbers thing)
• The unit of current is the ampere.
– The higher the amperage the more electrons that
pass a given point in the circuit every second.
– One ampere is equal to 6.25 x 1018 electrons
flowing per second.
– That’s 6,250,000,000,000,000,000 electrons
Resistance
• Resistance is the property of an electric circuit
that hinders or opposes the flow of electrons
• The unit of resistance is the ohm
• The four things that determine the resistance of
a conductor are:
•
•
•
•
Material
Length
Cross-sectional area
Temperature
Factors in a cirucit
• Potential difference, current, and resistance
are inter-related in a circuit.
• Changing one factor will have an effect on
another factor.
• Ohm’s Law shows that relationship between
voltage, amperage, and ohms.
4
Ohm’s Law
• Voltage = Amperage x Resistance (V = I x R)
– Remember this is a Very Important Rule
V
I
R
Ohm’s Law
• If voltage is increased, and resistance is
constant, what would happen to amperage?
• If voltage is constant, and the resistance is
increased, what would happen to amperage?
• If voltage is constant, and the amperage is
increased, what would happen to resistance?
Ohm’s Law
• If the total voltage is 60 volts, and the total
resistance is 20 ohms, what is the value of
the current flowing through the conductor?
• If the total resistance is 10 ohms and the
value of the current is 6 amps, what is the
total voltage?
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But Wait…… There’s More!
• One can also talk about current or a circuit in
terms of power.
• Electrical energy, just as any other form of
energy can perform work.
– Electrical energy is convertible to a definite amount
of work and/or heat.
– The power of a circuit is a measure of the amount of
work or heat produced.
– The unit of power is the watt.
Power of a Circuit
• P = IV or
IV = P
– Power (in watts) is equal to amps x volts
– Remember IVP
• P = I2R
– Since V = I x R, one can replace the V in P = IV
with I x R. That’s how we get this other formula.
– Power (in watts) is equal to amps squared x
resistance
– Amperage produces a lot of heat. This formula is
usually referred to as “power loss”
Power of a Circuit
• Power delivered to the x-ray generator is
essentially constant.
• One can not vary the wattage of the x-ray
circuits, but can manipulate the values of
amperage and voltage and/or resistance.
• Just remember that amps x volts has to
equal the wattage of the circuit.
6
The Basic X-Ray Circuit
• Two Circuits
– Tube Circuit
• Provides all the wiring for the production of x-rays
– Filament Circuit
• Provides a source of electrons so x-rays can be produced
Tube Circuit
• Source:
– Alternating current outlet (AC)
– 220 Volts
– Line voltage compensator adjusts incoming
voltage so that 220 volts is always delivered to
tube circuit.
Tube Circuit Source
Main
switch
AC Source
Image from: Stewart
Bushong’s Radiologic Science
for Technologists
7
Tube Circuit
• Autotransformer
– Also known as the kV selector
– Allows you to vary the voltage sent to the rest
of the tube circuit.
– Works on the principle of electromagnetic selfinduction
Autotransformer
Transformer Law
Ns
Vs
=
Np
Vp
Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit
• Exposure timers
– Determines the amount of time the tube circuit is
energized.
– Five main types:
•
•
•
•
•
Mechanical - obsolete
Synchronous - obsolete
Electronic
mAs timers
Automatic exposure controls (AEC)
8
Tube Circuit - Exposure Timers
Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit
• High Voltage Generator
– Is a step-up transformer
– Increases voltage values to kilovoltage values
– Works on the principle of electromagnetic
mutual induction
Tube Circuit
High Voltage Generator
Primary Side
Secondary Side
Image from: Stewart Bushong’s Radiologic Science for Technologists
9
Tube Circuit
• Rectification System
– Changes alternating current (AC) to pulsating
direct current (DC)
– Why? ? ? ?
Image from: Stewart Bushong’s
Radiologic Science for
Technologists
Tube Circuit
• X-Ray Tube
– Where x-ray production takes place.
Filament Circuit
• Source
– Same AC outlet as for tube circuit
Image from: Stewart Bushong’s Radiologic Science for Technologists
10
Filament Circuit
• “Rheostat”
– Is a variable resistor used to regulate the filament
current
– Known as the mA selector
– Modern tubes don’t use this anymore but utilize a
high-frequency circuit to control filament current.
mA Selector in Filament Circuit
Image from: Stewart Bushong’s Radiologic Science for Technologists
Filament Circuit
• Filament Transformer
– Is a step-down transformer
– Reduces the voltage and raises the amperage in
the filament circuit
Primary
Secondary
Image from: Stewart Bushong’s Radiologic Science for Technologists
11
Filament Circuit
• Filament
– Current of the filament circuit passes through
the filament in the x-ray tube.
– High amperage of filament circuit causes the
filament to become very hot.
– Electrons are “boiled off” of the filament in a
process called thermionic emission.
Filament of X-Ray Tube
Putting It All Together!!
Image from: Stewart Bushong’s Radiologic Science for Technologists
12