PRIMARY CIRCUIT OF THE
HIGH TENSION
TRANSFORMER
LEARNING
OBJECTIVES
Describe the Autotransformer
 Describe the H.T. Control devices
 Explain methods of Kilovoltage indication

INTRODUCTION
Primary circuit
 Is that circuit between the mains supply and the
primary winding of the high tension transformer,
no part of it is at high potential.
 The principal purpose of the primary circuit of
most X-ray units is to provide close controlled
variation and measurement of the voltage which
is to be supplied to the primary winding of the
high tension transformer.
THE AUTOTRANSFORMER
This is a type of transformer which has one
winding only and not two as in the case of H.T.T.
 A transformer with two windings works on the
principle of electromagnetic induction
between primary winding and the secondary
winding.


The autotransformer with its single winding
works on the principle of self induction.

Because there is only one winding the primary
and secondary circuits are metallic connection
with each other making it unsuitable for:
⚫
⚫

Transforming high voltages from one value to
another
Stepping up voltages to high values
Therefore it can be used to step voltages both up
and down from the mains supply value as long as
they are not used in high voltage circuits.
Advantages of A.T. over two winding
transformer
 They are smaller in size (relative simplicity)
 Are economical of copper wire (Fairly small)
 Costs less
 They can give/provide adjustable 2o voltage (i.e.
variable 2o output voltage)
Operated by manual controls sited on x-ray unit’s
control panel
 Able to maintain the volts per turn ratio at a
constant value when the input voltage varies
over quite a wide range of values
 Able to control the output of the HT T by varying
the voltage supply


Able to supply a number of circuits with a
suitable voltage because of it multiple tappings
Functions of the Autotransformer
 Compensate for changes in the mains voltage
 Provide voltage for the tube filament circuit
Provides the primary voltage for the high tension
transformer which can be altered to allow a
change in the x-ray beam quality
 Provides a suitable voltage for subsidiary circuits
 Provision of a kVp meter compensator to make a
pre-reading kVp meter read the on-load value

DESIGN
AND CONSTRUCTION
The Autotransformer consists essentially of a coil
of insulated wire wound around an enclosed soft
iron core.
 The wire is copper and has comparatively large
rectangular cross-section as it carries relatively
large currents.

At regular intervals – along the coil, tapings are
drawn off to metal buttons which pass out of the
autotransformer to terminals from which
connections can be made to the associated
electrical circuit.
 It (tapings) can be used as step-up or step-down
transformer.

O P E RAT IO N
If a mains supply is connected at A and B, the
magnetic flux in the core, due to the current
which will flow in this part of the winding will
link with all the turns on the complete winding
between Z and X.
 Each individual turn, whatever its position on
the core, will therefore have the same value to
E M F induced into it by self induction

The ratio of voltages between A –B and X-Y will
be proportional to the ratio of the number of
turns between these two points.
 For example if a supply of 400 volts is applied
across 200 turn between A and B then, ignoring
any transformer losses, 400 volts may be
obtained across points C and F,

between which there is the same number of turns
(i.e. 200 hence 2v per turn).
 Ratio of V:T = 400: 200
 Transformation ratio: Number of turns in primary winding
=
Primary volts
Number of turns in secondary winding
Secondary volts


If there are 300 turns between C and G then: 400
v = 200p
(Vs= Volts in 2o
Vs
300s

Hence
400 x 300 = 200 x Vs
Vs = 400 x 300
200
= 600V
AN AUTOTRANSFORMER
KILOVOLTAGE
AS A
CONTROL
OF
An autotransformer is the most commonly used
method of controlling K V in the diagnostic X-ray
equipment.
 The windings of an autotransformer has several
tappings which are conductors connected to it.

These conductors lead out of the winding of the
transformer and each conductor finishes in a
terminal or stud.
 These studs are used to connect a variable
number of turns of the Autotransformer into its
secondary circuit by means of a manual control
which moves a rotary switch from stud to stud.

This stud selector switch is marked on the control
panel of the X-ray set as the K V selector.
 The stud-selector switch gives a variable
secondary voltage as the output of the autotransformer which is then applied as the primary
input voltage of the high tension transformer
(HTT).

The figure above is a circuit diagram which
shows only a few tappings for the sake of clarity.
 In practice, there must be one tapping for every
K V value used in the X-ray set.

