3. Delay Circuit With Tube Stator
• This is an interlock which prevents an exposure
from occurring unless the anode has reached its
full speed of rotation.
• It also allows time for the filament to heat up to
the level necessary for thermionic emission to
occur.
• If the anode is subjected to the full load while it is
accelerating, it may be damaged nearly the same
way as if it were stationery.
4. • This is why it is necessary to hold the exposure
switch at prepare position long enough to
allow the anode to reach the desired speed.
• Sometimes however, radiographer may be
tempted to make the exposure in a hurry due
to the patients condition.
• If such an exposure occurs, the tube will
definitely be damaged, particularly if its
repeated on several occasions.
5. • The delay circuit ensures that such an
exposure does not begin until time needed for
the anode to rotate fully is over.
• Normally this time is 0.8 seconds for standard
speed anodes 2 seconds for high speed
anodes.
7. It consists of
1.Contacts in series with the exposure
contactor coil
2. A relay coil
3. A capacitor in parallel connection with the
relay.
8. • A variable resistor in series with the capacitor
when the unit is put into prepare POS, the delay
circuit is completed.
• Since the capacitor and the relay coil are in
parallel connections, the voltage across each of
them is equal.
• Immediately the circuit is completed, the
capacitor starts to charge through the resistor.
• The capacitor will therefore draw very heavy
currents for its charging, leaving very little current
for the relay.
9. • When the capacitor is fully charged it stops
drawing high currents therefore the current
through the relay increases. This energizes the
relay making it strong enough to close contact
S.
• When the exposure button is pressed, the
exposure contactor switch is closed and the
exposure begins.
10. • Delay time; is the time taken by the capacitor
to charge to its full potential. It is determined
by the value of the selected resistance.
• 0.8 Sec – Std speed anode
• 2 sec – high speed anode.
11. Overload Interlocks
These are circuits which prevent an exposure
from occurring if the selected exposure factors
are above the tube rating.
There are two types.
1. Mechanical interlocks
2. Electronic interlocks (analogue circuits)
12. Mechanical Interlocks
• They have their components arranged such
that the exposure controls physically obstruct
one another in a certain position. This is
achieved by use of a chain, a lever and a cam.
• A cam is a wheel of eccentric design used to
produce an alternating or a rectilinear
movement of some other parts of the
machinery. It’s a simple device which involves
no circuitry.
13. • A cam turned in a clockwise direction to a
certain position results in a linear excursion of
a pivoted bar. The arrangement is as shown
below.
14.
15. • The mA selector and the kV selector are
connected together by a chain which passes
also round a pulley at the free end of a
pivoted bar, B. The spring lever is spring-
loaded in order to hold the chain taut.
• Selection of a higher mA or kV, or both, winds
the chain round each of two pulleys, physically
shortening it.
16. • In this circumstances the lever moves
‘upwards’(as indicated by A arrow) to
accommodate the decreased length of the chain.
• Fitted to the time selector is a cam. Clockwise
rotation of the cam (as happens when greater
exposures times are chosen), eventually results in
the widest part of the cam reaching a position in
which it will bear on the lever B and prevent its
‘upwards’ movement i.e prevent further
shortening of the chain, thus selection of any
greater mA or kV or both.
17. If further clockwise rotation of the cam is
attempted against the resistance thus offered
(that is if the operator choses a longer interval
of exposure), the combine action of the lever
and the chain will turn the kilo-voltage control
anti-clockwise (lower the selected kilo-
voltage).
18. Application
• Not commonly used in modern X-ray units.
However it may still be found in some old
mobile units.
19. Electronic Overload Interlocks
(Analogue Circuit)
• Is the electronic type which functions by
adding together voltages which are each
representative of one of the factors
comprising the tube load (KV, MA, time).
20.
21. • T is a special transformer, its primary voltage is
obtained from the autotransformer by means of a
sliding contact which is ganged to the KV selector.
• The higher the KV selected, the higher the input
voltage to this transformer. The secondary
winding is itself tapped by another sliding
contact.
This contact is linked to the MA selector thus the
output voltage from the transformer is subject to
two independent voltages; a. KV selected
b. Selected MA
22. • Alterations in either of these factors will
correspondingly raise or lower the output
from the A/transformer.
• The output from T, is rectified and fed by way
of a series of resistors R, to supply the
negative bias from the grid of a triode valve.
23. • R is a variable resistor, and has a sliding contacts
which is ganged to the timer in such a way that,
when the timer is set for longer intervals of
exposure, the resistance from the circuit is
progressively diminished.
• As R decreases in value, the voltage drop across
it becomes less. In this way, the ultimate voltage
applied to the grid of the triode valve is an
aggregate of:
• Selected KV
24. • Selected MA
• Selected exposure time
• The grid voltage is raised or lowered whenever
any of this factors is raised or lowered. The
triode valve is conducting until the grid
voltage mounts to some predetermined value.
When this critical value is reached, the valve
becomes at once non-conductive i.e. an open
switch.
25. • The diagram shows that the current through
the triode valve also passes through the coil of
a normally open relay (the percentage load
relay) which has its contacts in series with the
exposure contactor coil.
• So long as the valve is conducting, the
percentage load relay contacts L is closed.
26. • The circuit to the exposure contactor coil can
be completed through the hand switch and
timer contacts, and radiographic exposure can
be made once the valve is non-conductive, the
relay coil is no longer energized, the
percentage load relay contact L opens and the
occurrence of the radiographic exposure is
prevented.
27. • This type of interlock allows the radiographer
a free choice of KV, MA and time
independently.
• If the aggregate voltage in the analogue circuit
is too high, a reduction in any of the tube
factors is effective.
28. Limitations Of Overload Interlock Circuits
• Overload interlocks indicates an overload
resulting from a given set of exposure or one
against overload due to factors such as:
• Overload occurrence due to repeated exposures
which are by themselves within safe limits.
• When the anode has accumulated excessive heat
due to repeated exposures. In order to be on the
safe side.
• It is important for the radiographer to stick to the
rating charts of every equipment.
