b. Functions of the Circuits in the Main Power ON/OFF Circuitry. The main
power source for the Porta Pak 90 is a 12v rechargeable NiCad battery.
(1) The main power ON switch applies battery power to several sections of the
machine for functions which are not dependent on high voltage, such as the synchronizer,
the monitor, paddle preamplifier, and floating power supply.
(2) Other voltages are available only when the panel or paddle charge button
is depressed. These voltages are applied through the closure of contacts on the power
relay K1, which remains latched on by the action of transistor Q9. In this way, such
actions as firing the paddles or pushing the disarm switch allow Q9 to be interrupted. The
relay opens allowing the whole cycle to be started again.
(3) The battery status is monitored continuously by a front panel meter
showing the user when the battery is approaching depletion of charge.
c. Circuits in the Battery Charger (NiCad).
(1) Because of the particular characteristics of NiCad batteries, it is important to
charge them using a constant current supply. The charging current should be
approximately 1/10 of the ampere-hour (AH) rating of the battery. In the Porta Pak 90,
these 2.5 AH batteries require a charging current of 250ma. A pass transistor Q1 supplies
the charging current through diode CR53 to the battery.
(2) The base of Q1 is driven by a network consisting of CR56, R115, and R85
which maintains a certain base current adjustable by R115. In order to make this network
independent of rectified line voltage variations, a zener diode CR80 keeps a constant
voltage of 12v across it, due to the current flowing to ground through R84.
(3) The combination of CR56 together with negative feedback resistor R86
and the constant current drive network maintains the charging current independent of line
voltage and temperature variations.
d. Functions of the Circuits in the 12v Floating Power Supply.
(1) The 12v floating power supply is needed to provide isolation from the
primary power. This is in order to operate all the controlling circuits in the high voltage or
secondary side of the defibrillator circuit, such as the high voltage regulator, the primary
dump circuit, and the delivered energy circuit. All other circuits are supplied either from the
battery directly or from power which is switched on only when the machine is charging.
(2) This latter voltage is supplemented by the regulated filtered output from
the primary side of the main power invertor, called the 12v booth supply, consisting of U4
and its associated components.