CONTINUOUS CONTROL

OF
KILOVOLTAGE
Previously K V control has been described as being
selected through a series of fixed steps prior to
exposure and it is not possible to change voltage
during exposure – exposure periods too short.
Therefore radiographers do not alter K V during
exposure but during fluoroscopy, the time
intervals during which the X-ray tube is
energized are longer.
 K V control in this case is of step less type i.e. is
continuously controlled.
 This is achieved by using a variac transformer.

VARIAC TRANSFORMER
KV
IN
CONTROL
OF
A variac transformer is an autotransformer of a
particular type.
 The insulation is a single winding wound on a
cylindrical core.
 The insulation is removed along a track on the
winding and a moving contact or brush slides
over the track.


This allows the ratio of turns in the primary and
secondary circuits of the transformer to be
varied.
In the above diagram the cylindrical core is
shown as a ring with the windings round it.
 The windings between P 1 and P 2 are the number
of turns of the transformer which are in the
primary circuit and primary voltage is applied
between them.

B is the moving contact or brush which rotates
round the annular track.
 No insulation is applied between it and the
winding and hence it makes electrical contacts
when it is in any given position.
 The turns of the transformer which are in the
secondary circuit vary for any given position of B,
being more when B is near P 2 and less when B is
near P 1 .

This makes it possible to obtain a continuously
variable secondary voltage from this transformer.
 Note: It is important to control tube voltage (KV)
due to the varying subject density.

Important considerations:
 K V determines radiation quality and therefore
the penetrating power of the X-ray beam.
 Subject type e.g.
-Thickness
-Density of tissue involved
-Pathology of area under examination
The presence of contrast media etc
 Contrast range required of exposed films
 Need to reduce absorbed dose to patients

CONTROL OF HIGH TENSION
1. Resistance Control of High Tension
 This is the simplest and the cheapest component
which can be used to vary the input voltage to
the primary winding of a transformer.
To obtain a change in the value of current (mA)
in the secondary circuit we must change the
value of the current in the primary circuit by
changing the value of the variable resistor.
 If kV in the 20 circuit has to be changed, the
value of the voltage in the 10 circuit must also be
changed – also by changing the value of the
resistor.

Formula
 Resistance (R) = voltage drop
Current
(V)
(I)
If the mains is connected to a supply of 210V and
the primary circuit requires 10 Amperes at 150
volts to produce and exposure of 60 kVp.
Then: R = Voltage drop = 210 –
150 volts
Current
10
= 60
10
=
6 ohms 
For example:
 If we wish to increase the secondary voltage from
60 kV to 80 kV but without increasing the mA,
then for 80 kV and 25mA, the voltage in the
primary circuit would have to be increased to
200v (at 10 amps).
This requires a voltage drop of only 10 volts and
so:
 Power in 10 = Power in 20
 V P x 10 Amp = 80KV x 25 M A

VP = 80 x 1000 x 25
1000
10 Amp
 = 200 volts
 This requires a voltage drop of 210 – 200= 10
volts only.
 It requires a resistance of V = 10 = 1 ohm
I 10

DISADVANTAGES
OF RESISTANCE CONTROL
The need to change the value of resistance for
different kV (When mA is constant) or mA (When
kV is constant) is a considerable nuisance to the
user.
 The value of current flowing through the variable
resistance would lead to variation in its
temperature and therefore its resistance value
for any particular setting may not be constant.

The resistance cannot provide voltages higher in
value than that of the supply to it.
 Note: Resistance method of voltage control is
used in some radiotherapy units, also used in:
1. Very low power portable diagnostic units
2. For the control of low value limited range
voltages to the step-down filament transformers
for X-ray tubes and valves.

TRANSFORMER
CONTROL OF HIGH TENSION
A step-up double wound transformer is used.
 The ratio of this control transformer can be
varied to provide voltages below and above the
supply by means of tapings from its secondary
winding.
 The voltages selected from the tapings by means
of a selector switch become the supply to the
primary windings of HTT.

AUTOTRANSFORMER
CONTROL OF
H.T.
When the mains supply is connected across some
of the turns (X-Y) of the winding of the
autotransformer TA, the current flowing through
the section of the winding produces magnetic
flux.
 This flux extends throughout the core and links
with every turn of the winding.

