the x-ray tube itself. In other units, rectification may be done by valve-tubes or modern
rectifiers. Some machines use two rectifiers, one placed on either side of the x-ray
tube. This produces efficient "half-wave" rectification. For still greater efficiency, four
rectifiers may be used in a bridge circuit (figure 2-23) to produce "full-wave" rectification.
(4) The x-ray tube is, of course, the primary component in the machine. All
other devices and mechanisms in an x-ray machine are for bringing the proper currents
to the tube at the proper times. The x-ray tube will be discussed at length later in this
THE TIMER OR CONTROL CIRCUIT
The timer circuit (figure 3-4) consists essentially of a timing device that can be
varied. This timing device activates a contactor that allows high-tension voltage to be
applied to the x-ray tube. The timer circuit draws its power from the main line, through
the main line switch and fuses. The timer itself is a modification of a push-button
switch, but has a timing mechanism that automatically cuts off the current after a preset
time. Note that the contactor is closed (and x-rays are produced) only when current
flows through the timer circuit. There are four main types of timers in general use at
present. These are the hand or mechanical timer, the synchronous timer, the impulse
timer, and the electronic timer. There is also another type, the photoelectric timer,
which is not so widely used.
a. The hand or mechanical timer has a spring-wound motor that is allowed to
run at constant speed, the timing dependent on how far the dial is initially set. The
mechanical timer has a theoretical minimum time of l/8 second; however, it is usually
not advisable to use such a timer for time intervals of less than 3/8 second because of
its inaccuracy at these shorter time intervals. This type of timer is found almost
exclusively on small portable machines, which do not require short accurate exposures.
b. The synchronous timer consists of a small synchronous motor, the revolutions
of which are counted and used as the timing factor. The synchronous timer can usually
be set for time intervals from as short as 1/20 second to as long as about 20 seconds
(the exact range depending upon the particular manufacturer's design, as is true of all
types of timers).
c. The impulse timer may operate at time intervals from as short as 1/120
second to as long as about 1/5 second. Because the maximum timing exposure of an
impulse timer is usually about 1/5 second, a synchronous timer is generally installed
along with it to provide for the longer exposures. The impulse timer is much more
accurate than the synchronous since it starts and stops the current at the zero point (no-
current interval) of the alternating current cycle.
d. Electronic timers use electronic circuitry to count the alternating current
pulsations, and they cover the entire range of times from milliseconds to several
seconds more accurately and reliable than impulse timers.