Physio Control LIFEPAK 9 Service Manual page 33

Description 1
Preamp Isolated Power Supply. The Isolated Power Supply provides the isolated
+15Vdc, +12Vdc, -15Vdc, +5 Vdc, and -12Vdc supplies required by the
Preamp circuitry. The drive for transformer T1 is generated by pulse-width
modulator US. Capacitor C12 and R28 set the frequency between 12kHz and
24kHz. The secondary of T1 is applied to full-wave bridge CR20. The
positive half of the ac input waveforms are stored at C87, forming the
isolated +15V supply. The negative ac waveforms are stored at C86, forming
the isolated -15V supply. Regulators U38 and U39 isolate more sensitive
circuitry from the heavier loads. Since the +5 Vdc loads are synchronous and
small, U37 doubles as a regulated supply and a reference source.
Front-End Protection. To protect against voltage surges, VSP1 through
VSP3 limit the input current to 2mA. Isolation for the paddles input is
provided by R131 and R132 which guard against excessive charge input to
the Preamp. Diodes CR18, CR19, and CR28 clamp the differential paddle
voltage when Kl is switched to the leads mode.
Lead Select. To select between paddles and the different patient leads, latch
U33 is loaded with the decoded data from counter U32. Flip-flop U33
controls relay K1, U43A, and analog switches U42 and U41. To select
between patient leads or paddles, U33 causes Q8 or Q9 to conduct, switching
the state of K1 to the desired position. Patient leads are selected when Q8
conducts; paddles are selected when Q9 conducts. If patient leads are
selected, analog switch U42 selects the lead (1,II,1I, or STD) requested by the
output of U33.
Optical Interface. Command pulses from the System microprocessor pass
through the the optical interface to perform three different functions:
synchronize the ECG conversion, select 1 of 5 patient cable leads, and induce
a +1mV calibration signal. The System microprocessor uses an internal
timer to transmit a pulse of variable duration (approximately 80ms to 1.2ms)
and a fixed-freguency period (540Hz). The variable-duration pulse from the
System microprocessor enables U4 to start counting, creating a 16kHz pulse
train at U4 pin 4.
The System microprocessor controls the count of the pulse train by
interrupting the output with a CLR signal as shown at TP6 in Figure 1-5.
The 16kHz pulse drives the infrared-emitting diode in U7 providing pulse
isolation. Comparator U34D performs waveform conditioning for the pulse
train as it passes to the Instruction Decoding circuit, U32 and U33.
© August 1993 Physio—Control All Rights Reserved 1-19