The E M F induced in the winding will produce a
potential difference between X-Z.
 The voltage between X and any one of the
tappings can be obtained between C and F by
means of the selector switch SW 2 and this
selected output voltage supply to the primary
winding of the HTT, ignoring transformer losses,


V P = TP and Vs = V P x T S
VS TS
TP

The voltmeter M 2 measures the voltage between
C-F, therefore given the step-up ratio of the high
tension transformer (HTT), the voltage across its
secondary winding will be given by the product
primary voltage and the transformer ratio –
TS/TP x Vp.
e.g. Let selected primary volts = 230v and
transformer ratio (HTT) = 59,000: 200
 Then; V S = 230 x 59000
200
 = 67,850V (R.M.S) Root mean square values of
K V peak.

= 67.85 K V (RMS)
 Converting to peak value:
= 67.85 x 1.414
= 95.9 K V peak

HIGH TENSION
KV
INDICATION
Importance; (of kv indication)
 It helps in knowing the penetrating power of the
beam being used
 It allows in the selection and controlling the
penetrating power of the beam in order to get the
desired density for a given examination.
 It helps in avoiding overload to the tube which
could be more damaging.
It helps in fault finding especially if the film
quality is deteriorating
Methods
 There are two methods of K V indication
 Calibrated auto-transformer
 Pre-reading K V meter

THE CALIBRATED AUTO-TRANSFORMER
When the selector switch is moved it corresponds
with the chosen value:
 e.g. if 200v (RMS) are taken from the
autotransformer and applied to the primary
windings of the HTT, Transformer ratio = 400: 1,
then the kV resulting from this position of the
selector switch will be; (assuming no transformer
losses):


200 x 400 = 80,000v
= 80 kV (R.M.S).
= 80 X 1.414
= 113.12 K V P
But, when the X-ray tube is passing current, the
current flowing through the tube is also flowing
as a load current through the windings of the
HTT.
 These windings have resistance and voltage must
be used to make load current flow against this
resistance.

Therefore:
 Voltage passing through the tube = calculated
voltage minus kV drop
 kV drop = voltage used to make the load current
flow through the windings.

K V drop and tube current
 High tension rectifiers connected between the
transformer and the X-ray tube increases the K V
drop.
 Therefore total kV drop = kV drop in transformer
+ kV drop in the rectifiers.
Since the total K V drop varies with the X-ray
tube current (mA), the kV available from selector
A must also vary with the mA.
 It will be lower from the highest tube load and
greatest at a tube current near zero.
 To compensate for this irregularity, a voltage
matching with the K V drop is added to the
primary circuit of the HTT, this is done by a
kilovoltage compensator K VC .

This compensator is ganged to the M A selector.
 When the M A selector is moved to select a higher
current, the K V C is moved in the direction
towards ‘H’, selecting more voltage to the
primary of the HTT.
 The control moves towards ‘L’ when low currents
(MA) are being selected.

PRE-READING KV
METER
This is a voltmeter which indicates to a
radiographer the kV obtainable from the
different positions of the kV Selector.
 The meter is connected to the out-put of the autotransformer and is energized by the voltage
applied to the primary winding of the HT T when
exposure begins.

Due to the kV drop, a meter reading compensator
is incorporated and ganged to mA selector.
 The winding M C is added to the auto
transformer, but wound in a direction opposite to
that of the main autotransformer winding.


It therefore produces a counter voltage which is
opposite in polarity to the output voltage from
the main autotransformer.
The pre-reading meter indicates the kV
obtainable from different position of kV selector.
The output of the high tension is into peak values
and the values indicated on the pre-reading kV
meter
 Thus;
 (Various voltages from tappings) x (step-up ratio
HTT) x 1.414.

Therefore voltage across the pre-reading K V
meter
 = output of Auto-transformer (-) counter voltage.
 kV is therefore selected after mA values so that
counter voltage remains unchanged for the
selected K V.

REFERENCES
Chesney D.N. and Chesney M.O. (1971) X-ray
equipment for student radiographers, Blackwell
Scientific Publications
 Carter P. H., Paterson A. M. (1994) Chesneys'
Equipment for Student Radiographers
 F. Jaundrell-Thompson & W. J . Ashworth (1965)
X Ray Physics and Equipment, Blackwell
Scientific Publications Ltd.

END