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CPI Canada Inc.
Theory of Operation
7
Use and disclosure is subject to the restrictions on the title page of this CPI document.
CMP 200
®
X-Ray Generator Service Manual Ch # 901476-09 Rev. C
Page 7-13
7.2.12 AEC
(Cont)
Refer to MD-0936, sheet 4:
This shows the signal processing circuits for AEC board assembly 737992. The AEC channel 1 to
channel 4 outputs are connected to the inverting input on U12A, which is a variable gain amplifier where
the gain of this stage depends on the number of AEC fields that are selected. With one field selected, the
gain of this stage will be at its maximum; and with three fields selected, the gain of this stage will be
divided by three. The variable gain of U12A compensates for the variable voltage output of the input
preamplifiers, thus keeping the signal output from U12A constant relative to the number of fields that are
selected.
The output of U12A feeds the inverting input of U11B. A sample and hold circuit is connected to the non-
inverting input on U11B. This circuit samples any electrical noise at the output of U12A during standby
operation, and subtracts this noise from the AEC signal during an AEC exposure. This ensures that the
output of U11B is proportional to the AEC chamber output only, and is not influenced by noise.
The output of U11B is connected to the input of U11A via the gain adjustment pots R1 to R4 and the
analog switches for channels 1 to 4.
U11A is an integrating amplifier where C31 starts to integrate the AEC voltage when the AEC exposure
starts. This is accomplished by opening the analog switches that are in series with R35, R38, R42, and
R59 at the start of the exposure. This produces a ramp whose slope is proportional to the input voltage at
U11A.
The AEC ramp is taken to the generator control board where it is processed as described earlier in this
section, and fed to comparator U10 on the AEC board where it is compared to the AEC reference voltage.
The output of U10 is normally high, switching low when the magnitude of the AEC ramp equals the
reference voltage. This PT stop signal is further processed on the generator control board.
The chamber-select signals exit on the generator control board as per sheet 1. Only one chamber may be
active at one time, and the active channel is indicated by DS1 to DS4 on the AEC board. The chamber-
select signals are inverted by U6, and then connected to the control inputs on analog switches U4D,
U14D, U5D, and U15D (sheet 3) for AEC channel selection and to the analog switches in series with R1
to R4 to select the required gain pot.
The field select signals from the generator control board are also active low. These signals are inverted
by U6, and then connected to the analog switches in the feedback loop of U12A, and to the analog
switches that select the fields on the previous sheet.
The start signal from the generator control board is inverted by U6 on the AEC board, and then inverted
again by Q2. The output of Q2 is connected to the analog switches in the feedback loop of U11A. The
extra inversion provided by Q2 is required to open those analog switches during an exposure.
Refer to MD-0936, sheet 5:
This is the functional schematic of AEC assembly 737998, used with ionization AEC chambers. The AEC
chambers are connected to J1 to J4. The AEC signal from the AEC chamber(s) is routed to the input of
analog switches U1A to U1D on the AEC board. These analog switches are controlled by the chamber-
select signals.
The chamber-select signals exit on the generator control board as per sheet 1. Only one chamber may be
active at one time, and the active channel is indicated by DS1 to DS4 on the AEC board. The chamber-
select signals are inverted by U6, and connected to the control inputs of the analog switches described in
the previous paragraph. The analog switch corresponding to the selected AEC channel will be closed
when that channel is selected, thus connecting the AEC signal to the input of U7A.