Module 8 - High Voltage Production & Control

X-ray production requires a

step-up high turns
ratio transformer

A HV transformer can increase 50-240 volts up
to

30-150 KV

Why is the voltage transformer immersed in high dielectric oil

To prevent arcing

The center tap for a HV Transformer creates

less potential reference to
ground for the HV cable

The diodes are located

in a tank of dielectric
oil along with the HV transformer

The diodes are also called

Sticks

What can be used to test the diodes

A megger

Diode sticks should always be replaced in

Pairs

Phase and Pulse

3 Phase

6 Pulse

Phase and Pulse

3 Phase

12 Pulse

High frequency inverter

Line Match

Selectable taps allow the installer to program
the input circuitry to produce the required
operating voltages based on the measured
value of the incoming line voltage

KV Selection Device

The component that delivers voltage to the primary of the
HV transformer

Pre reading KV meter

measures the
selected output of the KV control device and
displays the KV value that should be
produced when that voltage is delivered to
the high voltage transformer

Load Compensation

Compensates for the I2R load loss that occur
in the HV transformer

Usually done by increasing the primary
voltage to the HV transformer when mA is
increased

Primary Contacting

Method of controlling exposure time
Primary contactors are usually SCRs

Single Phase, Self Commutation

SCR is the primary contactor

Three phase self commutation

Self Commutation

removes the gate pulse and waits for the incoming voltage
to hit the zero crossover point. The SCR current will drop below the holding current and shut off
This method requires two SCRs for each phase of
incoming voltage

3 phased 12 pulsed

forced commutation

Forced Commutation

This method removes the SCR gate pulse and
applies a reverse voltage across the SCR.
The reverse bias is usually supplied by a bank
of capacitors that were charged prior to
exposure
Requires only two SCRs

Pre charge circuit

Used to charge the commutation capacitors prior
to exposure

Soft Start SCR (pre-contacting
SCR)

An SCR in series with a resistor is placed in
parallel with the contacting SCR and fired a
few milliseconds before the make SCR

Reduces KV overshoot

Maximum Filament Current
Limiter

limits the absolute value of filament
current, 5.0 to 6.5 amps

Standby Adjust

Sets the continuous filament current, while the
machine is in standby mode, 2.0-3.0 amps

Baseline Adjust

Sets the starting filament current values that will
produce the desired mA

Spacecharge Compensation

Makes electrons ready to shoot x rays

ma Sensing

monitors the current flowing form the
cathode to the anode

mA Stabilizer

Compares the actual mA to selected mA and
increases or decreases the filament mA to correct
for any differences

Tube Protector

Tube Protector 2 functions

• Prevents exceeding maximum

• Limit maximum generator output

Types of radiation sensors

• Photo multiplier

• Ionization Chamber

Control Logic

Control Logic
3 major areas

• General safety

• pre exposure

• exposure

General Safety

This area of control logic monitors the
condition of the x-ray system at turn on

Pre Exposure 1

boosting of filaments, starting anode
rotation, driving KV motor and charging commutation
capacitors

Pre exposure 2

feedback from the circuits in Pre-exposure
1 are added and sent to exposure area

Exposure

Bucky mechanism is started
Primary drive pulses are generated at
appropriate zero crossover of incoming line
voltage (core memory)