AC power supply.
(a) In ac operation, a conventional step-down power transformer (700
degrees 42) which delivers energy to a bridge rectifier CR30 through CR33 and, in turn, to
a filter C40. The resulting direct current is split in the following two ways: (1)
approximately 250 milliamperes for the constant current battery charge, when operating
and, (2) the rest feeds a voltage preregulator. It is capable of running the monitor and
chart recorder totally independent from the battery, with instantaneous switch-over from
battery power to line power as soon as the line cord is plugged into the outlet.
(b) If the preregulator is delivering less than 12.5v at its output, Q43 is
conducting very little even in cut-off condition. All or part of the current flowing through
RA7 is applied to the base of Q42, throwing this transistor in heavy conduction.
(c) The resulting collector current is driving the base of 041, reducing the
voltage drop across this pass transistor, which increases the voltage on its collector in
reference to ground.
(d) On the other hand, if the output voltage of the preregulator is above
the calibrated 12.5v, Q43 goes into heavy conduction. This deprives Q42 from its base
drive. Its collector current is reduced, and the current through Q41 reduces the output
voltage. Regulation of the circuit is better than 5 percent departure from the nominal value
against line voltage and load variations.
Battery charger (NiCad).
(a) Because of the particular characteristics of NiCad batteries, it is important
to charge them using a temperature-compensated, constant-current supply. The charging
current should be approximately 1/10 of the ampere-hour rating of the battery. In the Porta
Pak 90, these 2.2AH batteries require a charging current of 250ma. A pass transistor Q40
supplies the charging current through diode CR36 to the battery. The base of Q40 is
driven by a network consisting of CR34, RA2, and RA3 which maintains a certain base
current adjustable by RA2.
(b) In order to make this network independent of rectified line voltage
variations, a zener diode CR35 keeps a constant voltage of 12v across it. This is due to
the current flowing to ground through RA4. The combination of CR34 together with
negative feedback resistor RA1 and the constant current drive network maintains the
charging current very independent of line voltage and temperature variations.
On-off and low battery shut-off circuit.
(a) Both functions are combined into one building block. In battery
operation, only a high impedance voltage divider composed of RA15, RA16, and RA17 is
adjusted so as to present, under normal battery voltages, more than 1.35v to pin 3 of U21.